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getjscontent.js
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function getjscontent() {
return `
<html lang="en"><head>
<meta charset="utf-8">
<title>Google JavaScript Style Guide</title>
<style>
*{
margin:10px 10px 10px 10px
}
</style>
</head>
<body>
<div id="content">
<h1>Google JavaScript Style Guide</h1><div id="tocDiv" class="vertical_toc"><div class="toc_title"></div><h3 class="ignoreLink" id="toc">Table of Contents</h3><table class="columns"><tbody valign="top" class="toc"><tr><td class="two_columns"><dl><dt><a href="#introduction">1 Introduction</a></dt><dd><a href="#terminology-notes">1.1 Terminology notes</a></dd><dd><a href="#guide-notes">1.2 Guide notes</a></dd></dl><dl><dt><a href="#source-file-basics">2 Source file basics</a></dt><dd><a href="#file-name">2.1 File name</a></dd><dd><a href="#file-encoding">2.2 File encoding: UTF-8</a></dd><dd><a href="#special-characters">2.3 Special characters</a></dd></dl><dl><dt><a href="#source-file-structure">3 Source file structure</a></dt><dd><a href="#file-copyright">3.1 License or copyright information, if present</a></dd><dd><a href="#file-fileoverview">3.2 @fileoverview JSDoc, if present</a></dd><dd><a href="#file-goog-module">3.3 goog.module statement</a></dd><dd><a href="#file-goog-require">3.4 goog.require statements</a></dd><dd><a href="#file-implementation">3.5 The file’s implementation</a></dd></dl><dl><dt><a href="#formatting">4 Formatting</a></dt><dd><a href="#formatting-braces">4.1 Braces</a></dd><dd><a href="#formatting-block-indentation">4.2 Block indentation: +2 spaces</a></dd><dd><a href="#formatting-statements">4.3 Statements</a></dd><dd><a href="#formatting-column-limit">4.4 Column limit: 80</a></dd><dd><a href="#formatting-line-wrapping">4.5 Line-wrapping</a></dd><dd><a href="#formatting-whitespace">4.6 Whitespace</a></dd><dd><a href="#formatting-grouping-parentheses">4.7 Grouping parentheses: recommended</a></dd><dd><a href="#formatting-comments">4.8 Comments</a></dd></dl><dl><dt><a href="#language-features">5 Language features</a></dt><dd><a href="#features-local-variable-declarations">5.1 Local variable declarations</a></dd><dd><a href="#features-array-literals">5.2 Array literals</a></dd><dd><a href="#features-object-literals">5.3 Object literals</a></dd><dd><a href="#features-classes">5.4 Classes</a></dd><dd><a href="#features-functions">5.5 Functions</a></dd><dd><a href="#features-string-literals">5.6 String literals</a></dd><dd><a href="#features-number-literals">5.7 Number literals</a></dd><dd><a href="#features-control-structures">5.8 Control structures</a></dd><dd><a href="#features-this">5.9 this</a></dd><dd><a href="#disallowed-features">5.10 Disallowed features</a></dd></dl><dl><dt><a href="#naming">6 Naming</a></dt><dd><a href="#naming-rules-common-to-all-identifiers">6.1 Rules common to all identifiers</a></dd><dd><a href="#naming-rules-by-identifier-type">6.2 Rules by identifier type</a></dd><dd><a href="#naming-camel-case-defined">6.3 Camel case: defined</a></dd></dl><dl><dt><a href="#jsdoc">7 JSDoc</a></dt><dd><a href="#jsdoc-general-form">7.1 General form</a></dd><dd><a href="#jsdoc-markdown">7.2 Markdown</a></dd><dd><a href="#jsdoc-tags">7.3 JSDoc tags</a></dd><dd><a href="#jsdoc-line-wrapping">7.4 Line wrapping</a></dd><dd><a href="#jsdoc-top-file-level-comments">7.5 Top/file-level comments</a></dd><dd><a href="#jsdoc-class-comments">7.6 Class comments</a></dd><dd><a href="#jsdoc-enum-and-typedef-comments">7.7 Enum and typedef comments</a></dd><dd><a href="#jsdoc-method-and-function-comments">7.8 Method and function comments</a></dd><dd><a href="#jsdoc-property-comments">7.9 Property comments</a></dd><dd><a href="#jsdoc-type-annotations">7.10 Type annotations</a></dd><dd><a href="#jsdoc-visibility-annotations">7.11 Visibility annotations</a></dd></dl><dl><dt><a href="#policies">8 Policies</a></dt><dd><a href="#policies-be-consistent">8.1 Issues unspecified by Google Style: Be Consistent!</a></dd><dd><a href="#policies-compiler-warnings">8.2 Compiler warnings</a></dd><dd><a href="#policies-deprecation">8.3 Deprecation</a></dd><dd><a href="#policies-code-not-in-google-style">8.4 Code not in Google Style</a></dd><dd><a href="#policies-local-style-rules">8.5 Local style rules</a></dd><dd><a href="#policies-generated-code-mostly-exempt">8.6 Generated code: mostly exempt</a></dd></dl><dl><dt><a href="#appendices">9 Appendices</a></dt><dd><a href="#appendices-jsdoc-tag-reference">9.1 JSDoc tag reference</a></dd><dd><a href="#appendices-commonly-misunderstood-style-rules">9.2 Commonly misunderstood style rules</a></dd><dd><a href="#appendices-style-related-tools">9.3 Style-related tools</a></dd><dd><a href="#appendices-legacy-exceptions">9.4 Exceptions for legacy platforms</a></dd></dl></td></tr></tbody></table></div>
<h2 id="introduction" style="left: -46px; position: relative;">1 Introduction<a href="#introduction" alt="link to introduction"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h2>
<p>This document serves as the <strong>complete</strong> definition of Google’s coding standards
for source code in the JavaScript programming language. A JavaScript source file
is described as being <em>in Google Style</em> if and only if it adheres to the rules
herein.</p>
<p>Like other programming style guides, the issues covered span not only aesthetic
issues of formatting, but other types of conventions or coding standards as
