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<?xml version = '1.0'?>
<?xml-stylesheet type="text/xsl" href="styleguide.xsl"?>
<GUIDE title="Google JavaScript Style Guide">
<p class="revision">
Please note: there's a newer version of this guide that includes
ECMAScript 6th Edition features. It lives <a href="jsguide.html">here</a>.
You should probably be using that for new code.
Revision 2.93
</p>
<address>
Aaron Whyte<br/>
Bob Jervis<br/>
Dan Pupius<br/>
Erik Arvidsson<br/>
Fritz Schneider<br/>
Robby Walker<br/>
</address>
<OVERVIEW>
<CATEGORY title="Important Note">
<STYLEPOINT title="Displaying Hidden Details in this Guide">
<SUMMARY>
This style guide contains many details that are initially
hidden from view. They are marked by the triangle icon, which you
see here on your left. Click it now.
You should see "Hooray" appear below.
</SUMMARY>
<BODY>
<p>
Hooray! Now you know you can expand points to get more
details. Alternatively, there's a "toggle all" at the
top of this document.
</p>
</BODY>
</STYLEPOINT>
</CATEGORY>
<CATEGORY title="Background">
<p>
JavaScript is the main client-side scripting language used
by many of Google's open-source
projects.
This style guide is a list of <em>do</em>s and <em>don't</em>s for
JavaScript programs.
</p>
</CATEGORY>
</OVERVIEW>
<CATEGORY title="JavaScript Language Rules">
<STYLEPOINT title="var">
<SUMMARY>
Declarations with <code>var</code>: Always
</SUMMARY>
<BODY>
<DECISION>
When you fail to specify <code>var</code>,
the variable gets placed in the global context, potentially clobbering
existing values. Also, if there's no declaration, it's hard to tell in
what scope a variable lives (e.g., it could be in the Document or
Window just as easily as in the local scope). So always declare with
<code>var</code>.
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Constants">
<SUMMARY>
<ul>
<li>Use <code>NAMES_LIKE_THIS</code> for constant <em>values</em>.</li>
<li>Use <code>@const</code> to indicate a constant (non-overwritable)
<em>pointer</em> (a variable or property).</li>
<li>Never use the
<a href="https://developer.mozilla.org/en/JavaScript/Reference/Statements/const">
<code>const</code> keyword</a>
as it's not supported in Internet Explorer.</li>
</ul>
</SUMMARY>
<BODY>
<DECISION>
<SUBSECTION title="Constant values">
<p>If a value is intended to be <em>constant</em>
and <em>immutable</em>, it should be given a name
in <code>CONSTANT_VALUE_CASE</code>.
<code>ALL_CAPS</code> additionally implies <code>@const</code>
(that the value is not overwritable).
</p>
<p>Primitive types (<code>number</code>, <code>string</code>,
<code>boolean</code>) are constant values.</p>
<p><code>Objects</code>'
immutability is more subjective — objects should be
considered immutable only if they do not demonstrate observable
state change. This is not enforced by the compiler.</p>
</SUBSECTION>
<SUBSECTION title="Constant pointers (variables and properties)">
<p>The <code>@const</code> annotation on a variable or property
implies that it is not overwritable. This is enforced by the
compiler at build time. This behavior is consistent with the
<a href="https://developer.mozilla.org/en/JavaScript/Reference/Statements/const">
<code>const</code> keyword</a> (which we do not use due to the
lack of support in Internet Explorer).</p>
<p>A <code>@const</code> annotation on a method additionally
implies that the method cannot not be overridden in subclasses.
</p>
<p>A <code>@const</code> annotation on a constructor implies the
class cannot be subclassed (akin to <code>final</code> in Java).
</p>
</SUBSECTION>
<SUBSECTION title="Examples">
<p>Note that <code>@const</code> does not necessarily imply
<code>CONSTANT_VALUES_CASE</code>.
However, <code>CONSTANT_VALUES_CASE</code>
<em>does</em> imply <code>@const</code>.
</p>
<CODE_SNIPPET>
/**
* Request timeout in milliseconds.
* @type {number}
*/
goog.example.TIMEOUT_IN_MILLISECONDS = 60;
</CODE_SNIPPET>
<p>The number of seconds in a minute never changes. It is a
constant value. <code>ALL_CAPS</code>
also implies <code>@const</code>, so the constant cannot be
overwritten.
</p>
<p>The open source compiler will allow the symbol to be
overwritten because the constant is
<em>not</em> marked as <code>@const</code>.</p>
<CODE_SNIPPET>
/**
* Map of URL to response string.
* @const
*/
MyClass.fetchedUrlCache_ = new goog.structs.Map();
</CODE_SNIPPET>
<CODE_SNIPPET>
/**
* Class that cannot be subclassed.
* @const
* @constructor
*/
sloth.MyFinalClass = function() {};
</CODE_SNIPPET>
<p>In this case, the pointer can never be overwritten, but
value is highly mutable and <b>not</b> constant (and thus in
<code>camelCase</code>, not <code>ALL_CAPS</code>).</p>
</SUBSECTION>
</DECISION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Semicolons">
<SUMMARY>
Always use semicolons.
</SUMMARY>
<BODY>
<p>Relying on implicit insertion can cause subtle, hard to debug
problems. Don't do it. You're better than that.</p>
<p>There are a couple places where missing semicolons are particularly
dangerous:</p>
<BAD_CODE_SNIPPET>
// 1.
MyClass.prototype.myMethod = function() {
return 42;
} // No semicolon here.
(function() {
// Some initialization code wrapped in a function to create a scope for locals.
})();
var x = {
'i': 1,
'j': 2
} // No semicolon here.
// 2. Trying to do one thing on Internet Explorer and another on Firefox.
// I know you'd never write code like this, but throw me a bone.
[ffVersion, ieVersion][isIE]();
var THINGS_TO_EAT = [apples, oysters, sprayOnCheese] // No semicolon here.
// 3. conditional execution a la bash
-1 == resultOfOperation() || die();
</BAD_CODE_SNIPPET>
<SUBSECTION title="So what happens?">
<ol>
<li>JavaScript error - first the function returning 42 is called
with the second function as a parameter, then the number 42 is
"called" resulting in an error.</li>
<li>You will most likely get a 'no such property in undefined'
error at runtime as it tries to call
<code>x[ffVersion, ieVersion][isIE]()</code>.</li>
<li><code>die</code> is always called since the array minus 1 is
<code>NaN</code> which is never equal to anything (not even if
<code>resultOfOperation()</code> returns <code>NaN</code>) and
<code>THINGS_TO_EAT</code> gets assigned the result of
<code>die()</code>.</li>
</ol>
</SUBSECTION>
<SUBSECTION title="Why?">
<p>JavaScript requires statements to end with a semicolon, except when
it thinks it can safely infer their existence. In each of these
examples, a function declaration or object or array literal is used
inside a statement. The closing brackets are not enough to signal
the end of the statement. Javascript never ends a statement if the
next token is an infix or bracket operator.</p>
<p>This has really surprised people, so make sure your assignments end
with semicolons.</p>
</SUBSECTION>
<SUBSECTION title="Clarification: Semicolons and functions">
<p>Semicolons should be included at the end of function expressions,
but not at the end of function declarations. The distinction is
best illustrated with an example:</p>
<CODE_SNIPPET>
var foo = function() {
return true;
}; // semicolon here.
function foo() {
return true;
} // no semicolon here.
</CODE_SNIPPET>
</SUBSECTION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Nested functions">
<SUMMARY>Yes</SUMMARY>
<BODY>
<p>Nested functions can be very useful, for example in the creation of
continuations and for the task of hiding helper functions. Feel free
to use them.</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Function Declarations Within Blocks">
<SUMMARY>No</SUMMARY>
<BODY>
<p>Do not do this:</p>
<BAD_CODE_SNIPPET>
if (x) {
function foo() {}
}
</BAD_CODE_SNIPPET>
<p>While most script engines support Function Declarations within blocks
it is not part of ECMAScript (see
<a href="http://www.ecma-international.org/publications/standards/Ecma-262.htm">ECMA-262</a>,
clause 13 and 14). Worse implementations are inconsistent with each
other and with future EcmaScript proposals. ECMAScript only allows for
Function Declarations in the root statement list of a script or
function. Instead use a variable initialized with a Function
Expression to define a function within a block:</p>
<CODE_SNIPPET>
if (x) {
var foo = function() {};
}
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Exceptions">
<SUMMARY>Yes</SUMMARY>
<BODY>
<p>You basically can't avoid exceptions if you're doing something
non-trivial (using an application development framework, etc.).
Go for it.</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Custom exceptions">
<SUMMARY>Yes</SUMMARY>
<BODY>
<p>Without custom exceptions, returning error information from a
function that also returns a value can be tricky, not to mention
inelegant. Bad solutions include passing in a reference type to hold
error information or always returning Objects with a potential
error member. These basically amount to a primitive exception
handling hack. Feel free to use custom exceptions when
appropriate.</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Standards features">
<SUMMARY>Always preferred over non-standards features</SUMMARY>
<BODY>
<p>For maximum portability and compatibility, always prefer standards
features over non-standards features (e.g.,
<code>string.charAt(3)</code> over <code>string[3]</code> and element
access with DOM functions instead of using an application-specific
shorthand).</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Wrapper objects for primitive types">
<SUMMARY>No</SUMMARY>
<BODY>
<p>There's no reason to use wrapper objects for primitive types, plus
they're dangerous:</p>
<BAD_CODE_SNIPPET>
var x = new Boolean(false);
if (x) {
alert('hi'); // Shows 'hi'.
}
</BAD_CODE_SNIPPET>
<p>Don't do it!</p>
<p>However type casting is fine.</p>
<CODE_SNIPPET>
var x = Boolean(0);
if (x) {
alert('hi'); // This will never be alerted.
}
typeof Boolean(0) == 'boolean';
typeof new Boolean(0) == 'object';
</CODE_SNIPPET>
<p>This is very useful for casting things to
<code>number</code>, <code>string</code> and <code>boolean</code>.</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Multi-level prototype hierarchies">
<SUMMARY>Not preferred</SUMMARY>
<BODY>
<p>Multi-level prototype hierarchies are how JavaScript implements
inheritance. You have a multi-level hierarchy if you have a
user-defined class D with another user-defined class B as its
prototype. These hierarchies are much harder to get right than they
first appear! </p>
<p>For that reason, it is best to use <code>goog.inherits()</code> from
<a href="https://code.google.com/closure/library/">
the Closure Library
</a>
or a similar library function.
</p>
<CODE_SNIPPET>
function D() {
goog.base(this)
}
goog.inherits(D, B);
D.prototype.method = function() {
...