well. However, this document focuses primarily on the hard-and-fast rules that
we follow universally, and avoids giving advice that isn't clearly enforceable
(whether by human or tool). </p>
<h3 id="terminology-notes" style="left: -46px; position: relative;">1.1 Terminology notes<a href="#terminology-notes" alt="link to terminology-notes"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<p>In this document, unless otherwise clarified:</p>
<ol>
<li><p>The term <em>comment</em> always refers to <em>implementation</em> comments. We do not use
the phrase <q>documentation comments</q>, instead using the common term “JSDoc”
for both human-readable text and machine-readable annotations within
<code>/** … */</code>.</p></li>
<li><p>This Style Guide uses <a href="http://tools.ietf.org/html/rfc2119">RFC 2119</a> terminology when using the phrases <em>must</em>,
<em>must not</em>, <em>should</em>, <em>should not</em>, and <em>may</em>. The terms <em>prefer</em> and
<em>avoid</em> correspond to <em>should</em> and <em>should not</em>, respectively. Imperative
and declarative statements are prescriptive and correspond to <em>must</em>.</p></li>
</ol>
<p>Other <q>terminology notes</q> will appear occasionally throughout the document.</p>
<h3 id="guide-notes" style="left: -46px; position: relative;">1.2 Guide notes<a href="#guide-notes" alt="link to guide-notes"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<p>Example code in this document is <strong>non-normative</strong>. That is, while the examples
are in Google Style, they may not illustrate the <em>only</em> stylish way to represent
the code. Optional formatting choices made in examples must not be enforced as
rules.</p>
<h2 id="source-file-basics" style="left: -46px; position: relative;">2 Source file basics<a href="#source-file-basics" alt="link to source-file-basics"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h2>
<h3 id="file-name" style="left: -46px; position: relative;">2.1 File name<a href="#file-name" alt="link to file-name"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<p>File names must be all lowercase and may include underscores (<code>_</code>) or dashes
(<code>-</code>), but no additional punctuation. Follow the convention that your project
uses. Filenames’ extension must be <code>.js</code>.</p>
<h3 id="file-encoding" style="left: -46px; position: relative;">2.2 File encoding: UTF-8<a href="#file-encoding" alt="link to file-encoding"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<p>Source files are encoded in <strong>UTF-8</strong>.</p>
<h3 id="special-characters" style="left: -46px; position: relative;">2.3 Special characters<a href="#special-characters" alt="link to special-characters"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<h4 id="whitespace-characters" style="left: -46px; position: relative;">2.3.1 Whitespace characters<a href="#whitespace-characters" alt="link to whitespace-characters"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Aside from the line terminator sequence, the ASCII horizontal space character
(0x20) is the only whitespace character that appears anywhere in a source
file. This implies that</p>
<ol>
<li><p>All other whitespace characters in string literals are escaped, and</p></li>
<li><p>Tab characters are <strong>not</strong> used for indentation.</p></li>
</ol>
<h4 id="special-escape-sequences" style="left: -46px; position: relative;">2.3.2 Special escape sequences<a href="#special-escape-sequences" alt="link to special-escape-sequences"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>For any character that has a special escape sequence (<code>\\'</code>, <code>\\"</code>, <code>\\</code>, <code>\\b</code>,
<code>\\f</code>, <code>\\n</code>, <code>\\r</code>, <code>\\t</code>, <code>\\v</code>), that sequence is used rather than the
corresponding numeric escape (e.g <code>\\x0a</code>, <code>\\u000a</code>, or <code>\\u{a}</code>). Legacy octal
escapes are never used.</p>
<h4 id="non-ascii-characters" style="left: -46px; position: relative;">2.3.3 Non-ASCII characters<a href="#non-ascii-characters" alt="link to non-ascii-characters"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>For the remaining non-ASCII characters, either the actual Unicode character
(e.g. <code>∞</code>) or the equivalent hex or Unicode escape (e.g. <code>\\u221e</code>) is used,
depending only on which makes the code <strong>easier to read and understand</strong>.</p>
<p class="tip">Tip: In the Unicode escape case, and occasionally even when actual Unicode
characters are used, an explanatory comment can be very helpful.</p>
<table>
<thead>
<tr>
<th>Example
</th><th>Discussion
</th></tr></thead><tbody>
<tr>
<td><code class="prettyprint lang-js">const units = 'μs';</code>
</td><td>Best: perfectly clear even without a comment.
</td></tr><tr>
<td>
<code class="prettyprint lang-js">const units = '\\u03bcs'; // 'μs'
</code>
</td><td>Allowed, but there’s no reason to do this.
</td></tr><tr>
<td>
<code class="prettyprint lang-js">const units = '\\u03bcs'; // Greek letter mu, 's'
</code>
</td><td>Allowed, but awkward and prone to mistakes.
</td></tr><tr>
<td><code class="badcode">const units = '\\u03bcs';</code>
</td><td>Poor: the reader has no idea what this is.
</td></tr><tr>
<td>
<code class="prettyprint lang-js">return '\\ufeff' + content; // byte order mark
</code>
</td><td>
Good: use escapes for non-printable characters, and comment if
necessary.