};
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Method and property definitions">
<SUMMARY><code>/** @constructor */
function SomeConstructor() {
this.someProperty = 1;
}
Foo.prototype.someMethod = function() { ... };</code></SUMMARY>
<BODY>
<p>While there are several ways to attach methods and properties to an
object created via "new", the preferred style for methods
is:</p>
<CODE_SNIPPET>
Foo.prototype.bar = function() {
/* ... */
};
</CODE_SNIPPET>
<p>The preferred style for other properties is to initialize the field
in the constructor:</p>
<CODE_SNIPPET>
/** @constructor */
function Foo() {
this.bar = value;
}
</CODE_SNIPPET>
<SUBSECTION title="Why?">
<p>Current JavaScript engines optimize based on the "shape"
of an object, <a href="https://developers.google.com/v8/design#prop_access">
adding a property to an object (including overriding
a value set on the prototype) changes the shape and can degrade
performance.</a></p>
</SUBSECTION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="delete">
<SUMMARY>Prefer <code>this.foo = null</code>.</SUMMARY>
<BODY>
<CODE_SNIPPET>
Foo.prototype.dispose = function() {
this.property_ = null;
};
</CODE_SNIPPET>
<p>Instead of:</p>
<BAD_CODE_SNIPPET>
Foo.prototype.dispose = function() {
delete this.property_;
};
</BAD_CODE_SNIPPET>
<p>In modern JavaScript engines, changing the number of properties on an
object is much slower than reassigning the values. The delete keyword
should be avoided except when it is necessary to remove a property
from an object's iterated list of keys, or to change the result of
<code>if (key in obj)</code>.</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Closures">
<SUMMARY>Yes, but be careful.</SUMMARY>
<BODY>
<p>The ability to create closures is perhaps the most useful and often
overlooked feature of JS. Here is
<a href="http://jibbering.com/faq/faq_notes/closures.html">
a good description of how closures work</a>.</p>
<p>One thing to keep in mind, however, is that a closure keeps a pointer
to its enclosing scope. As a result, attaching a closure to a DOM
element can create a circular reference and thus, a memory leak. For
example, in the following code:</p>
<BAD_CODE_SNIPPET>
function foo(element, a, b) {
element.onclick = function() { /* uses a and b */ };
}
</BAD_CODE_SNIPPET>
<p>the function closure keeps a reference to <code>element</code>,
<code>a</code>, and <code>b</code> even if it never uses
<code>element</code>. Since <code>element</code> also keeps a
reference to the closure, we have a cycle that won't be cleaned up by
garbage collection. In these situations, the code can be structured
as follows:</p>
<CODE_SNIPPET>
function foo(element, a, b) {
element.onclick = bar(a, b);
}
function bar(a, b) {
return function() { /* uses a and b */ };
}
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="eval()">
<SUMMARY>
Only for code loaders and REPL (Read–eval–print loop)
</SUMMARY>
<BODY>
<p><code>eval()</code> makes for confusing semantics and is dangerous
to use if the string being <code>eval()</code>'d contains user input.
There's usually a better, clearer, and safer way to write your code,
so its use is generally not permitted.</p>
<p>For RPC you can always use JSON and read the result using
<code>JSON.parse()</code> instead of <code>eval()</code>.</p>
<p>Let's assume we have a server that returns something like this:</p>
<CODE_SNIPPET>
{
"name": "Alice",
"id": 31502,
"email": "looking_glass@example.com"
}
</CODE_SNIPPET>
<BAD_CODE_SNIPPET>
var userInfo = eval(feed);
var email = userInfo['email'];
</BAD_CODE_SNIPPET>
<p>If the feed was modified to include malicious JavaScript code, then
if we use <code>eval</code> then that code will be executed.</p>
<CODE_SNIPPET>
var userInfo = JSON.parse(feed);
var email = userInfo['email'];
</CODE_SNIPPET>
<p>With <code>JSON.parse</code>, invalid JSON (including all executable
JavaScript) will cause an exception to be thrown.</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="with() {}">
<SUMMARY>No</SUMMARY>
<BODY>
<p>Using <code>with</code> clouds the semantics of your program.
Because the object of the <code>with</code> can have properties that
collide with local variables, it can drastically change the meaning
of your program. For example, what does this do?</p>
<BAD_CODE_SNIPPET>
with (foo) {
var x = 3;
return x;
}
</BAD_CODE_SNIPPET>
<p>Answer: anything. The local variable <code>x</code> could be
clobbered by a property of <code>foo</code> and perhaps it even has
a setter, in which case assigning <code>3</code> could cause lots of
other code to execute. Don't use <code>with</code>.</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="this">
<SUMMARY>
Only in object constructors, methods, and in setting up closures
</SUMMARY>
<BODY>
<p>The semantics of <code>this</code> can be tricky. At times it refers
to the global object (in most places), the scope of the caller (in
<code>eval</code>), a node in the DOM tree (when attached using an
event handler HTML attribute), a newly created object (in a
constructor), or some other object (if function was
<code>call()</code>ed or <code>apply()</code>ed).</p>
<p>Because this is so easy to get wrong, limit its use to those places
where it is required:</p>
<ul>
<li>in constructors</li>
<li>in methods of objects (including in the creation of closures)</li>
</ul>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="for-in loop">
<SUMMARY>
Only for iterating over keys in an object/map/hash
</SUMMARY>
<BODY>
<p><code>for-in</code> loops are often incorrectly used to loop over
the elements in an <code>Array</code>. This is however very error
prone because it does not loop from <code>0</code> to
<code>length - 1</code> but over all the present keys in the object
and its prototype chain. Here are a few cases where it fails:</p>
<BAD_CODE_SNIPPET>
function printArray(arr) {
for (var key in arr) {
print(arr[key]);
}
}
printArray([0,1,2,3]); // This works.
var a = new Array(10);
printArray(a); // This is wrong.
a = document.getElementsByTagName('*');
printArray(a); // This is wrong.
a = [0,1,2,3];
a.buhu = 'wine';
printArray(a); // This is wrong again.
a = new Array;
a[3] = 3;
printArray(a); // This is wrong again.
</BAD_CODE_SNIPPET>
<p>Always use normal for loops when using arrays.</p>
<CODE_SNIPPET>
function printArray(arr) {
var l = arr.length;
for (var i = 0; i < l; i++) {
print(arr[i]);
}
}
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Associative Arrays">
<SUMMARY>
Never use <code>Array</code> as a map/hash/associative array
</SUMMARY>
<BODY>
<p>Associative <code>Array</code>s are not allowed... or more precisely
you are not allowed to use non number indexes for arrays. If you need
a map/hash use <code>Object</code> instead of <code>Array</code> in
these cases because the features that you want are actually features
of <code>Object</code> and not of <code>Array</code>.
<code>Array</code> just happens to extend <code>Object</code> (like
any other object in JS and therefore you might as well have used
<code>Date</code>, <code>RegExp</code> or <code>String</code>).</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Multiline string literals">
<SUMMARY>No</SUMMARY>
<BODY>
<p>Do not do this:</p>
<BAD_CODE_SNIPPET>
var myString = 'A rather long string of English text, an error message \
actually that just keeps going and going -- an error \
message to make the Energizer bunny blush (right through \
those Schwarzenegger shades)! Where was I? Oh yes, \
you\'ve got an error and all the extraneous whitespace is \
just gravy. Have a nice day.';
</BAD_CODE_SNIPPET>
<p>The whitespace at the beginning of each line can't be safely stripped
at compile time; whitespace after the slash will result in tricky
errors. </p>
<p>Use string concatenation instead:</p>
<CODE_SNIPPET>
var myString = 'A rather long string of English text, an error message ' +
'actually that just keeps going and going -- an error ' +
'message to make the Energizer bunny blush (right through ' +
'those Schwarzenegger shades)! Where was I? Oh yes, ' +
'you\'ve got an error and all the extraneous whitespace is ' +
'just gravy. Have a nice day.';
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Array and Object literals">
<SUMMARY>Yes</SUMMARY>
<BODY>
<p>Use <code>Array</code> and <code>Object</code> literals instead of
<code>Array</code> and <code>Object</code> constructors.</p>
<p>Array constructors are error-prone due to their arguments.</p>
<BAD_CODE_SNIPPET>
// Length is 3.
var a1 = new Array(x1, x2, x3);
// Length is 2.
var a2 = new Array(x1, x2);
// If x1 is a number and it is a natural number the length will be x1.
// If x1 is a number but not a natural number this will throw an exception.
// Otherwise the array will have one element with x1 as its value.
var a3 = new Array(x1);
// Length is 0.
var a4 = new Array();
</BAD_CODE_SNIPPET>
<p>Because of this, if someone changes the code to pass 1 argument
instead of 2 arguments, the array might not have the expected
length.</p>
<p>To avoid these kinds of weird cases, always use the more readable
array literal.</p>
<CODE_SNIPPET>
var a = [x1, x2, x3];
var a2 = [x1, x2];
var a3 = [x1];
var a4 = [];
</CODE_SNIPPET>
<p>Object constructors don't have the same problems, but for readability
and consistency object literals should be used.</p>
<BAD_CODE_SNIPPET>
var o = new Object();
var o2 = new Object();
o2.a = 0;
o2.b = 1;
o2.c = 2;
o2['strange key'] = 3;
</BAD_CODE_SNIPPET>
<p>Should be written as:</p>
<CODE_SNIPPET>
var o = {};
var o2 = {
a: 0,
b: 1,
c: 2,
'strange key': 3
};
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Modifying prototypes of builtin objects">
<SUMMARY>No</SUMMARY>
<BODY>
<p>Modifying builtins like <code>Object.prototype</code> and
<code>Array.prototype</code> are strictly forbidden. Modifying other
builtins like <code>Function.prototype</code> is less dangerous but
still leads to hard to debug issues in production and should be
avoided.</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Internet Explorer's Conditional Comments">
<SUMMARY>No</SUMMARY>
<BODY>
<p>Don't do this:</p>
<BAD_CODE_SNIPPET>
var f = function () {
/*@cc_on if (@_jscript) { return 2* @*/ 3; /*@ } @*/
};
</BAD_CODE_SNIPPET>
<p>Conditional Comments hinder automated tools as they can vary the
JavaScript syntax tree at runtime.</p>
</BODY>
</STYLEPOINT>
</CATEGORY>
<CATEGORY title="JavaScript Style Rules">
<STYLEPOINT title="Naming">
<SUMMARY>
<p>In general, use
<code>functionNamesLikeThis</code>,
<code>variableNamesLikeThis</code>,
<code>ClassNamesLikeThis</code>,
<code>EnumNamesLikeThis</code>,
<code>methodNamesLikeThis</code>,
<code>CONSTANT_VALUES_LIKE_THIS</code>,
<code>foo.namespaceNamesLikeThis.bar</code>, and
<code>filenameslikethis.js</code>.
</p>
</SUMMARY>
<BODY>
<SUBSECTION title="Properties and methods">
<ul>
<li><em>Private</em> properties and methods should be named with a
trailing underscore.
</li>
<li><em>Protected</em> properties and methods should be
named without a trailing underscore (like public ones).</li>
</ul>
<p>For more information on <em>private</em> and <em>protected</em>,
read the section on
<a href="#Visibility__private_and_protected_fields_">
visibility</a>.
</p>
</SUBSECTION>
<SUBSECTION title="Method and function parameter">
<p>Optional function arguments start with <code>opt_</code>.</p>
<p>Functions that take a variable number of arguments should have the
last argument named <code>var_args</code>. You may not refer to
<code>var_args</code> in the code; use the <code>arguments</code>
array.</p>
<p>Optional and variable arguments can also be specified in
<code>@param</code> annotations. Although either convention is
acceptable to the compiler, using both together is preferred.</p>
</SUBSECTION>
<SUBSECTION title="Getters and Setters">
<p>EcmaScript 5 getters and setters for properties are discouraged.
However, if they are used, then getters must not change observable
state.</p>
<BAD_CODE_SNIPPET>
/**
* WRONG -- Do NOT do this.
*/
var foo = { get next() { return this.nextId++; } };
</BAD_CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Accessor functions">
<p>Getters and setters methods for properties are not required.