</td></tr></tbody></table>
<p class="tip">Tip: Never make your code less readable simply out of fear that some programs
might not handle non-ASCII characters properly. If that happens, those programs
are <strong>broken</strong> and they must be <strong>fixed</strong>.</p>
<h2 id="source-file-structure" style="left: -46px; position: relative;">3 Source file structure<a href="#source-file-structure" alt="link to source-file-structure"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h2>
<p>A source file consists of, <strong>in order</strong>:</p>
<ol>
<li>License or copyright information, if present</li>
<li><code>@fileoverview</code> JSDoc, if present</li>
<li><code>goog.module</code> statement</li>
<li><code>goog.require</code> statements</li>
<li>The file’s implementation</li>
</ol>
<p><strong>Exactly one blank line</strong> separates each section that is present, except the
file's implementation, which may be preceded by 1 or 2 blank lines.</p>
<h3 id="file-copyright" style="left: -46px; position: relative;">3.1 License or copyright information, if present<a href="#file-copyright" alt="link to file-copyright"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<p>If license or copyright information belongs in a file, it belongs here.</p>
<h3 id="file-fileoverview" style="left: -46px; position: relative;">3.2 <code>@fileoverview</code> JSDoc, if present<a href="#file-fileoverview" alt="link to file-fileoverview"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<p>See <a href="#jsdoc-top-file-level-comments">7.5 Top/file-level comments</a> for formatting rules.</p>
<h3 id="file-goog-module" style="left: -46px; position: relative;">3.3 <code>goog.module</code> statement<a href="#file-goog-module" alt="link to file-goog-module"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<p>All files must declare exactly one <code>goog.module</code> name on a single line: lines
containing a <code>goog.module</code> declaration must not be wrapped, and are therefore an
exception to the 80-column limit.</p>
<p>The entire argument to goog.module is what defines a namespace. It is the
package name (an identifier that reflects the fragment of the directory
structure where the code lives) plus, optionally, the main class/enum/interface
that it defines concatenated to the end.</p>
<p>Example</p>
<pre class="language-js prettyprint">goog.module('search.urlHistory.UrlHistoryService');
</pre>
<h4 id="naming-hierarchy" style="left: -46px; position: relative;">3.3.1 Hierarchy<a href="#naming-hierarchy" alt="link to naming-hierarchy"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Module namespaces may never be named as a <em>direct</em> child of another module's
namespace.</p>
<p class="illegal">Illegal:</p>
<pre class="language-js prettyprint badcode">goog.module('foo.bar'); // 'foo.bar.qux' would be fine, though
goog.module('foo.bar.baz');
</pre>
<p>The directory hierarchy reflects the namespace hierarchy, so that deeper-nested
children are subdirectories of higher-level parent directories. Note that this
implies that owners of “parent” namespace groups are necessarily aware of all
child namespaces, since they exist in the same directory.</p>
<h4 id="file-set-test-only" style="left: -46px; position: relative;">3.3.2 <code>goog.setTestOnly</code><a href="#file-set-test-only" alt="link to file-set-test-only"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>The single <code>goog.module</code> statement may optionally be followed by a call to
goog.setTestOnly().</p>
<h4 id="file-declare-legacy-namespace" style="left: -46px; position: relative;">3.3.3 <code>goog.module.declareLegacyNamespace</code><a href="#file-declare-legacy-namespace" alt="link to file-declare-legacy-namespace"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>The single <code>goog.module</code> statement may optionally be followed by a call to
<code>goog.module.declareLegacyNamespace();</code>. Avoid
<code>goog.module.declareLegacyNamespace()</code> when possible.</p>
<p class="example">Example:</p>
<pre class="language-js prettyprint">goog.module('my.test.helpers');
goog.module.declareLegacyNamespace();
goog.setTestOnly();
</pre>
<p><code>goog.module.declareLegacyNamespace</code> exists to ease the transition from
traditional object hierarchy-based namespaces but comes with some naming
restrictions. As the child module name must be created after the parent
namespace, this name <strong>must not</strong> be a child or parent of any other
<code>goog.module</code> (for example, <code>goog.module('parent');</code> and
<code>goog.module('parent.child');</code> cannot both exist safely, nor can
<code>goog.module('parent');</code> and <code>goog.module('parent.child.grandchild');</code>).</p>
<h4 id="file-es6-modules" style="left: -46px; position: relative;">3.3.4 ES6 Modules<a href="#file-es6-modules" alt="link to file-es6-modules"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Do not use ES6 modules yet (i.e. the <code>export</code> and <code>import</code> keywords), as their
semantics are not yet finalized. Note that this policy will be revisited once
the semantics are fully-standard.</p>
<h3 id="file-goog-require" style="left: -46px; position: relative;">3.4 <code>goog.require</code> statements<a href="#file-goog-require" alt="link to file-goog-require"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<p>Imports are done with <code>goog.require</code> statements, grouped together immediately
following the module declaration. Each <code>goog.require</code> is assigned to a single
constant alias, or else destructured into several constant aliases. These
aliases are the only acceptable way to refer to the <code>require</code>d dependency,
whether in code or in type annotations: the fully qualified name is never used
except as the argument to <code>goog.require</code>. Alias names should match the final
dot-separated component of the imported module name when possible, though
additional components may be included (with appropriate casing such that the
alias' casing still correctly identifies its type) if necessary to
disambiguate, or if it significantly improves readability. <code>goog.require</code>
statements may not appear anywhere else in the file.</p>
<p>If a module is imported only for its side effects, the assignment may be
omitted, but the fully qualified name may not appear anywhere else in the file.
A comment is required to explain why this is needed and suppress a compiler
warning.</p>
<p>The lines are sorted according to the following rules: All requires with a name
on the left hand side come first, sorted alphabetically by those names. Then
destructuring requires, again sorted by the names on the left hand side.
Finally, any <code>goog.require</code> calls that are standalone (generally these are for
modules imported just for their side effects).</p>
<p class="tip">Tip: There’s no need to memorize this order and enforce it manually. You can
rely on your IDE to report requires
that are not sorted correctly.</p>
<p>If a long alias or module name would cause a line to exceed the 80-column limit,
it <strong>must not</strong> be wrapped: goog.require lines are an exception to the 80-column
limit.</p>
<p class="example">Example:</p>
<pre class="language-js prettyprint">const MyClass = goog.require('some.package.MyClass');
const NsMyClass = goog.require('other.ns.MyClass');
const googAsserts = goog.require('goog.asserts');
const testingAsserts = goog.require('goog.testing.asserts');
const than80columns = goog.require('pretend.this.is.longer.than80columns');
const {clear, forEach, map} = goog.require('goog.array');
/** @suppress {extraRequire} Initializes MyFramework. */
goog.require('my.framework.initialization');
</pre>
<p class="illegal">Illegal:</p>
<pre class="language-js badcode prettyprint">const randomName = goog.require('something.else'); // name must match
const {clear, forEach, map} = // don't break lines
goog.require('goog.array');
function someFunction() {
const alias = goog.require('my.long.name.alias'); // must be at top level
// …
}
</pre>
<h4 id="file-goog-forward-declare" style="left: -46px; position: relative;">3.4.1 <code>goog.forwardDeclare</code><a href="#file-goog-forward-declare" alt="link to file-goog-forward-declare"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p><code>goog.forwardDeclare</code> is not needed very often, but is a valuable tool to break
circular dependencies or to reference late loaded code. These statements are
grouped together and immediately follow any <code>goog.require</code> statements. A
<code>goog.forwardDeclare</code> statement must follow the same style rules as a
<code>goog.require</code> statement.</p>
<h3 id="file-implementation" style="left: -46px; position: relative;">3.5 The file’s implementation<a href="#file-implementation" alt="link to file-implementation"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<p>The actual implementation follows after all dependency information is declared
(separated by at least one blank line).</p>
<p>This may consist of any module-local declarations (constants, variables,
classes, functions, etc), as well as any exported symbols.