However, if they are used, then getters must be named
<code>getFoo()</code> and setters must be named
<code>setFoo(value)</code>. (For boolean getters,
<code>isFoo()</code> is also acceptable, and often sounds more
natural.)</p>
</SUBSECTION>
<SUBSECTION title="Namespaces">
<p>JavaScript has no inherent packaging or namespacing support.</p>
<p>Global name conflicts are difficult to debug, and can cause
intractable problems when two projects try to integrate. In order
to make it possible to share common JavaScript code, we've adopted
conventions to prevent collisions. </p>
<SUBSUBSECTION title="Use namespaces for global code">
<p><em>ALWAYS</em> prefix identifiers in the global scope with a
unique pseudo namespace related to the project or library. If you
are working on "Project Sloth", a reasonable pseudo namespace
would be <code>sloth.*</code>.</p>
<CODE_SNIPPET>
var sloth = {};
sloth.sleep = function() {
...
};
</CODE_SNIPPET>
<p>Many JavaScript libraries, including
<a href="https://code.google.com/closure/library/">
the Closure Library
</a>
and
<a href="http://www.dojotoolkit.org/">
Dojo toolkit
</a>
give you high-level functions for declaring your namespaces.
Be consistent about how you declare your namespaces.</p>
<CODE_SNIPPET>
goog.provide('sloth');
sloth.sleep = function() {
...
};
</CODE_SNIPPET>
</SUBSUBSECTION>
<SUBSUBSECTION title="Respect namespace ownership">
<p>When choosing a child-namespace, make sure that the owners of the
parent namespace know what you are doing. If you start a project
that creates hats for sloths, make sure that the Sloth team knows
that you're using <code>sloth.hats</code>.</p>
</SUBSUBSECTION>
<SUBSUBSECTION title="Use different namespaces for external code and internal code">
<p>"External code" is code that comes from outside your codebase,
and is compiled independently. Internal and external names should
be kept strictly separate. If you're using an external library
that makes things available in <code>foo.hats.*</code>, your
internal code should not define all its symbols in
<code>foo.hats.*</code>, because it will break if the other
team defines new symbols.</p>
<BAD_CODE_SNIPPET>
foo.require('foo.hats');
/**
* WRONG -- Do NOT do this.
* @constructor
* @extends {foo.hats.RoundHat}
*/
foo.hats.BowlerHat = function() {
};
</BAD_CODE_SNIPPET>
<p>If you need to define new APIs on an external namespace, then you
should explicitly export the public API functions, and only those
functions. Your internal code should call the internal APIs by
their internal names, for consistency and so that the compiler
can optimize them better.</p>
<CODE_SNIPPET>
foo.provide('googleyhats.BowlerHat');
foo.require('foo.hats');
/**
* @constructor
* @extends {foo.hats.RoundHat}
*/
googleyhats.BowlerHat = function() {
...
};
goog.exportSymbol('foo.hats.BowlerHat', googleyhats.BowlerHat);
</CODE_SNIPPET>
</SUBSUBSECTION>
<SUBSUBSECTION title="Alias long type names to improve readability">
<p>Use local aliases for fully-qualified types if doing so improves
readability. The name of a local alias should match the last part
of the type.</p>
<CODE_SNIPPET>
/**
* @constructor
*/
some.long.namespace.MyClass = function() {
};
/**
* @param {some.long.namespace.MyClass} a
*/
some.long.namespace.MyClass.staticHelper = function(a) {
...
};
myapp.main = function() {
var MyClass = some.long.namespace.MyClass;
var staticHelper = some.long.namespace.MyClass.staticHelper;
staticHelper(new MyClass());
};
</CODE_SNIPPET>
<p>Do not create local aliases of namespaces. Namespaces should only
be aliased using <a href="#goog-scope">goog.scope</a>.</p>
<BAD_CODE_SNIPPET>
myapp.main = function() {
var namespace = some.long.namespace;
namespace.MyClass.staticHelper(new namespace.MyClass());
};
</BAD_CODE_SNIPPET>
<p>Avoid accessing properties of an aliased type, unless it is an
enum.</p>
<CODE_SNIPPET>
/** @enum {string} */
some.long.namespace.Fruit = {
APPLE: 'a',
BANANA: 'b'
};
myapp.main = function() {
var Fruit = some.long.namespace.Fruit;
switch (fruit) {
case Fruit.APPLE:
...
case Fruit.BANANA:
...
}
};
</CODE_SNIPPET>
<BAD_CODE_SNIPPET>
myapp.main = function() {
var MyClass = some.long.namespace.MyClass;
MyClass.staticHelper(null);
};
</BAD_CODE_SNIPPET>
<p>Never create aliases in the global scope. Use them only in
function blocks.</p>
</SUBSUBSECTION>
</SUBSECTION>
<SUBSECTION title="Filenames">
<p>Filenames should be all lowercase in order to avoid confusion on
case-sensitive platforms. Filenames should end in <code>.js</code>,
and should contain no punctuation except for <code>-</code> or
<code>_</code> (prefer <code>-</code> to <code>_</code>).</p>
</SUBSECTION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Custom toString() methods">
<SUMMARY>
Must always succeed without side effects.
</SUMMARY>
<BODY>
<p>You can control how your objects string-ify themselves by defining a
custom <code>toString()</code> method. This is fine, but you need
to ensure that your method (1) always succeeds and (2) does not have
side-effects. If your method doesn't meet these criteria, it's very
easy to run into serious problems. For example, if
<code>toString()</code> calls a method that does an
<code>assert</code>, <code>assert</code> might try to output the name
of the object in which it failed, which of course requires calling
<code>toString()</code>.</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Deferred initialization">
<SUMMARY>OK</SUMMARY>
<BODY>
<p>It isn't always possible to initialize variables at the point of
declaration, so deferred initialization is fine.</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Explicit scope">
<SUMMARY>Always</SUMMARY>
<BODY>
<p>Always use explicit scope - doing so increases portability and
clarity. For example, don't rely on <code>window</code> being in the
scope chain. You might want to use your function in another
application for which <code>window</code> is not the content
window.</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Code formatting">
<SUMMARY>Expand for more information.</SUMMARY>
<BODY>
<p>We follow the <a href="cppguide.html#Formatting">C++ formatting
rules</a> in spirit, with the following additional clarifications.</p>
<SUBSECTION title="Curly Braces">
<p>Because of implicit semicolon insertion, always start your curly
braces on the same line as whatever they're opening. For
example:</p>
<CODE_SNIPPET>
if (something) {
// ...
} else {
// ...
}
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Array and Object Initializers">
<p>Single-line array and object initializers are allowed when they
fit on a line:</p>
<CODE_SNIPPET>
var arr = [1, 2, 3]; // No space after [ or before ].
var obj = {a: 1, b: 2, c: 3}; // No space after { or before }.
</CODE_SNIPPET>
<p>Multiline array initializers and object initializers are indented
2 spaces, with the braces on their own line, just like blocks.</p>
<CODE_SNIPPET>
// Object initializer.
var inset = {
top: 10,
right: 20,
bottom: 15,
left: 12
};
// Array initializer.
this.rows_ = [
'"Slartibartfast" <fjordmaster@magrathea.com>',
'"Zaphod Beeblebrox" <theprez@universe.gov>',
'"Ford Prefect" <ford@theguide.com>',
'"Arthur Dent" <has.no.tea@gmail.com>',
'"Marvin the Paranoid Android" <marv@googlemail.com>',
'the.mice@magrathea.com'
];
// Used in a method call.
goog.dom.createDom(goog.dom.TagName.DIV, {
id: 'foo',
className: 'some-css-class',
style: 'display:none'
}, 'Hello, world!');
</CODE_SNIPPET>
<p>Long identifiers or values present problems for aligned
initialization lists, so always prefer non-aligned initialization.
For example:</p>
<CODE_SNIPPET>
CORRECT_Object.prototype = {
a: 0,
b: 1,
lengthyName: 2
};
</CODE_SNIPPET>
<p>Not like this:</p>
<BAD_CODE_SNIPPET>
WRONG_Object.prototype = {
a : 0,
b : 1,
lengthyName: 2
};
</BAD_CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Function Arguments">
<p>When possible, all function arguments should be listed on the same
line. If doing so would exceed the 80-column limit, the arguments
must be line-wrapped in a readable way. To save space, you may wrap
as close to 80 as possible, or put each argument on its own line to
enhance readability. The indentation may be either four spaces, or
aligned to the parenthesis. Below are the most common patterns for
argument wrapping:</p>
<CODE_SNIPPET>
// Four-space, wrap at 80. Works with very long function names, survives
// renaming without reindenting, low on space.
goog.foo.bar.doThingThatIsVeryDifficultToExplain = function(
veryDescriptiveArgumentNumberOne, veryDescriptiveArgumentTwo,
tableModelEventHandlerProxy, artichokeDescriptorAdapterIterator) {
// ...
};
// Four-space, one argument per line. Works with long function names,
// survives renaming, and emphasizes each argument.
goog.foo.bar.doThingThatIsVeryDifficultToExplain = function(
veryDescriptiveArgumentNumberOne,
veryDescriptiveArgumentTwo,
tableModelEventHandlerProxy,
artichokeDescriptorAdapterIterator) {
// ...
};
// Parenthesis-aligned indentation, wrap at 80. Visually groups arguments,
// low on space.
function foo(veryDescriptiveArgumentNumberOne, veryDescriptiveArgumentTwo,
tableModelEventHandlerProxy, artichokeDescriptorAdapterIterator) {
// ...
}
// Parenthesis-aligned, one argument per line. Emphasizes each
// individual argument.
function bar(veryDescriptiveArgumentNumberOne,
veryDescriptiveArgumentTwo,
tableModelEventHandlerProxy,
artichokeDescriptorAdapterIterator) {
// ...
}
</CODE_SNIPPET>
<p>When the function call is itself indented, you're free to start the
4-space indent relative to the beginning of the original statement
or relative to the beginning of the current function call.
The following are all acceptable indentation styles.</p>
<CODE_SNIPPET>
if (veryLongFunctionNameA(
veryLongArgumentName) ||
veryLongFunctionNameB(
veryLongArgumentName)) {
veryLongFunctionNameC(veryLongFunctionNameD(
veryLongFunctioNameE(
veryLongFunctionNameF)));
}
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Passing Anonymous Functions">
<p>When declaring an anonymous function in the list of arguments for
a function call, the body of the function is indented two spaces
from the left edge of the statement, or two spaces from the left
edge of the function keyword. This is to make the body of the
anonymous function easier to read (i.e. not be all squished up into
the right half of the screen).</p>
<CODE_SNIPPET>
prefix.something.reallyLongFunctionName('whatever', function(a1, a2) {
if (a1.equals(a2)) {
someOtherLongFunctionName(a1);
} else {
andNowForSomethingCompletelyDifferent(a2.parrot);
}
});
var names = prefix.something.myExcellentMapFunction(
verboselyNamedCollectionOfItems,
function(item) {
return item.name;
});
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Aliasing with goog.scope">
<a name="goog-scope"/>
<p>
<a href="https://docs.google.com/document/pub?id=1ETFAuh2kaXMVL-vafUYhaWlhl6b5D9TOvboVg7Zl68Y"><code>goog.scope</code></a>
may be used to shorten references to
namespaced symbols in programs using
<a href="https://code.google.com/closure/library/">the Closure
Library</a>.</p>
<p>Only one <code>goog.scope</code> invocation may be added per
file. Always place it in the global scope.</p>
<p>The opening <code>goog.scope(function() {</code> invocation
must be preceded by exactly one blank line and follow any
<code>goog.provide</code> statements, <code>goog.require</code>
statements, or top-level comments. The invocation must be closed on
the last line in the file. Append <code>// goog.scope</code> to the
closing statement of the scope. Separate the comment from the
semicolon by two spaces.</p>
<p>Similar to C++ namespaces, do not indent under goog.scope
declarations. Instead, continue from the 0 column.</p>
<p>Only alias names that will not be re-assigned to another object
(e.g., most constructors, enums, and namespaces). Do not do
this (see below for how to alias a constructor):</p>
<BAD_CODE_SNIPPET>
goog.scope(function() {
var Button = goog.ui.Button;
Button = function() { ... };
...