</p>
<h2 id="formatting" style="left: -46px; position: relative;">4 Formatting<a href="#formatting" alt="link to formatting"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h2>
<p><strong>Terminology Note</strong>: <em>block-like construct</em> refers to the body of a class,
function, method, or brace-delimited block of code. Note that, by
<a href="#features-array-literals">5.2 Array literals</a> and <a href="#features-object-literals">5.3 Object literals</a>, any array or
object literal may optionally be treated as if it were a block-like construct.</p>
<p class="tip">Tip: Use <code>clang-format</code>. The JavaScript community has invested effort to make
sure clang-format <q>does the right thing</q> on JavaScript files. <code>clang-format</code> has
integration with several popular
editors.</p>
<h3 id="formatting-braces" style="left: -46px; position: relative;">4.1 Braces<a href="#formatting-braces" alt="link to formatting-braces"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<h4 id="formatting-braces-all" style="left: -46px; position: relative;">4.1.1 Braces are used for all control structures<a href="#formatting-braces-all" alt="link to formatting-braces-all"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Braces are required for all control structures (i.e. <code>if</code>, <code>else</code>, <code>for</code>, <code>do</code>,
<code>while</code>, as well as any others), even if the body contains only a single
statement. The first statement of a non-empty block must begin on its own line.</p>
<p class="illegal">Illegal:</p>
<pre class="language-js badcode prettyprint">if (someVeryLongCondition())
doSomething();
for (let i = 0; i < foo.length; i++) bar(foo[i]);
</pre>
<p class="exception"><strong>Exception</strong>: A simple if statement that can fit entirely on a single line with
no wrapping (and that doesn’t have an else) may be kept on a single line with no
braces when it improves readability. This is the only case in which a control
structure may omit braces and newlines.</p>
<pre class="language-js prettyprint">if (shortCondition()) return;
</pre>
<h4 id="formatting-nonempty-blocks" style="left: -46px; position: relative;">4.1.2 Nonempty blocks: K&R style<a href="#formatting-nonempty-blocks" alt="link to formatting-nonempty-blocks"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Braces follow the Kernighan and Ritchie style (<q><a href="http://www.codinghorror.com/blog/2012/07/new-programming-jargon.html">Egyptian brackets</a></q>) for
<em>nonempty</em> blocks and block-like constructs:</p>
<ul>
<li>No line break before the opening brace.</li>
<li>Line break after the opening brace.</li>
<li>Line break before the closing brace.</li>
<li>Line break after the closing brace <em>if</em> that brace terminates a statement or
the body of a function or class statement, or a class method. Specifically,
there is <em>no</em> line break after the brace if it is followed by <code>else</code>, <code>catch</code>,
<code>while</code>, or a comma, semicolon, or right-parenthesis.</li>
</ul>
<p class="example">Example:</p>
<pre class="language-js prettyprint">class InnerClass {
constructor() {}
/** @param {number} foo */
method(foo) {
if (condition(foo)) {
try {
// Note: this might fail.
something();
} catch (err) {
recover();
}
}
}
}
</pre>
<h4 id="formatting-empty-blocks" style="left: -46px; position: relative;">4.1.3 Empty blocks: may be concise<a href="#formatting-empty-blocks" alt="link to formatting-empty-blocks"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>An empty block or block-like construct <em>may</em> be closed immediately after it is
opened, with no characters, space, or line break in between (i.e. <code>{}</code>),
<strong>unless</strong> it is a part of a <em>multi-block statement</em> (one that directly contains
multiple blocks: <code>if</code>/<code>else</code> or <code>try</code>/<code>catch</code>/<code>finally</code>).</p>
<p class="example">Example:</p>
<pre class="language-js prettyprint">function doNothing() {}
</pre>
<p class="illegal">Illegal:</p>
<pre class="language-js prettyprint badcode">if (condition) {
// …
} else if (otherCondition) {} else {
// …
}
try {
// …
} catch (e) {}
</pre>
<h3 id="formatting-block-indentation" style="left: -46px; position: relative;">4.2 Block indentation: +2 spaces<a href="#formatting-block-indentation" alt="link to formatting-block-indentation"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<p>Each time a new block or block-like construct is opened, the indent increases by
two spaces. When the block ends, the indent returns to the previous indent
level. The indent level applies to both code and comments throughout the
block. (See the example in <a href="#formatting-nonempty-blocks">4.1.2 Nonempty blocks: K&R style</a>).</p>
<h4 id="formatting-array-literals" style="left: -46px; position: relative;">4.2.1 Array literals: optionally <q>block-like</q><a href="#formatting-array-literals" alt="link to formatting-array-literals"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Any array literal may optionally be formatted as if it were a “block-like
construct.” For example, the following are all valid (<strong>not</strong> an exhaustive
list):</p>
<pre class="language-js prettyprint columns">const a = [
0,
1,
2,
];
const b =
[0, 1, 2];
</pre>
<pre class="language-js prettyprint columns">const c = [0, 1, 2];
someMethod(foo, [
0, 1, 2,
], bar);
</pre>
<p>Other combinations are allowed, particularly when emphasizing semantic groupings
between elements, but should not be used only to reduce the vertical size of
larger arrays.</p>
<h4 id="formatting-object-literals" style="left: -46px; position: relative;">4.2.2 Object literals: optionally <q>block-like</q><a href="#formatting-object-literals" alt="link to formatting-object-literals"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Any object literal may optionally be formatted as if it were a “block-like
construct.” The same examples apply as <a href="#formatting-array-literals">4.2.1 Array literals: optionally block-like</a>. For
example, the following are all valid (<strong>not</strong> an exhaustive list):</p>
<pre class="language-js prettyprint columns">const a = {
a: 0,
b: 1,
};
const b =
{a: 0, b: 1};
</pre>
<pre class="language-js prettyprint columns">const c = {a: 0, b: 1};
someMethod(foo, {
a: 0, b: 1,
}, bar);
</pre>
<h4 id="formatting-class-literals" style="left: -46px; position: relative;">4.2.3 Class literals<a href="#formatting-class-literals" alt="link to formatting-class-literals"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Class literals (whether declarations or expressions) are indented as blocks. Do
not add semicolons after methods, or after the closing brace of a class
<em>declaration</em> (statements—such as assignments—that contain class <em>expressions</em>
are still terminated with a semicolon). Use the <code>extends</code> keyword, but not the
<code>@extends</code> JSDoc annotation unless the class extends a templatized type.</p>
<p class="example">Example:</p>
<pre class="language-js prettyprint columns">class Foo {
constructor() {
/** @type {number} */
this.x = 42;
}
/** @return {number} */
method() {
return this.x;
}
}
Foo.Empty = class {};
</pre>
<pre class="language-js prettyprint columns">/** @extends {Foo<string>} */
foo.Bar = class extends Foo {
/** @override */
method() {
return super.method() / 2;
}
};
/** @interface */
class Frobnicator {
/** @param {string} message */
frobnicate(message) {}
}
</pre>
<h4 id="formatting-function-expressions" style="left: -46px; position: relative;">4.2.4 Function expressions<a href="#formatting-function-expressions" alt="link to formatting-function-expressions"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>When declaring an anonymous function in the list of arguments for a function
call, the body of the function is indented two spaces more than the preceding
indentation depth.</p>
<p class="example">Example:</p>
<pre class="language-js prettyprint">prefix.something.reallyLongFunctionName('whatever', (a1, a2) => {
// Indent the function body +2 relative to indentation depth
// of the 'prefix' statement one line above.