</BAD_CODE_SNIPPET>
<p>Names must be the same as the last property of the global that they
are aliasing.</p>
<CODE_SNIPPET>
goog.provide('my.module.SomeType');
goog.require('goog.dom');
goog.require('goog.ui.Button');
goog.scope(function() {
var Button = goog.ui.Button;
var dom = goog.dom;
// Alias new types <b>after</b> the constructor declaration.
my.module.SomeType = function() { ... };
var SomeType = my.module.SomeType;
// Declare methods on the prototype as usual:
SomeType.prototype.findButton = function() {
// Button as aliased above.
this.button = new Button(dom.getElement('my-button'));
};
...
}); // goog.scope
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Indenting wrapped lines">
<p>Except for <a href="#Array_and_Object_literals">array literals,
object literals</a>, and anonymous functions, all wrapped lines
should be indented either left-aligned to a sibling expression
above, or four spaces (not two spaces) deeper than a parent
expression (where "sibling" and "parent" refer to parenthesis
nesting level).
</p>
<CODE_SNIPPET>
someWonderfulHtml = '<div class="' + getClassesForWonderfulHtml()'">' +
getEvenMoreHtml(someReallyInterestingValues, moreValues,
evenMoreParams, 'a duck', true, 72,
slightlyMoreMonkeys(0xfff)) +
'</div>';
thisIsAVeryLongVariableName =
hereIsAnEvenLongerOtherFunctionNameThatWillNotFitOnPrevLine();
thisIsAVeryLongVariableName = siblingOne + siblingTwo + siblingThree +
siblingFour + siblingFive + siblingSix + siblingSeven +
moreSiblingExpressions + allAtTheSameIndentationLevel;
thisIsAVeryLongVariableName = operandOne + operandTwo + operandThree +
operandFour + operandFive * (
aNestedChildExpression + shouldBeIndentedMore);
someValue = this.foo(
shortArg,
'Some really long string arg - this is a pretty common case, actually.',
shorty2,
this.bar());
if (searchableCollection(allYourStuff).contains(theStuffYouWant) &&
!ambientNotification.isActive() && (client.isAmbientSupported() ||
client.alwaysTryAmbientAnyways())) {
ambientNotification.activate();
}
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Blank lines">
<p>Use newlines to group logically related pieces of code.
For example:</p>
<CODE_SNIPPET>
doSomethingTo(x);
doSomethingElseTo(x);
andThen(x);
nowDoSomethingWith(y);
andNowWith(z);
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Binary and Ternary Operators">
<p>Always put the operator on the preceding line. Otherwise,
line breaks and indentation follow the same rules as in other
Google style guides. This operator placement was initially agreed
upon out of concerns about automatic semicolon insertion. In fact,
semicolon insertion cannot happen before a binary operator, but new
code should stick to this style for consistency.</p>
<CODE_SNIPPET>
var x = a ? b : c; // All on one line if it will fit.
// Indentation +4 is OK.
var y = a ?
longButSimpleOperandB : longButSimpleOperandC;
// Indenting to the line position of the first operand is also OK.
var z = a ?
moreComplicatedB :
moreComplicatedC;
</CODE_SNIPPET>
<p>This includes the dot operator.</p>
<CODE_SNIPPET>
var x = foo.bar().
doSomething().
doSomethingElse();
</CODE_SNIPPET>
</SUBSECTION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Parentheses">
<SUMMARY>Only where required</SUMMARY>
<BODY>
<p>Use sparingly and in general only where required by the syntax
and semantics.</p>
<p>Never use parentheses for unary operators such as
<code>delete</code>, <code>typeof</code> and <code>void</code> or
after keywords such as <code>return</code>, <code>throw</code> as
well as others (<code>case</code>, <code>in</code> or
<code>new</code>).</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Strings">
<SUMMARY>Prefer ' over "</SUMMARY>
<BODY>
<p>For consistency single-quotes (') are preferred to double-quotes (").
This is helpful when creating strings that include HTML:</p>
<CODE_SNIPPET>
var msg = 'This is <a href="http://foo">some HTML</a>';
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Visibility (private and protected fields)">
<SUMMARY>Encouraged, use JSDoc annotations <code>@private</code> and
<code>@protected</code></SUMMARY>
<BODY>
<p>We recommend the use of the JSDoc annotations <code>@private</code> and
<code>@protected</code> to indicate visibility levels for classes,
functions, and properties.</p>
<p>The --jscomp_warning=visibility compiler flag turns on compiler
warnings for visibility violations. See
<a href="https://code.google.com/p/closure-compiler/wiki/Warnings">
Closure Compiler
Warnings</a>.
</p>
<p><code>@private</code> global variables and functions are only
accessible to code in the same file.</p>
<p>Constructors marked <code>@private</code> may only be instantiated by
code in the same file and by their static and instance members.
<code>@private</code> constructors may also be accessed anywhere in the
same file for their public static properties and by the
<code>instanceof</code> operator.</p>
<p>Global variables, functions, and constructors should never be
annotated <code>@protected</code>.</p>
<CODE_SNIPPET>
// File 1.
// AA_PrivateClass_ and AA_init_ are accessible because they are global
// and in the same file.
/**
* @private
* @constructor
*/
AA_PrivateClass_ = function() {
};
/** @private */
function AA_init_() {
return new AA_PrivateClass_();
}
AA_init_();
</CODE_SNIPPET>
<p><code>@private</code> properties are accessible to all code in the
same file, plus all static methods and instance methods of that class
that "owns" the property, if the property belongs to a class. They
cannot be accessed or overridden from a subclass in a different file.</p>
<p><code>@protected</code> properties are accessible to all code in the
same file, plus any static methods and instance methods of any subclass
of a class that "owns" the property.</p>
<p>Note that these semantics differ from those of C++ and Java, in that
they grant private and protected access to all code in the same file,
not just in the same class or class hierarchy. Also, unlike in C++,
private properties cannot be overridden by a subclass.
</p>
<CODE_SNIPPET>
// File 1.
/** @constructor */
AA_PublicClass = function() {
/** @private */
this.privateProp_ = 2;
/** @protected */
this.protectedProp = 4;
};
/** @private */
AA_PublicClass.staticPrivateProp_ = 1;
/** @protected */
AA_PublicClass.staticProtectedProp = 31;
/** @private */
AA_PublicClass.prototype.privateMethod_ = function() {};
/** @protected */
AA_PublicClass.prototype.protectedMethod = function() {};
// File 2.
/**
* @return {number} The number of ducks we've arranged in a row.
*/
AA_PublicClass.prototype.method = function() {
// Legal accesses of these two properties.
return this.privateProp_ + AA_PublicClass.staticPrivateProp_;
};
// File 3.
/**
* @constructor
* @extends {AA_PublicClass}
*/
AA_SubClass = function() {
// Legal access of a protected static property.
AA_PublicClass.staticProtectedProp = this.method();
};
goog.inherits(AA_SubClass, AA_PublicClass);
/**
* @return {number} The number of ducks we've arranged in a row.
*/
AA_SubClass.prototype.method = function() {
// Legal access of a protected instance property.
return this.protectedProp;
};
</CODE_SNIPPET>
<p>Notice that in JavaScript, there is no distinction between a type
(like <code>AA_PrivateClass_</code>) and the constructor for that
type. There is no way to express both that a type is public and its
constructor is private (because the constructor could easily be aliased
in a way that would defeat the privacy check).</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="JavaScript Types">
<SUMMARY>Encouraged and enforced by the compiler.</SUMMARY>
<BODY>
<a name="JsTypes"/>
<p>When documenting a type in JSDoc, be as specific and accurate as
possible. The types we support are based on the
<a href="http://wiki.ecmascript.org/doku.php?id=spec:spec">
EcmaScript 4 spec</a>.</p>
<SUBSECTION title="The JavaScript Type Language">
<p>The ES4 proposal contained a language for specifying JavaScript
types. We use this language in JsDoc to express the types of
function parameters and return values.</p>
<p>As the ES4 proposal has evolved, this language has changed. The
compiler still supports old syntaxes for types, but those syntaxes
are deprecated.</p>
<p/>
<table border="1" style="border-collapse:collapse" cellpadding="4">
<thead>
<tr>
<th>Syntax Name</th>
<th>Syntax</th>
<th>Description</th>
<th>Deprecated Syntaxes</th>
</tr>
</thead>
<tbody>
<tr>
<td>Primitive Type</td>
<td>
There are 5 primitive types in JavaScript:
<code>{null}</code>,
<code>{undefined}</code>,
<code>{boolean}</code>,
<code>{number}</code>, and
<code>{string}</code>.
</td>
<td>Simply the name of a type.</td>
<td/>
</tr>
<tr>
<td>Instance Type</td>
<td>
<code>{Object}</code><br/>
An instance of Object or null.<p/>
<code>{Function}</code><br/>
An instance of Function or null.<p/>
<code>{EventTarget}</code><br/>
An instance of a constructor that implements the EventTarget
interface, or null.
</td>
<td>An instance of a constructor or interface function.<p/>
Constructor functions are functions defined with the
<code>@constructor</code> JSDoc tag.
Interface functions are functions defined with the
<code>@interface</code> JSDoc tag.<p/>
By default, instance types will accept null. This is the only
type syntax that makes the type nullable. Other type syntaxes
in this table will not accept null.
</td>
<td/>
</tr>
<tr>
<td>Enum Type</td>
<td>
<code>{goog.events.EventType}</code><br/>
One of the properties of the object literal initializer
of <code>goog.events.EventType</code>.
</td>
<td>An enum must be initialized as an object literal, or as
an alias of another enum, annotated with the <code>@enum</code>
JSDoc tag. The properties of this literal are the instances
of the enum. The syntax of the enum is defined
<a href="#enums">below</a>.<p/>
Note that this is one of the few things in our type system
that were not in the ES4 spec.
</td>
<td/>
</tr>
<tr>
<td>Type Application</td>
<td>
<code>{Array.<string>}</code><br/>An array of strings.<p/>
<code>{Object.<string, number>}</code>
<br/>An object in which the keys are strings and the values
are numbers.
</td>
<td>Parameterizes a type, by applying a set of type arguments
to that type. The idea is analogous to generics in Java.
</td>
<td/>
</tr>
<tr>
<td>Type Union</td>
<td>
<code>{(number|boolean)}</code><br/>A number or a boolean.
</td>
<td>Indicates that a value might have type A OR type B.<p/>
The parentheses may be omitted at the top-level
expression, but the parentheses should be included in
sub-expressions to avoid ambiguity.<br/>
<code>{number|boolean}</code><br/>
<code>{function(): (number|boolean)}</code>
</td>
<td>
<code>{(number,boolean)}</code>,<br/>
<code>{(number||boolean)}</code>
</td>
</tr>
<tr>
<td>Nullable type</td>
<td>
<code>{?number}</code><br/> A number or null.
</td>
<td>Shorthand for the union of the null type with any
other type. This is just syntactic sugar.
</td>
<td>
<code>{number?}</code>
</td>
</tr>
<tr>
<td>Non-nullable type</td>
<td>
<code>{!Object}</code><br/> An Object, but never the
<code>null</code> value.
</td>
<td>Filters null out of nullable types. Most often used
with instance types, which are nullable by default.
</td>
<td>
<code>{Object!}</code>
</td>
</tr>
<tr>
<td>Record Type</td>
<td>
<code>{{myNum: number, myObject}}</code>
<br/>An anonymous type with the given type members.
</td>
<td>
<p>Indicates that the value has the specified members with the
specified types. In this case, <code>myNum</code> with a
type <code>number</code> and <code>myObject</code> with any
type.</p>
<p>Notice that the braces are part of the type syntax. For
example, to denote an <code>Array</code> of objects that
have a <code>length</code> property, you might write
<code>Array.<{length}></code>.</p>
</td>
<td/>
</tr>
<tr>
<td>Function Type</td>
<td>
<code>{function(string, boolean)}</code><br/>
A function that takes two arguments (a string and a boolean),
and has an unknown return value.<br/>
</td>
<td>Specifies a function.</td>
<td/>
</tr>
<tr>
<td>Function Return Type</td>
<td>
<code>{function(): number}</code><br/>
A function that takes no arguments and returns a number.<br/>
</td>
<td>Specifies a function return type.</td>
<td/>
</tr>
<tr>
<td>Function <code>this</code> Type</td>
<td>
<code>{function(this:goog.ui.Menu, string)}</code><br/>
A function that takes one argument (a string), and executes
in the context of a goog.ui.Menu.
</td>
<td>Specifies the context type of a function type.</td>
<td/>
</tr>
<tr>
<td>Function <code>new</code> Type</td>
<td>
<code>{function(new:goog.ui.Menu, string)}</code><br/>
A constructor that takes one argument (a string), and
creates a new instance of goog.ui.Menu when called
with the 'new' keyword.
</td>
<td>Specifies the constructed type of a constructor.</td>
<td/>
</tr>
<tr>
<td>Variable arguments</td>
<td>
<code>{function(string, ...[number]): number}</code><br/>
A function that takes one argument (a string), and then a
variable number of arguments that must be numbers.
</td>
<td>Specifies variable arguments to a function.</td>
<td/>
</tr>
<tr>
<td>
<a name="var-args-annotation"/>
Variable arguments (in <code>@param</code> annotations)
</td>
<td>
<code>@param {...number} var_args</code><br/>
A variable number of arguments to an annotated function.
</td>
<td>
Specifies that the annotated function accepts a variable
number of arguments.
</td>
<td/>
</tr>
<tr>
<td>Function <a href="#optional">optional arguments</a></td>
<td>
<code>{function(?string=, number=)}</code><br/>
A function that takes one optional, nullable string and one
optional number as arguments. The <code>=</code> syntax is
only for <code>function</code> type declarations.
</td>
<td>Specifies optional arguments to a function.</td>
<td/>
</tr>
<tr>
<td>
<a name="optional-arg-annotation"/>
Function <a href="#optional">optional arguments</a>
(in <code>@param</code> annotations)
</td>
<td>
<code>@param {number=} opt_argument</code><br/>
An optional parameter of type <code>number</code>.
</td>
<td>Specifies that the annotated function accepts an optional
argument.</td>
<td/>
</tr>
<tr>
<td>The ALL type</td>
<td><code>{*}</code></td>
<td>Indicates that the variable can take on any type.</td>
<td/>
</tr>
<tr>
<td>The UNKNOWN type</td>
<td><code>{?}</code></td>
<td>Indicates that the variable can take on any type,
and the compiler should not type-check any uses of it.</td>
<td/>
</tr>
</tbody>
</table>
</SUBSECTION>
<SUBSECTION title="Types in JavaScript">
<p/>
<table border="1" style="border-collapse:collapse" cellpadding="4">
<thead>
<tr>
<th>Type Example</th>
<th>Value Examples</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>number</td>
<td>
<CODE_SNIPPET>
1
1.0
-5
1e5
Math.PI
</CODE_SNIPPET>
</td>
<td/>
</tr>
<tr>
<td>Number</td>
<td>
<CODE_SNIPPET>
new Number(true)
</CODE_SNIPPET>
</td>
<td>
<a href="#Wrapper_objects_for_primitive_types">
Number object
</a>
</td>
</tr>
<tr>
<td>string</td>
<td>
<CODE_SNIPPET>
'Hello'
"World"
String(42)
</CODE_SNIPPET>
</td>
<td>
String value
</td>
</tr>
<tr>
<td>String</td>
<td>
<CODE_SNIPPET>
new String('Hello')
new String(42)
</CODE_SNIPPET>
</td>
<td>
<a href="#Wrapper_objects_for_primitive_types">
String object
</a>
</td>
</tr>
<tr>
<td>boolean</td>
<td>
<CODE_SNIPPET>
true
false
Boolean(0)
</CODE_SNIPPET>
</td>
<td>
Boolean value
</td>
</tr>
<tr>
<td>Boolean</td>
<td>
<CODE_SNIPPET>
new Boolean(true)
</CODE_SNIPPET>
</td>
<td>
<a href="#Wrapper_objects_for_primitive_types">
Boolean object
</a>
</td>
</tr>
<tr>
<td>RegExp</td>
<td>
<CODE_SNIPPET>
new RegExp('hello')
/world/g
</CODE_SNIPPET></td><td>
</td>
</tr>
<tr>
<td>Date</td>
<td>
<CODE_SNIPPET>
new Date
new Date()
</CODE_SNIPPET></td>
<td/>
</tr>
<tr>
<td>
null
</td>
<td>
<CODE_SNIPPET>
null
</CODE_SNIPPET>
</td>
<td/>
</tr>
<tr>
<td>
undefined
</td>
<td>
<CODE_SNIPPET>
undefined
</CODE_SNIPPET>
</td>
<td/>
</tr>
<tr>
<td>void</td>
<td>
<CODE_SNIPPET>
function f() {
return;
}
</CODE_SNIPPET>
</td>
<td>No return value</td>
</tr>
<tr>
<td>Array</td>
<td>
<CODE_SNIPPET>
['foo', 0.3, null]
[]
</CODE_SNIPPET>
</td>
<td>Untyped Array</td>
</tr>
<tr>
<td>Array.<number></td>
<td>
<CODE_SNIPPET>
[11, 22, 33]
</CODE_SNIPPET>
</td>
<td>
An Array of numbers
</td>
</tr>
<tr>
<td>Array.<Array.<string>></td>
<td>
<CODE_SNIPPET>
[['one', 'two', 'three'], ['foo', 'bar']]
</CODE_SNIPPET>
</td>
<td>Array of Arrays of strings</td>
</tr>
<tr>
<td>Object</td>
<td>
<CODE_SNIPPET>
{}
{foo: 'abc', bar: 123, baz: null}
</CODE_SNIPPET>
</td>
<td/>
</tr>
<tr>
<td>Object.<string></td>
<td>
<CODE_SNIPPET>
{'foo': 'bar'}
</CODE_SNIPPET>
</td>
<td>
An Object in which the values are strings.
</td>
</tr>
<tr>
<td>Object.<number, string></td>
<td>
<CODE_SNIPPET>
var obj = {};
obj[1] = 'bar';
</CODE_SNIPPET>
</td>
<td>
An Object in which the keys are numbers and the values are
strings. <p/>Note that in JavaScript, the keys are always
implicitly converted to strings, so
<code>obj['1'] == obj[1]</code>.
So the key will always be a string in for...in loops. But the
compiler will verify the type of the key when indexing into
the object.
</td>
</tr>
<tr>
<td>Function</td>
<td>
<CODE_SNIPPET>
function(x, y) {
return x * y;
}
</CODE_SNIPPET>
</td>
<td>
<a href="#Wrapper_objects_for_primitive_types">
Function object
</a>
</td>
</tr>
<tr>
<td>function(number, number): number</td>
<td>
<CODE_SNIPPET>
function(x, y) {
return x * y;
}
</CODE_SNIPPET>
</td>
<td>function value</td>
</tr>
<tr>
<td><a name="constructor-tag">SomeClass</a></td>
<td>
<CODE_SNIPPET>
/** @constructor */
function SomeClass() {}
new SomeClass();
</CODE_SNIPPET>
</td>
<td/>
</tr>
<tr>
<td>SomeInterface</td>
<td>
<CODE_SNIPPET>
/** @interface */
function SomeInterface() {}
SomeInterface.prototype.draw = function() {};
</CODE_SNIPPET>
</td>
<td/>
</tr>
<tr>
<td>project.MyClass</td>
<td>
<CODE_SNIPPET>
/** @constructor */
project.MyClass = function () {}
new project.MyClass()
</CODE_SNIPPET>
</td>
<td/>
</tr>
<tr>
<td>project.MyEnum</td>
<td>
<CODE_SNIPPET>
/** @enum {string} */
project.MyEnum = {
/** The color blue. */
BLUE: '#0000dd',
/** The color red. */
RED: '#dd0000'
};
</CODE_SNIPPET>
</td>
<td><a name="enums">Enumeration</a><p/>
JSDoc comments on enum values are optional.
</td>
</tr>
<tr>
<td>Element</td>
<td>
<CODE_SNIPPET>
document.createElement('div')
</CODE_SNIPPET>
</td>
<td>Elements in the DOM.</td>
</tr>
<tr>
<td>Node</td>
<td>
<CODE_SNIPPET>
document.body.firstChild
</CODE_SNIPPET>
</td>
<td>Nodes in the DOM.</td>
</tr>
<tr>
<td>HTMLInputElement</td>
<td>
<CODE_SNIPPET>
htmlDocument.getElementsByTagName('input')[0]
</CODE_SNIPPET>
</td>
<td>A specific type of DOM element.</td>
</tr>
</tbody>
</table>
</SUBSECTION>
<SUBSECTION title="Type Casts">
<p>In cases where type-checking doesn't accurately infer the type of
an expression, it is possible to add a type cast comment by adding a
type annotation comment and enclosing the expression in
parentheses. The parentheses are required.</p>
<CODE_SNIPPET>
/** @type {number} */ (x)
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Nullable vs. Optional Parameters and Properties">
<a name="optional"/>
<p>Because JavaScript is a loosely-typed language, it is very
important to understand the subtle differences between optional,
nullable, and undefined function parameters and class
properties.</p>
<p>Instances of classes and interfaces are nullable by default.
For example, the following declaration</p>
<CODE_SNIPPET>
/**
* Some class, initialized with a value.
* @param {Object} value Some value.
* @constructor
*/
function MyClass(value) {
/**
* Some value.
* @type {Object}
* @private
*/
this.myValue_ = value;
}
</CODE_SNIPPET>
<p>tells the compiler that the <code>myValue_</code> property holds
either an Object or null. If <code>myValue_</code> must never be
null, it should be declared like this:</p>
<CODE_SNIPPET>
/**
* Some class, initialized with a non-null value.
* @param {!Object} value Some value.
* @constructor
*/
function MyClass(value) {
/**
* Some value.
* @type {!Object}
* @private
*/
this.myValue_ = value;
}
</CODE_SNIPPET>
<p>This way, if the compiler can determine that somewhere in the code
<code>MyClass</code> is initialized with a null value, it will issue
a warning.</p>
<p>Optional parameters to functions may be undefined at runtime, so if
they are assigned to class properties, those properties must be
declared accordingly:</p>
<CODE_SNIPPET>
/**
* Some class, initialized with an optional value.
* @param {Object=} opt_value Some value (optional).
* @constructor
*/
function MyClass(opt_value) {
/**
* Some value.