if (a1.equals(a2)) {
someOtherLongFunctionName(a1);
} else {
andNowForSomethingCompletelyDifferent(a2.parrot);
}
});
some.reallyLongFunctionCall(arg1, arg2, arg3)
.thatsWrapped()
.then((result) => {
// Indent the function body +2 relative to the indentation depth
// of the '.then()' call.
if (result) {
result.use();
}
});
</pre>
<h4 id="formatting-switch-statements" style="left: -46px; position: relative;">4.2.5 Switch statements<a href="#formatting-switch-statements" alt="link to formatting-switch-statements"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>As with any other block, the contents of a switch block are indented +2.</p>
<p>After a switch label, a newline appears, and the indentation level is increased
+2, exactly as if a block were being opened. An explicit block may be used if
required by lexical scoping. The following switch label returns to the previous
indentation level, as if a block had been closed.</p>
<p>A blank line is optional between a <code>break</code> and the following case.</p>
<p class="example">Example:</p>
<pre class="language-js prettyprint">switch (animal) {
case Animal.BANDERSNATCH:
handleBandersnatch();
break;
case Animal.JABBERWOCK:
handleJabberwock();
break;
default:
throw new Error('Unknown animal');
}
</pre>
<h3 id="formatting-statements" style="left: -46px; position: relative;">4.3 Statements<a href="#formatting-statements" alt="link to formatting-statements"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<h4 id="formatting-one-statement-perline" style="left: -46px; position: relative;">4.3.1 One statement per line<a href="#formatting-one-statement-perline" alt="link to formatting-one-statement-perline"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Each statement is followed by a line-break.</p>
<h4 id="formatting-semicolons-are-required" style="left: -46px; position: relative;">4.3.2 Semicolons are required<a href="#formatting-semicolons-are-required" alt="link to formatting-semicolons-are-required"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Every statement must be terminated with a semicolon. Relying on automatic
semicolon insertion is forbidden.</p>
<h3 id="formatting-column-limit" style="left: -46px; position: relative;">4.4 Column limit: 80<a href="#formatting-column-limit" alt="link to formatting-column-limit"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<p>JavaScript code has a column limit of 80 characters. Except as noted below, any
line that would exceed this limit must be line-wrapped, as explained in
<a href="#formatting-line-wrapping">4.5 Line-wrapping</a>.</p>
<p class="exceptions"><strong>Exceptions:</strong></p>
<ol>
<li>Lines where obeying the column limit is not possible (for example, a long URL
in JSDoc or a shell command intended to be copied-and-pasted).</li>
<li><code>goog.module</code> and <code>goog.require</code> statements (see <a href="#file-goog-module">3.3 goog.module statement</a> and
<a href="#file-goog-require">3.4 goog.require statements</a>).</li>
</ol>
<h3 id="formatting-line-wrapping" style="left: -46px; position: relative;">4.5 Line-wrapping<a href="#formatting-line-wrapping" alt="link to formatting-line-wrapping"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<p><strong>Terminology Note</strong>: <em>Line-wrapping</em> is defined as breaking a single expression
into multiple lines.</p>
<p>There is no comprehensive, deterministic formula showing <em>exactly</em> how to
line-wrap in every situation. Very often there are several valid ways to
line-wrap the same piece of code.</p>
<p class="note">Note: While the typical reason for line-wrapping is to avoid overflowing the
column limit, even code that would in fact fit within the column limit may be
line-wrapped at the author's discretion.</p>
<p class="tip">Tip: Extracting a method or local variable may solve the problem without the
need to line-wrap.</p>
<h4 id="formatting-where-to-break" style="left: -46px; position: relative;">4.5.1 Where to break<a href="#formatting-where-to-break" alt="link to formatting-where-to-break"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>The prime directive of line-wrapping is: prefer to break at a <strong>higher syntactic
level</strong>. </p>
<p class="preferred">Preferred:</p>
<pre class="language-js prettyprint">currentEstimate =
calc(currentEstimate + x * currentEstimate) /
2.0f;
</pre>
<p class="discouraged">Discouraged:</p>
<pre class="language-js prettyprint badcode">currentEstimate = calc(currentEstimate + x *
currentEstimate) / 2.0f;
</pre>
<p>In the preceding example, the syntactic levels from highest to lowest are as
follows: assignment, division, function call, parameters, number constant.</p>
<p>Operators are wrapped as follows:</p>
<ol>
<li>When a line is broken at an operator the break comes after the symbol. (Note
that this is not the same practice used in Google style for Java.)