* @type {Object|undefined}
* @private
*/
this.myValue_ = opt_value;
}
</CODE_SNIPPET>
<p>This tells the compiler that <code>myValue_</code> may hold an
Object, null, or remain undefined.</p>
<p>Note that the optional parameter <code>opt_value</code> is declared
to be of type <code>{Object=}</code>, not
<code>{Object|undefined}</code>. This is because optional
parameters may, by definition, be undefined. While there is no harm
in explicitly declaring an optional parameter as possibly undefined,
it is both unnecessary and makes the code harder to read.</p>
<p>Finally, note that being nullable and being optional are orthogonal
properties. The following four declarations are all different:</p>
<CODE_SNIPPET>
/**
* Takes four arguments, two of which are nullable, and two of which are
* optional.
* @param {!Object} nonNull Mandatory (must not be undefined), must not be null.
* @param {Object} mayBeNull Mandatory (must not be undefined), may be null.
* @param {!Object=} opt_nonNull Optional (may be undefined), but if present,
* must not be null!
* @param {Object=} opt_mayBeNull Optional (may be undefined), may be null.
*/
function strangeButTrue(nonNull, mayBeNull, opt_nonNull, opt_mayBeNull) {
// ...
};
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Typedefs">
<a name="Typedefs"/>
<p>Sometimes types can get complicated. A function that accepts
content for an Element might look like:</p>
<CODE_SNIPPET>
/**
* @param {string} tagName
* @param {(string|Element|Text|Array.<Element>|Array.<Text>)} contents
* @return {!Element}
*/
goog.createElement = function(tagName, contents) {
...
};
</CODE_SNIPPET>
<p>You can define commonly used type expressions with a
<code>@typedef</code> tag. For example,</p>
<CODE_SNIPPET>
/** @typedef {(string|Element|Text|Array.<Element>|Array.<Text>)} */
goog.ElementContent;
/**
* @param {string} tagName
* @param {goog.ElementContent} contents
* @return {!Element}
*/
goog.createElement = function(tagName, contents) {
...
};
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Template types">
<a name="Template_types"/>
<p>The compiler has limited support for template types. It can only
infer the type of <code>this</code> inside an anonymous function
literal from the type of the <code>this</code> argument and whether the
<code>this</code> argument is missing.</p>
<CODE_SNIPPET>
/**
* @param {function(this:T, ...)} fn
* @param {T} thisObj
* @param {...*} var_args
* @template T
*/
goog.bind = function(fn, thisObj, var_args) {
...
};
// Possibly generates a missing property warning.
goog.bind(function() { this.someProperty; }, new SomeClass());
// Generates an undefined this warning.
goog.bind(function() { this.someProperty; });
</CODE_SNIPPET>
</SUBSECTION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Comments">
<SUMMARY>Use JSDoc</SUMMARY>
<BODY>
<p>
We follow the
<a href="cppguide.html#Comments">
C++ style for comments</a> in spirit.
</p>
<p>All files, classes, methods and properties should be documented with
<a href="https://code.google.com/p/jsdoc-toolkit/">JSDoc</a>
comments with the appropriate <a href="#JSDoc_Tag_Reference">tags</a>
and <a href="#JsTypes">types</a>. Textual descriptions for properties,
methods, method parameters and method return values should be included
unless obvious from the property, method, or parameter name.
</p>
<p>Inline comments should be of the <code>//</code> variety.</p>
<p>Complete sentences are recommended but not required.
Complete sentences should use appropriate capitalization
and punctuation.</p>
<SUBSECTION title="Comment Syntax">
<p>The JSDoc syntax is based on
<a href="https://www.oracle.com/technetwork/java/javase/documentation/index-137868.html">
JavaDoc</a>. Many tools extract metadata from JSDoc comments to
perform code validation and optimizations. These comments must be
well-formed.</p>
<CODE_SNIPPET>
/**
* A JSDoc comment should begin with a slash and 2 asterisks.
* Inline tags should be enclosed in braces like {@code this}.
* @desc Block tags should always start on their own line.
*/
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="JSDoc Indentation">
<p>If you have to line break a block tag, you should treat this as
breaking a code statement and indent it four spaces.</p>
<CODE_SNIPPET>
/**
* Illustrates line wrapping for long param/return descriptions.
* @param {string} foo This is a param with a description too long to fit in
* one line.
* @return {number} This returns something that has a description too long to
* fit in one line.
*/
project.MyClass.prototype.method = function(foo) {
return 5;
};
</CODE_SNIPPET>
<p>You should not indent the <code>@fileoverview</code> command. You do not have to
indent the <code>@desc</code> command.</p>
<p>Even though it is not preferred, it is also acceptable to line up
the description.</p>
<CODE_SNIPPET>
/**
* This is NOT the preferred indentation method.
* @param {string} foo This is a param with a description too long to fit in
* one line.
* @return {number} This returns something that has a description too long to
* fit in one line.
*/
project.MyClass.prototype.method = function(foo) {
return 5;
};
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="HTML in JSDoc">
<p>Like JavaDoc, JSDoc supports many HTML tags, like <code>,
<pre>, <tt>, <strong>, <ul>, <ol>,
<li>, <a>, and others.</p>
<p>This means that plaintext formatting is not respected. So, don't
rely on whitespace to format JSDoc:</p>
<BAD_CODE_SNIPPET>
/**
* Computes weight based on three factors:
* items sent
* items received
* last timestamp
*/
</BAD_CODE_SNIPPET>
<p>It'll come out like this:</p>
<BAD_CODE_SNIPPET>
Computes weight based on three factors: items sent items received last timestamp
</BAD_CODE_SNIPPET>
<p>Instead, do this:</p>
<CODE_SNIPPET>
/**
* Computes weight based on three factors:
* <ul>
* <li>items sent
* <li>items received
* <li>last timestamp
* </ul>
*/
</CODE_SNIPPET>
The <a href="https://www.oracle.com/technetwork/java/javase/documentation/index-137868.html">
JavaDoc</a> style guide is a useful resource on how to write
well-formed doc comments.
</SUBSECTION>
<SUBSECTION title="Top/File-Level Comments">
<p>
A <a href="copyright.html">copyright notice</a> and author information are optional.
File overviews are generally recommended whenever a file consists of
more than a single class definition. The top level comment is
designed to orient readers unfamiliar with the code to what is in
this file. If present, it should provide a description of the
file's contents and any dependencies or compatibility information.
As an example:
</p>
<CODE_SNIPPET>
/**
* @fileoverview Description of file, its uses and information
* about its dependencies.
*/
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Class Comments">
<p>Classes must be documented with a description and a
<a href="#constructor-tag">type tag that
identifies the constructor</a>.
</p>
<CODE_SNIPPET>
/**
* Class making something fun and easy.
* @param {string} arg1 An argument that makes this more interesting.
* @param {Array.<number>} arg2 List of numbers to be processed.
* @constructor
* @extends {goog.Disposable}
*/
project.MyClass = function(arg1, arg2) {
// ...
};
goog.inherits(project.MyClass, goog.Disposable);
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Method and Function Comments">
<p>Parameter and return types should be documented. The method
description may be omitted if it is obvious from the parameter
or return type descriptions. Method descriptions should start
with a sentence written in the third person declarative voice.</p>
<CODE_SNIPPET>
/**
* Operates on an instance of MyClass and returns something.
* @param {project.MyClass} obj Instance of MyClass which leads to a long
* comment that needs to be wrapped to two lines.
* @return {boolean} Whether something occurred.
*/
function PR_someMethod(obj) {
// ...
}
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Property Comments">
<CODE_SNIPPET>
/** @constructor */
project.MyClass = function() {
/**
* Maximum number of things per pane.
* @type {number}
*/
this.someProperty = 4;
}
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="JSDoc Tag Reference">
<a name="JSDoc_Tag_Reference"/>
<p/>
<table border="1" style="border-collapse:collapse" cellpadding="4">
<thead>
<tr>
<th>Tag</th>
<th>Template & Examples</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>
<a name="tag-author">@author</a>
</td>
<td>
<code>@author username@google.com (first last)</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* @fileoverview Utilities for handling textareas.
* @author kuth@google.com (Uthur Pendragon)
*/
</CODE_SNIPPET>
</td>
<td>
Document the author of a file or the owner of a test,
generally only used in the <code>@fileoverview</code> comment.
</td>
</tr>
<tr>
<td><a name="tag-code">@code</a></td>
<td>
<code>{@code ...}</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* Moves to the next position in the selection.
* Throws {@code goog.iter.StopIteration} when it
* passes the end of the range.
* @return {Node} The node at the next position.
*/
goog.dom.RangeIterator.prototype.next = function() {
// ...
};
</CODE_SNIPPET>
</td>
<td>
Indicates that a term in a JSDoc description is code so it may
be correctly formatted in generated documentation.
</td>
</tr>
<tr>
<td><a name="tag-const">@const</a></td>
<td>
<code>@const</code><br/>
<code>@const {type}</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/** @const */ var MY_BEER = 'stout';
/**
* My namespace's favorite kind of beer.
* @const {string}
*/
mynamespace.MY_BEER = 'stout';
/** @const */ MyClass.MY_BEER = 'stout';
/**
* Initializes the request.
* @const
*/
mynamespace.Request.prototype.initialize = function() {
// This method cannot be overridden in a subclass.
};
</CODE_SNIPPET>
</td>
<td>
<p>Marks a variable (or property) as read-only and suitable
for inlining.</p>
<p>A <code>@const</code> variable is an immutable pointer to
a value. If a variable or property marked as
<code>@const</code> is overwritten, JSCompiler will give
warnings.</p>
<p>The type declaration of a constant value can be omitted
if it can be clearly inferred. An additional comment about
the variable is optional.</p>
<p>When <code>@const</code> is applied to a method, it
implies the method is not only not overwritable, but also
that the method is <em>finalized</em> —
not overridable in subclasses.</p>
<p>For more on <code>@const</code>, see the
<a href="#Constants">Constants</a> section.</p>
</td>
</tr>
<tr>
<td><a name="tag-constructor">@constructor</a></td>
<td>
<code>@constructor</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* A rectangle.
* @constructor
*/
function GM_Rect() {
...
}
</CODE_SNIPPET>
</td>
<td>
Used in a class's documentation to indicate the constructor.
</td>
</tr>
<tr>
<td><a name="tag-define">@define</a></td>
<td>
<code>@define {Type} description</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/** @define {boolean} */
var TR_FLAGS_ENABLE_DEBUG = true;
/**
* @define {boolean} Whether we know at compile-time that
* the browser is IE.
*/
goog.userAgent.ASSUME_IE = false;
</CODE_SNIPPET>
</td>
<td>
Indicates a constant that can be overridden by the compiler at
compile-time. In the example, the compiler flag
<code>--define='goog.userAgent.ASSUME_IE=true'</code>
could be specified in the BUILD file to indicate that the
constant <code>goog.userAgent.ASSUME_IE</code> should be replaced
with <code>true</code>.
</td>
</tr>
<tr>
<td><a name="tag-deprecated">@deprecated</a></td>
<td>
<code>@deprecated Description</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* Determines whether a node is a field.
* @return {boolean} True if the contents of
* the element are editable, but the element
* itself is not.
* @deprecated Use isField().
*/
BN_EditUtil.isTopEditableField = function(node) {
// ...
};
</CODE_SNIPPET>
</td>
<td>
Used to tell that a function, method or property should not be
used any more. Always provide instructions on what callers
should use instead.
</td>
</tr>
<tr>
<td><a name="tag-dict">@dict</a></td>
<td>
<code>@dict Description</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* @constructor
* @dict
*/
function Foo(x) {
this['x'] = x;
}
var obj = new Foo(123);
var num = obj.x; // warning
(/** @dict */ { x: 1 }).x = 123; // warning
</CODE_SNIPPET>
</td>
<td>
When a constructor (<code>Foo</code> in the example) is
annotated with <code>@dict</code>, you can only use the
bracket notation to access the properties of <code>Foo</code>
objects.
The annotation can also be used directly on object literals.
</td>
</tr>
<tr>
<td><a name="tag-enum">@enum</a></td>
<td>
<code>@enum {Type}</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* Enum for tri-state values.
* @enum {number}
*/
project.TriState = {
TRUE: 1,
FALSE: -1,
MAYBE: 0
};
</CODE_SNIPPET>
</td>
</tr>
<tr>
<td><a name="tag-export">@export</a></td>
<td>
<code>@export</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/** @export */
foo.MyPublicClass.prototype.myPublicMethod = function() {
// ...
};
</CODE_SNIPPET>
</td>
<td>
<p>Given the code on the left, when the compiler is run with
the <code>--generate_exports</code> flag, it will generate the
code:</p>
<CODE_SNIPPET>
goog.exportSymbol('foo.MyPublicClass.prototype.myPublicMethod',
foo.MyPublicClass.prototype.myPublicMethod);
</CODE_SNIPPET>
<p>which will export the symbols to uncompiled code.
Code that uses the <code>@export</code> annotation must either</p>
<ol>
<li>include <code>//javascript/closure/base.js</code>, or</li>
<li>define both <code>goog.exportSymbol</code> and
<code>goog.exportProperty</code> with the same method
signature in their own codebase.</li>
</ol>
</td>
</tr>
<tr>
<td><a name="tag-expose">@expose</a></td>
<td>
<code>@expose</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/** @expose */
MyClass.prototype.exposedProperty = 3;
</CODE_SNIPPET>
</td>
<td>
<p>
Declares an exposed property. Exposed properties
will not be removed, or renamed, or collapsed,
or optimized in any way by the compiler. No properties
with the same name will be able to be optimized either.
</p>
<p>
<code>@expose</code> should never be used in library code,
because it will prevent that property from ever getting
removed.
</p>
</td>
</tr>
<tr>
<td><a name="tag-extends">@extends</a></td>
<td>
<code>
@extends Type<br/>
@extends {Type}
</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* Immutable empty node list.
* @constructor
* @extends goog.ds.BasicNodeList
*/
goog.ds.EmptyNodeList = function() {
...
};
</CODE_SNIPPET>
</td>
<td>
Used with <code>@constructor</code> to indicate that a class
inherits from another class. Curly braces around the type are
optional.
</td>
</tr>
<tr>
<td><a name="tag-externs">@externs</a></td>
<td>
<code>@externs</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* @fileoverview This is an externs file.
* @externs
*/
var document;
</CODE_SNIPPET>
</td>
<td>
<p>
Declares an
externs file.
</p>
</td>
</tr>
<tr>
<td><a name="tag-fileoverview">@fileoverview</a></td>
<td>
<code>@fileoverview Description</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* @fileoverview Utilities for doing things that require this very long
* but not indented comment.
* @author kuth@google.com (Uthur Pendragon)
*/
</CODE_SNIPPET>
</td>
<td>Makes the comment block provide file level information.</td>
</tr>
<tr>
<td><a name="tag-implements">@implements</a></td>
<td>
<code>
@implements Type<br/>
@implements {Type}
</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* A shape.
* @interface
*/
function Shape() {};
Shape.prototype.draw = function() {};
/**
* @constructor
* @implements {Shape}
*/
function Square() {};
Square.prototype.draw = function() {
...
};
</CODE_SNIPPET>
</td>
<td>
Used with <code>@constructor</code> to indicate that a class
implements an interface. Curly braces around the type are
optional.
</td>
</tr>
<tr>
<td><a name="tag-inheritDoc">@inheritDoc</a></td>
<td>
<code>@inheritDoc</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/** @inheritDoc */
project.SubClass.prototype.toString() {
// ...
};
</CODE_SNIPPET>
</td>
<td>
<p style="font-weight:bold">Deprecated. Use
<code>@override</code> instead.</p>
Indicates that a method or property of a subclass
intentionally hides a method or property of the superclass,
and has exactly the same documentation. Notice that
<code>@inheritDoc</code> implies <code>@override</code>
</td>
</tr>
<tr>
<td><a name="tag-interface">@interface</a></td>
<td>
<code>@interface</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* A shape.
* @interface
*/
function Shape() {};
Shape.prototype.draw = function() {};
/**
* A polygon.
* @interface
* @extends {Shape}
*/
function Polygon() {};
Polygon.prototype.getSides = function() {};
</CODE_SNIPPET>
</td>
<td>
Used to indicate that the function defines an interface.
</td>
</tr>
<tr>
<td><a name="tag-lends">@lends</a></td>
<td>
<code>@lends objectName</code><br/>
<code>@lends {objectName}</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
goog.object.extend(
Button.prototype,
/** @lends {Button.prototype} */ {
isButton: function() { return true; }
});
</CODE_SNIPPET>
</td>
<td>
Indicates that the keys of an object literal should
be treated as properties of some other object. This annotation
should only appear on object literals.<p/>
Notice that the name in braces is not a type name like
in other annotations. It's an object name. It names
the object on which the properties are "lent".
For example, <code>@type {Foo}</code> means "an instance of Foo",
but <code>@lends {Foo}</code> means "the constructor Foo".<p/>
The <a href="https://code.google.com/p/jsdoc-toolkit/wiki/TagLends">
JSDoc Toolkit docs</a> have more information on this
annotation.
</td>
</tr>
<tr>
<td><a name="tag-license">@license</a> or
<a name="tag-preserve">@preserve</a></td>
<td>
<code>@license Description</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* @preserve Copyright 2009 SomeThirdParty.
* Here is the full license text and copyright
* notice for this file. Note that the notice can span several
* lines and is only terminated by the closing star and slash:
*/
</CODE_SNIPPET>
</td>
<td>
Anything marked by <code>@license</code> or
<code>@preserve</code> will be retained by the compiler and
output at the top of the compiled code for that file. This
annotation allows important notices (such as legal licenses or
copyright text) to survive compilation unchanged. Line breaks
are preserved.
</td>
</tr>
<tr>
<td><a name="tag-noalias">@noalias</a></td>
<td>
<code>@noalias</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/** @noalias */
function Range() {}
</CODE_SNIPPET>
</td>
<td>
Used in an externs file to indicate to the compiler that the
variable or function should not be aliased as part of the
alias externals pass of the compiler.
</td>
</tr>
<tr>
<td><a name="tag-nocompile">@nocompile</a></td>
<td>
<code>@nocompile</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/** @nocompile */
// JavaScript code
</CODE_SNIPPET>
</td>
<td>
Used at the top of a file to tell the compiler to parse this
file but not compile it.
Code that is not meant for compilation and should be omitted
from compilation tests (such as bootstrap code) uses this
annotation.
Use sparingly.
</td>
</tr>
<tr>
<td><a name="tag-nosideeffects">@nosideeffects</a></td>
<td>
<code>@nosideeffects</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/** @nosideeffects */
function noSideEffectsFn1() {
// ...
}
/** @nosideeffects */
var noSideEffectsFn2 = function() {
// ...
};
/** @nosideeffects */
a.prototype.noSideEffectsFn3 = function() {
// ...
};
</CODE_SNIPPET>
</td>
<td>
This annotation can be used as part of function and
constructor declarations to indicate that calls to the
declared function have no side-effects. This annotation
allows the compiler to remove calls to these functions if the
return value is not used.
</td>
</tr>
<tr>
<td><a name="tag-override">@override</a></td>
<td>
<code>@override</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* @return {string} Human-readable representation of project.SubClass.
* @override
*/
project.SubClass.prototype.toString = function() {
// ...
};
</CODE_SNIPPET>
</td>
<td>
Indicates that a method or property of a subclass
intentionally hides a method or property of the superclass. If
no other documentation is included, the method or property
also inherits documentation from its superclass.
</td>
</tr>
<tr>
<td><a name="tag-param">@param</a></td>
<td>
<code>@param {Type} varname Description</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* Queries a Baz for items.
* @param {number} groupNum Subgroup id to query.
* @param {string|number|null} term An itemName,
* or itemId, or null to search everything.
*/
goog.Baz.prototype.query = function(groupNum, term) {
// ...
};
</CODE_SNIPPET>
</td>
<td>
Used with method, function and constructor calls to document
the arguments of a function.<p/>
<a href="#JsTypes">Type</a>
names must be enclosed in curly braces. If the type
is omitted, the compiler will not type-check the parameter.
</td>
</tr>
<tr>
<td><a name="tag-private">@private</a></td>
<td>
<code>@private</code><br/>
<code>@private {type}</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* Handlers that are listening to this logger.
* @private {!Array.<Function>}
*/
this.handlers_ = [];
</CODE_SNIPPET>
</td>
<td>
Used in conjunction with a trailing underscore on the method
or property name to indicate that the member is
<a href="#Visibility__private_and_protected_fields_">private</a> and final.
</td>
</tr>
<tr>
<td><a name="tag-protected">@protected</a></td>
<td>
<code>@protected</code><br/>
<code>@protected {type}</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* Sets the component's root element to the given element.
* @param {Element} element Root element for the component.
* @protected
*/
goog.ui.Component.prototype.setElementInternal = function(element) {
// ...
};
</CODE_SNIPPET>
</td>
<td>
Used to indicate that the member or property is
<a href="#Visibility__private_and_protected_fields_">protected</a>.
Should be used in conjunction with names with no trailing
underscore.
</td>
</tr>
<tr>
<td><a name="tag-public">@public</a></td>
<td>
<code>@public</code><br/>
<code>@public {type}</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* Whether to cancel the event in internal capture/bubble processing.
* @public {boolean}
* @suppress {visiblity} Referencing this outside this package is strongly
* discouraged.
*/
goog.events.Event.prototype.propagationStopped_ = false;
</CODE_SNIPPET>
</td>
<td>
Used to indicate that the member or property is public. Variables and
properties are public by default, so this annotation is rarely necessary.
Should only be used in legacy code that cannot be easily changed to
override the visibility of members that were named as private variables.
</td>
</tr>
<tr>
<td><a name="tag-return">@return</a></td>
<td>
<code>@return {Type} Description</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* @return {string} The hex ID of the last item.
*/
goog.Baz.prototype.getLastId = function() {
// ...
return id;
};
</CODE_SNIPPET>
</td>
<td>
Used with method and function calls to document the return
type. When writing descriptions for boolean parameters,
prefer "Whether the component is visible" to "True if the
component is visible, false otherwise". If there is no return
value, do not use an <code>@return</code> tag.<p/>
<a href="#JsTypes">Type</a>
names must be enclosed in curly braces. If the type
is omitted, the compiler will not type-check the return value.
</td>
</tr>
<tr>
<td><a name="tag-see">@see</a></td>
<td>
<code>@see Link</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* Adds a single item, recklessly.
* @see #addSafely
* @see goog.Collect
* @see goog.RecklessAdder#add
...
</CODE_SNIPPET>
</td>
<td>Reference a lookup to another class function or method.</td>
</tr>
<tr>
<td><a name="tag-struct">@struct</a></td>
<td>
<code>@struct Description</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* @constructor
* @struct
*/
function Foo(x) {
this.x = x;
}
var obj = new Foo(123);
var num = obj['x']; // warning
obj.y = "asdf"; // warning
Foo.prototype = /** @struct */ {
method1: function() {}
};
Foo.prototype.method2 = function() {}; // warning
</CODE_SNIPPET>
</td>
<td>
When a constructor (<code>Foo</code> in the example) is
annotated with <code>@struct</code>, you can only use the dot
notation to access the properties of <code>Foo</code> objects.