<ol>
<li>This does not apply to the <q>dot</q> (<code>.</code>), which is not actually an
operator.</li>
</ol></li>
<li>A method or constructor name stays attached to the open parenthesis (<code>(</code>)
that follows it.</li>
<li>A comma (<code>,</code>) stays attached to the token that precedes it.</li>
</ol>
<blockquote>
<p class="note">Note: The primary goal for line wrapping is to have clear code, not
necessarily code that fits in the smallest number of lines.</p>
</blockquote>
<h4 id="formatting-indent" style="left: -46px; position: relative;">4.5.2 Indent continuation lines at least +4 spaces<a href="#formatting-indent" alt="link to formatting-indent"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>When line-wrapping, each line after the first (each <em>continuation line</em>) is
indented at least +4 from the original line, unless it falls under the rules of
block indentation.</p>
<p>When there are multiple continuation lines, indentation may be varied beyond +4
as appropriate. In general, continuation lines at a deeper syntactic level are
indented by larger multiples of 4, and two lines use the same indentation level
if and only if they begin with syntactically parallel elements.</p>
<p><a href="#formatting-horizontal-alignment">4.6.3 Horizontal alignment: discouraged</a> addresses the discouraged practice of
using a variable number of spaces to align certain tokens with previous lines.</p>
<h3 id="formatting-whitespace" style="left: -46px; position: relative;">4.6 Whitespace<a href="#formatting-whitespace" alt="link to formatting-whitespace"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<h4 id="formatting-vertical-whitespace" style="left: -46px; position: relative;">4.6.1 Vertical whitespace<a href="#formatting-vertical-whitespace" alt="link to formatting-vertical-whitespace"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>A single blank line appears:</p>
<ol>
<li>Between consecutive methods in a class or object literal
<ol>
<li>Exception: A blank line between two consecutive properties definitions in
an object literal (with no other code between them) is optional. Such
blank lines are used as needed to create <em>logical groupings</em> of fields.</li>
</ol></li>
<li>Within method bodies, sparingly to create <em>logical groupings</em> of statements.
Blank lines at the start or end of a function body are not allowed.</li>
<li><em>Optionally</em> before the first or after the last method in a class or object
literal (neither encouraged nor discouraged).</li>
<li>As required by other sections of this document (e.g.
<a href="#file-goog-require">3.4 goog.require statements</a>).</li>
</ol>
<p><em>Multiple</em> consecutive blank lines are permitted, but never required (nor
encouraged).</p>
<h4 id="formatting-horizontal-whitespace" style="left: -46px; position: relative;">4.6.2 Horizontal whitespace<a href="#formatting-horizontal-whitespace" alt="link to formatting-horizontal-whitespace"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Use of horizontal whitespace depends on location, and falls into three broad
categories: <em>leading</em> (at the start of a line), <em>trailing</em> (at the end of a
line), and <em>internal</em>. Leading whitespace (i.e., indentation) is addressed
elsewhere. Trailing whitespace is forbidden.</p>
<p>Beyond where required by the language or other style rules, and apart from
literals, comments, and JSDoc, a single internal ASCII space also appears in the
following places <strong>only</strong>.</p>
<ol>
<li>Separating any reserved word (such as <code>if</code>, <code>for</code>, or <code>catch</code>) from an open
parenthesis (<code>(</code>) that follows it on that line.</li>
<li>Separating any reserved word (such as <code>else</code> or <code>catch</code>) from a closing
curly brace (<code>}</code>) that precedes it on that line.</li>
<li>Before any open curly brace (<code>{</code>), with two exceptions:
<ol>
<li>Before an object literal that is the first argument of a function or the
first element in an array literal (e.g. <code>foo({a: [{c: d}]})</code>).</li>
<li>In a template expansion, as it is forbidden by the language
(e.g. <code>abc${1 + 2}def</code>).</li>
</ol></li>
<li>On both sides of any binary or ternary operator.</li>
<li>After a comma (<code>,</code>) or semicolon (<code>;</code>). Note that spaces are <em>never</em> allowed
before these characters.</li>
<li>After the colon (<code>:</code>) in an object literal.</li>
<li>On both sides of the double slash (<code>//</code>) that begins an end-of-line
comment. Here, multiple spaces are allowed, but not required.</li>
<li>After an open-JSDoc comment character and on both sides of close characters
(e.g. for short-form type declarations or casts: <code>this.foo = /** @type
{number} */ (bar);</code> or <code>function(/** string */ foo) {</code>).</li>
</ol>
<h4 id="formatting-horizontal-alignment" style="left: -46px; position: relative;">4.6.3 Horizontal alignment: discouraged<a href="#formatting-horizontal-alignment" alt="link to formatting-horizontal-alignment"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p><strong>Terminology Note</strong>: <em>Horizontal alignment</em> is the practice of adding a
variable number of additional spaces in your code with the goal of making
certain tokens appear directly below certain other tokens on previous lines.</p>
<p>This practice is permitted, but it is <strong>generally discouraged</strong> by Google
Style. It is not even required to <em>maintain</em> horizontal alignment in places
where it was already used.</p>
<p>Here is an example without alignment, followed by one with alignment. Both are
allowed, but the latter is discouraged:</p>
<pre class="language-js prettyprint">{
tiny: 42, // this is great
longer: 435, // this too
};
{
tiny: 42, // permitted, but future edits
longer: 435, // may leave it unaligned
};
</pre>
<p class="tip">Tip: Alignment can aid readability, but it creates problems for future
maintenance. Consider a future change that needs to touch just one line. This
change may leave the formerly-pleasing formatting mangled, and that is
allowed. More often it prompts the coder (perhaps you) to adjust whitespace on
nearby lines as well, possibly triggering a cascading series of
reformattings. That one-line change now has a <q>blast radius.</q> This can at worst
result in pointless busywork, but at best it still corrupts version history
information, slows down reviewers and exacerbates merge conflicts.</p>
<h4 id="formatting-function-arguments" style="left: -46px; position: relative;">4.6.4 Function arguments<a href="#formatting-function-arguments" alt="link to formatting-function-arguments"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Prefer to put all function arguments on the same line as the function name. 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. Indentation should be four spaces. Aligning to the parenthesis is allowed, but discouraged. Below are the most common patterns for argument wrapping:</p>
<pre class="language-js prettyprint">// Arguments start on a new line, indented four spaces. Preferred when the
// arguments don't fit on the same line with the function name (or the keyword
// "function") but fit entirely on the second line. Works with very long
// function names, survives renaming without reindenting, low on space.