Also, you cannot add new properties to <code>Foo</code>
objects after they have been created.
The annotation can also be used directly on object literals.
</td>
</tr>
<tr>
<td><a name="tag-supported">@supported</a></td>
<td>
<code>@supported Description</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* @fileoverview Event Manager
* Provides an abstracted interface to the
* browsers' event systems.
* @supported So far tested in IE6 and FF1.5
*/
</CODE_SNIPPET>
</td>
<td>
Used in a fileoverview to indicate what browsers are supported
by the file.
</td>
</tr>
<tr>
<td><a name="tag-suppress">@suppress</a></td>
<td>
<code>
@suppress {warning1|warning2}
</code>
<code>
@suppress {warning1,warning2}
</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* @suppress {deprecated}
*/
function f() {
deprecatedVersionOfF();
}
</CODE_SNIPPET>
</td>
<td>
Suppresses warnings from tools. Warning categories are
separated by <code>|</code> or <code>,</code>.
</td>
</tr>
<tr>
<td><a name="tag-template">@template</a></td>
<td>
<code>@template</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* @param {function(this:T, ...)} fn
* @param {T} thisObj
* @param {...*} var_args
* @template T
*/
goog.bind = function(fn, thisObj, var_args) {
...
};
</CODE_SNIPPET>
</td>
<td>
This annotation can be used to declare a
<a href="#Template_types">template typename</a>.
</td>
</tr>
<tr>
<td><a name="tag-this">@this</a></td>
<td>
<code>
@this Type<br/>
@this {Type}
</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
pinto.chat.RosterWidget.extern('getRosterElement',
/**
* Returns the roster widget element.
* @this pinto.chat.RosterWidget
* @return {Element}
*/
function() {
return this.getWrappedComponent_().getElement();
});
</CODE_SNIPPET>
</td>
<td>
The type of the object in whose context a particular method is
called. Required when the <code>this</code> keyword is referenced
from a function that is not a prototype method.
</td>
</tr>
<tr>
<td><a name="tag-type">@type</a></td>
<td>
<code>
@type Type<br/>
@type {Type}
</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/**
* The message hex ID.
* @type {string}
*/
var hexId = hexId;
</CODE_SNIPPET>
</td>
<td>
Identifies the <a href="#JsTypes">type</a> of a variable,
property, or expression. Curly braces are not required around
most types, but some projects mandate them for all types, for
consistency.
</td>
</tr>
<tr>
<td><a name="tag-typedef">@typedef</a></td>
<td>
<code>@typedef</code>
<p><i>For example:</i></p>
<CODE_SNIPPET>
/** @typedef {(string|number)} */
goog.NumberLike;
/** @param {goog.NumberLike} x A number or a string. */
goog.readNumber = function(x) {
...
}
</CODE_SNIPPET>
</td>
<td>
This annotation can be used to declare an alias of a more
<a href="#Typedefs">complex type</a>.
</td>
</tr>
</tbody>
</table>
<p>
You may also see other types of JSDoc annotations in third-party
code. These annotations appear in the
<a href="https://code.google.com/p/jsdoc-toolkit/wiki/TagReference">
JSDoc Toolkit Tag Reference
</a>
but are currently discouraged in Google code. You should consider
them "reserved" names for future use. These include:
<ul>
<li>@augments</li>
<li>@argument</li>
<li>@borrows</li>
<li>@class</li>
<li>@constant</li>
<li>@constructs</li>
<li>@default</li>
<li>@event</li>
<li>@example</li>
<li>@field</li>
<li>@function</li>
<li>@ignore</li>
<li>@inner</li>
<li>@link</li>
<li>@memberOf</li>
<li>@name</li>
<li>@namespace</li>
<li>@property</li>
<li>@public</li>
<li>@requires</li>
<li>@returns</li>
<li>@since</li>
<li>@static</li>
<li>@version</li>
</ul>
</p>
</SUBSECTION>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Providing Dependencies With goog.provide">
<SUMMARY>
Only provide top-level symbols.
</SUMMARY>
<BODY>
<p>
All members defined on a class should be in the same file. So, only
top-level classes should be provided in a file that contains multiple
members defined on the same class (e.g. enums, inner classes, etc).
</p>
<p>Do this:</p>
<CODE_SNIPPET>
goog.provide('namespace.MyClass');
</CODE_SNIPPET>
<p>Not this:</p>
<BAD_CODE_SNIPPET>
goog.provide('namespace.MyClass');
goog.provide('namespace.MyClass.Enum');
goog.provide('namespace.MyClass.InnerClass');
goog.provide('namespace.MyClass.TypeDef');
goog.provide('namespace.MyClass.CONSTANT');
goog.provide('namespace.MyClass.staticMethod');
</BAD_CODE_SNIPPET>
<p>
Members on namespaces may also be provided:
</p>
<CODE_SNIPPET>
goog.provide('foo.bar');
goog.provide('foo.bar.method');
goog.provide('foo.bar.CONSTANT');
</CODE_SNIPPET>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Compiling">
<SUMMARY>Required</SUMMARY>
<BODY>
<p>Use of JS compilers such as the
<a href="https://code.google.com/closure/compiler/">Closure Compiler</a>
is required for all customer-facing code.</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Tips and Tricks">
<SUMMARY>JavaScript tidbits</SUMMARY>
<BODY>
<SUBSECTION title="True and False Boolean Expressions">
<p>The following are all false in boolean expressions:</p>
<ul>
<li><code>null</code></li>
<li><code>undefined</code></li>
<li><code>''</code> the empty string</li>
<li><code>0</code> the number</li>
</ul>
<p>But be careful, because these are all true:</p>
<ul>
<li><code>'0'</code> the string</li>
<li><code>[]</code> the empty array</li>
<li><code>{}</code> the empty object</li>
</ul>
<p>This means that instead of this:</p>
<BAD_CODE_SNIPPET>
while (x != null) {
</BAD_CODE_SNIPPET>
<p>you can write this shorter code (as long as you don't expect x to
be 0, or the empty string, or false):</p>
<CODE_SNIPPET>
while (x) {
</CODE_SNIPPET>
<p>And if you want to check a string to see if it is null or empty,
you could do this:</p>
<BAD_CODE_SNIPPET>
if (y != null && y != '') {
</BAD_CODE_SNIPPET>
<p>But this is shorter and nicer:</p>
<CODE_SNIPPET>
if (y) {
</CODE_SNIPPET>
<p><strong>Caution:</strong> There are many unintuitive things about
boolean expressions. Here are some of them:</p>
<ul>
<li><code>
Boolean('0') == true<br/>
'0' != true</code></li>
<li><code>
0 != null<br/>
0 == []<br/>
0 == false</code></li>
<li><code>
Boolean(null) == false<br/>
null != true<br/>
null != false</code></li>
<li><code>
Boolean(undefined) == false<br/>
undefined != true<br/>
undefined != false</code></li>
<li><code>
Boolean([]) == true<br/>
[] != true<br/>
[] == false</code></li>
<li><code>
Boolean({}) == true<br/>
{} != true<br/>
{} != false</code></li>
</ul>
</SUBSECTION>
<SUBSECTION title="Conditional (Ternary) Operator (?:)">
<p>Instead of this:</p>
<CODE_SNIPPET>
if (val) {
return foo();
} else {
return bar();
}
</CODE_SNIPPET>
<p>you can write this:</p>
<CODE_SNIPPET>
return val ? foo() : bar();
</CODE_SNIPPET>
<p>The ternary conditional is also useful when generating HTML:</p>
<CODE_SNIPPET>
var html = '<input type="checkbox"' +
(isChecked ? ' checked' : '') +
(isEnabled ? '' : ' disabled') +
' name="foo">';
</CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="&& and ||">
<p>These binary boolean operators are short-circuited, and evaluate
to the last evaluated term.</p>
<p>"||" has been called the 'default' operator, because instead of
writing this:</p>
<BAD_CODE_SNIPPET>
/** @param {*=} opt_win */
function foo(opt_win) {
var win;
if (opt_win) {
win = opt_win;
} else {
win = window;
}
// ...
}
</BAD_CODE_SNIPPET>
<p>you can write this:</p>
<CODE_SNIPPET>
/** @param {*=} opt_win */
function foo(opt_win) {
var win = opt_win || window;
// ...
}
</CODE_SNIPPET>
<p>"&&" is also useful for shortening code. For instance,
instead of this:</p>
<BAD_CODE_SNIPPET>
if (node) {
if (node.kids) {
if (node.kids[index]) {
foo(node.kids[index]);
}
}
}
</BAD_CODE_SNIPPET>
<p>you could do this:</p>
<CODE_SNIPPET>
if (node && node.kids && node.kids[index]) {
foo(node.kids[index]);
}
</CODE_SNIPPET>
<p>or this:</p>
<CODE_SNIPPET>
var kid = node && node.kids && node.kids[index];
if (kid) {
foo(kid);
}
</CODE_SNIPPET>
<p>However, this is going a little too far:</p>
<BAD_CODE_SNIPPET>
node && node.kids && node.kids[index] && foo(node.kids[index]);
</BAD_CODE_SNIPPET>
</SUBSECTION>
<SUBSECTION title="Iterating over Node Lists">
<p>Node lists are often implemented as node iterators with a filter.
This means that getting a property like length is O(n), and
iterating over the list by re-checking the length will be
O(n^2).</p>
<BAD_CODE_SNIPPET>
var paragraphs = document.getElementsByTagName('p');
for (var i = 0; i < paragraphs.length; i++) {
doSomething(paragraphs[i]);
}
</BAD_CODE_SNIPPET>
<p>It is better to do this instead:</p>
<CODE_SNIPPET>
var paragraphs = document.getElementsByTagName('p');
for (var i = 0, paragraph; paragraph = paragraphs[i]; i++) {
doSomething(paragraph);
}
</CODE_SNIPPET>
<p>This works well for all collections and arrays as long as the array
does not contain things that are treated as boolean false.</p>
<p>In cases where you are iterating over the childNodes you can also
use the firstChild and nextSibling properties.</p>
<CODE_SNIPPET>
var parentNode = document.getElementById('foo');
for (var child = parentNode.firstChild; child; child = child.nextSibling) {
doSomething(child);
}
</CODE_SNIPPET>
</SUBSECTION>
</BODY>
</STYLEPOINT>
</CATEGORY>
<PARTING_WORDS>
<p>
<em>BE CONSISTENT</em>.
</p>
<p>
If you're editing code, take a few minutes to look at the code
around you and determine its style. If they use spaces around
all their arithmetic operators, you should too. If their
comments have little boxes of hash marks around them, make your
comments have little boxes of hash marks around them too.
</p>
<p>
The point of having style guidelines is to have a common vocabulary
of coding so people can concentrate on what you're saying rather
than on how you're saying it. We present global style rules here so
people know the vocabulary, but local style is also important. If
code you add to a file looks drastically different from the existing
code around it, it throws readers out of their rhythm when they go to
read it. Avoid this.
</p>
</PARTING_WORDS>
<p align="right">
Revision 2.93
</p>
<address>
Aaron Whyte<br/>
Bob Jervis<br/>
Dan Pupius<br/>
Erik Arvidsson<br/>
Fritz Schneider<br/>
Robby Walker<br/>
</address>
</GUIDE>