doSomething(
descriptiveArgumentOne, descriptiveArgumentTwo, descriptiveArgumentThree) {
// …
}
// If the argument list is longer, wrap at 80. Uses less vertical space,
// but violates the rectangle rule and is thus not recommended.
doSomething(veryDescriptiveArgumentNumberOne, veryDescriptiveArgumentTwo,
tableModelEventHandlerProxy, artichokeDescriptorAdapterIterator) {
// …
}
// Four-space, one argument per line. Works with long function names,
// survives renaming, and emphasizes each argument.
doSomething(
veryDescriptiveArgumentNumberOne,
veryDescriptiveArgumentTwo,
tableModelEventHandlerProxy,
artichokeDescriptorAdapterIterator) {
// …
}
</pre>
<h3 id="formatting-grouping-parentheses" style="left: -46px; position: relative;">4.7 Grouping parentheses: recommended<a href="#formatting-grouping-parentheses" alt="link to formatting-grouping-parentheses"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<p>Optional grouping parentheses are omitted only when the author and reviewer
agree that there is no reasonable chance that the code will be misinterpreted
without them, nor would they have made the code easier to read. It is <em>not</em>
reasonable to assume that every reader has the entire operator precedence table
memorized.</p>
<p>Do not use unnecessary parentheses around the entire expression following
<code>delete</code>, <code>typeof</code>, <code>void</code>, <code>return</code>, <code>throw</code>, <code>case</code>, <code>in</code>, <code>of</code>, or <code>yield</code>.</p>
<p>Parentheses are required for type casts: <code>/** @type {!Foo} */ (foo)</code>.</p>
<h3 id="formatting-comments" style="left: -46px; position: relative;">4.8 Comments<a href="#formatting-comments" alt="link to formatting-comments"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<p>This section addresses <em>implementation comments</em>. JSDoc is addressed separately
in <a href="#jsdoc">7 JSDoc</a>.</p>
<h4 id="formatting-block-comment-style" style="left: -46px; position: relative;">4.8.1 Block comment style<a href="#formatting-block-comment-style" alt="link to formatting-block-comment-style"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Block comments are indented at the same level as the surrounding code. They may
be in <code>/* … */</code> or <code>//</code>-style. For multi-line <code>/* … */</code> comments, subsequent
lines must start with * aligned with the <code>*</code> on the previous line, to make
comments obvious with no extra context. “Parameter name” comments should appear
after values whenever the value and method name do not sufficiently convey the
meaning.</p>
<pre class="language-js prettyprint">/*
* This is
* okay.
*/
// And so
// is this.
/* This is fine, too. */
someFunction(obviousParam, true /* shouldRender */, 'hello' /* name */);
</pre>
<p>Comments are not enclosed in boxes drawn with asterisks or other characters.</p>
<p>Do not use JSDoc (<code>/** … */</code>) for any of the above implementation comments.</p>
<h2 id="language-features" style="left: -46px; position: relative;">5 Language features<a href="#language-features" alt="link to language-features"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h2>
<p>JavaScript includes many dubious (and even dangerous) features. This section
delineates which features may or may not be used, and any additional constraints
on their use.</p>
<h3 id="features-local-variable-declarations" style="left: -46px; position: relative;">5.1 Local variable declarations<a href="#features-local-variable-declarations" alt="link to features-local-variable-declarations"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<h4 id="features-use-const-and-let" style="left: -46px; position: relative;">5.1.1 Use <code>const</code> and <code>let</code><a href="#features-use-const-and-let" alt="link to features-use-const-and-let"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Declare all local variables with either <code>const</code> or <code>let</code>. Use const by default,
unless a variable needs to be reassigned. The <code class="badcode">var</code>
keyword must not be used.</p>
<h4 id="features-one-variable-per-declaration" style="left: -46px; position: relative;">5.1.2 One variable per declaration<a href="#features-one-variable-per-declaration" alt="link to features-one-variable-per-declaration"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Every local variable declaration declares only one variable: declarations such
as <code class="badcode">let a = 1, b = 2;</code> are not used.</p>
<h4 id="features-declared-when-needed" style="left: -46px; position: relative;">5.1.3 Declared when needed, initialized as soon as possible<a href="#features-declared-when-needed" alt="link to features-declared-when-needed"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Local variables are <strong>not</strong> habitually declared at the start of their containing
block or block-like construct. Instead, local variables are declared close to
the point they are first used (within reason), to minimize their scope.</p>
<h4 id="features-declare-types-as-needed" style="left: -46px; position: relative;">5.1.4 Declare types as needed<a href="#features-declare-types-as-needed" alt="link to features-declare-types-as-needed"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>JSDoc type annotations may be added either on the line above the declaration, or
else inline before the variable name.</p>
<p class="example">Example:</p>
<pre class="language-js prettyprint">const /** !Array<number> */ data = [];
/** @type {!Array<number>} */
const data = [];
</pre>
<p class="tip">Tip: There are many cases where the compiler can infer a templatized type but
not its parameters. This is particularly the case when the initializing literal
or constructor call does not include any values of the template parameter type
(e.g., empty arrays, objects, <code>Map</code>s, or <code>Set</code>s), or if the variable is modified
in a closure. Local variable type annotations are particularly helpful in these
cases since otherwise the compiler will infer the template parameter as unknown.</p>
<h3 id="features-array-literals" style="left: -46px; position: relative;">5.2 Array literals<a href="#features-array-literals" alt="link to features-array-literals"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<h4 id="features-arrays-trailing-comma" style="left: -46px; position: relative;">5.2.1 Use trailing commas<a href="#features-arrays-trailing-comma" alt="link to features-arrays-trailing-comma"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Include a trailing comma whenever there is a line break between the final
element and the closing bracket.</p>
<p class="example">Example:</p>
<pre class="language-js prettyprint">const values = [
'first value',
'second value',
];
</pre>
<h4 id="features-arrays-ctor" style="left: -46px; position: relative;">5.2.2 Do not use the variadic <code>Array</code> constructor<a href="#features-arrays-ctor" alt="link to features-arrays-ctor"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>The constructor is error-prone if arguments are added or removed. Use a literal
instead.</p>
<p class="illegal">Illegal:</p>
<pre class="language-js prettyprint badcode">const a1 = new Array(x1, x2, x3);
const a2 = new Array(x1, x2);
const a3 = new Array(x1);
const a4 = new Array();
</pre>
<p>This works as expected except for the third case: if <code>x1</code> is a whole number then
<code>a3</code> is an array of size <code>x1</code> where all elements are <code>undefined</code>. If <code>x1</code> is any
other number, then an exception will be thrown, and if it is anything else then
it will be a single-element array.</p>
<p>Instead, write</p>
<pre class="language-js prettyprint">const a1 = [x1, x2, x3];
const a2 = [x1, x2];
const a3 = [x1];
const a4 = [];
</pre>
<p>Explicitly allocating an array of a given length using <code>new Array(length)</code> is
allowed when appropriate.</p>
<h4 id="features-arrays-non-numeric-properties" style="left: -46px; position: relative;">5.2.3 Non-numeric properties<a href="#features-arrays-non-numeric-properties" alt="link to features-arrays-non-numeric-properties"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Do not define or use non-numeric properties on an array (other than
<code>length</code>). Use a <code>Map</code> (or <code>Object</code>) instead.</p>
<h4 id="features-arrays-destructuring" style="left: -46px; position: relative;">5.2.4 Destructuring<a href="#features-arrays-destructuring" alt="link to features-arrays-destructuring"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Array literals may be used on the left-hand side of an assignment to perform
destructuring (such as when unpacking multiple values from a single array or
iterable). A final <q>rest</q> element may be included (with no space between the
<code>...</code> and the variable name). Elements should be omitted if they are unused.</p>
<pre class="language-js prettyprint">const [a, b, c, ...rest] = generateResults();
let [, b,, d] = someArray;
</pre>
<p>Destructuring may also be used for function parameters (note that a parameter
name is required but ignored). Always specify <code>[]</code> as the default value if a
destructured array parameter is optional, and provide default values on the left
hand side:</p>
<pre class="language-js prettyprint">/** @param {!Array<number>=} param1 */
function optionalDestructuring([a = 4, b = 2] = []) { … };
</pre>
<p class="illegal">Illegal:</p>
<pre class="language-js prettyprint badcode">function badDestructuring([a, b] = [4, 2]) { … };
</pre>
<p class="tip">Tip: For (un)packing multiple values into a function’s parameter or return,
prefer object destructuring to array destructuring when possible, as it allows
naming the individual elements and specifying a different type for each.*</p>
<h4 id="features-arrays-spread-operator" style="left: -46px; position: relative;">5.2.5 Spread operator<a href="#features-arrays-spread-operator" alt="link to features-arrays-spread-operator"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Array literals may include the spread operator (<code>...</code>) to flatten elements out
of one or more other iterables. The spread operator should be used instead of
more awkward constructs with <code>Array.prototype</code>. There is no space after the
<code>...</code>.</p>
<p class="example">Example:</p>
<pre class="language-js prettyprint">[...foo] // preferred over Array.prototype.slice.call(foo)
[...foo, ...bar] // preferred over foo.concat(bar)
</pre>
<h3 id="features-object-literals" style="left: -46px; position: relative;">5.3 Object literals<a href="#features-object-literals" alt="link to features-object-literals"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h3>
<h4 id="features-objects-use-trailing-comma" style="left: -46px; position: relative;">5.3.1 Use trailing commas<a href="#features-objects-use-trailing-comma" alt="link to features-objects-use-trailing-comma"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Include a trailing comma whenever there is a line break between the final
property and the closing brace.</p>
<h4 id="features-objects-ctor" style="left: -46px; position: relative;">5.3.2 Do not use the <code>Object</code> constructor<a href="#features-objects-ctor" alt="link to features-objects-ctor"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>While <code>Object</code> does not have the same problems as <code>Array</code>, it is still
disallowed for consistency. Use an object literal (<code>{}</code> or <code>{a: 0, b: 1, c: 2}</code>)
instead.</p>
<h4 id="features-objects-mixing-keys" style="left: -46px; position: relative;">5.3.3 Do not mix quoted and unquoted keys<a href="#features-objects-mixing-keys" alt="link to features-objects-mixing-keys"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Object literals may represent either <em>structs</em> (with unquoted keys and/or
symbols) or <em>dicts</em> (with quoted and/or computed keys). Do not mix these key
types in a single object literal.</p>
<p class="illegal">Illegal:</p>
<pre class="language-js prettyprint badcode">{
a: 42, // struct-style unquoted key
'b': 43, // dict-style quoted key
}
</pre>
<h4 id="features-objects-computed-property-names" style="left: -46px; position: relative;">5.3.4 Computed property names<a href="#features-objects-computed-property-names" alt="link to features-objects-computed-property-names"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Computed property names (e.g., <code>{['key' + foo()]: 42}</code>) are allowed, and are
considered dict-style (quoted) keys (i.e., must not be mixed with non-quoted
keys) unless the computed property is a symbol (e.g., <code>[Symbol.iterator]</code>).
Enum values may also be used for computed keys, but should not be mixed with
non-enum keys in the same literal.</p>
<h4 id="features-objects-method-shorthand" style="left: -46px; position: relative;">5.3.5 Method shorthand<a href="#features-objects-method-shorthand" alt="link to features-objects-method-shorthand"><img src="https://google.github.io/styleguide/include/link.png" width="21" height="21" style="float:left;position:relative;bottom:5px;"></a></h4>
<p>Methods can be defined on object literals using the method shorthand (<code>{method()
{… }}</code>) in place of a colon immediately followed by a <code>function</code> or arrow
function literal.</p>
<p class="example">Example:</p>
<pre class="language-js prettyprint">return {
stuff: 'candy',
method() {
return this.stuff; // Returns 'candy'
},
};
</pre>
<p>Note that <code>this</code> in a method shorthand or <code>function</code> refers to the object
literal itself whereas <code>this</code> in an arrow function refers to the scope outside
the object literal.</p>
<p class="example">Example:</p>
<pre class="language-js prettyprint">class {
getObjectLiteral() {
this.stuff = 'fruit';