Google JavaScript Style Guide

1 Introduction

This document serves as the complete definition of Google’s coding standards for source code in the JavaScript programming language. A JavaScript source file is described as being in Google Style if and only if it adheres to the rules herein.

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).

1.1 Terminology notes

In this document, unless otherwise clarified:

1. The term comment always refers to implementation comments. We do not use the phrase documentation comments, instead using the common term “JSDoc” for both human-readable text and machine-readable annotations within /** … */.

2. This Style Guide uses RFC 2119 terminology when using the phrases must, must not, should, should not, and may. The terms prefer and avoid correspond to should and should not, respectively. Imperative and declarative statements are prescriptive and correspond to must.

Other terminology notes will appear occasionally throughout the document.

1.2 Guide notes

Example code in this document is non-normative. That is, while the examples are in Google Style, they may not illustrate the only stylish way to represent the code. Optional formatting choices made in examples must not be enforced as rules.

2 Source file basics

2.1 File name

File names must be all lowercase and may include underscores (_) or dashes (-), but no additional punctuation. Follow the convention that your project uses. Filenames’ extension must be .js.

2.2 File encoding: UTF-8

Source files are encoded in UTF-8.

2.3 Special characters

2.3.1 Whitespace characters

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

1. All other whitespace characters in string literals are escaped, and

2. Tab characters are not used for indentation.

2.3.2 Special escape sequences

For any character that has a special escape sequence (\', \", \\, \b, \f, \n, \r, \t, \v), that sequence is used rather than the corresponding numeric escape (e.g \x0a, \u000a, or \u{a}). Legacy octal escapes are never used.

2.3.3 Non-ASCII characters

For the remaining non-ASCII characters, either the actual Unicode character (e.g. ∞) or the equivalent hex or Unicode escape (e.g. \u221e) is used, depending only on which makes the code easier to read and understand.

Tip: In the Unicode escape case, and occasionally even when actual Unicode characters are used, an explanatory comment can be very helpful.

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 broken and they must be fixed.

3 Source file structure

A source file consists of, in order:

1. License or copyright information, if present
2. @fileoverview JSDoc, if present
3. goog.module statement
4. goog.require statements
5. The file’s implementation

Exactly one blank line separates each section that is present, except the file's implementation, which may be preceded by 1 or 2 blank lines.

3.1 License or copyright information, if present

If license or copyright information belongs in a file, it belongs here.

3.2 @fileoverview JSDoc, if present

See ?? for formatting rules.

3.3 goog.module statement

All files must declare exactly one goog.module name on a single line: lines containing a goog.module declaration must not be wrapped, and are therefore an exception to the 80-column limit.

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.

Example

goog.module('search.urlHistory.UrlHistoryService');

3.3.1 Hierarchy

Module namespaces may never be named as a direct child of another module's namespace.

Illegal:

goog.module('foo.bar');   // 'foo.bar.qux' would be fine, though
goog.module('foo.bar.baz');

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.

3.3.2 goog.setTestOnly

The single goog.module statement may optionally be followed by a call to goog.setTestOnly().

3.3.3 goog.module.declareLegacyNamespace

The single goog.module statement may optionally be followed by a call to goog.module.declareLegacyNamespace();. Avoid goog.module.declareLegacyNamespace() when possible.

Example:

goog.module('my.test.helpers');
goog.module.declareLegacyNamespace();
goog.setTestOnly();

goog.module.declareLegacyNamespace 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 must not be a child or parent of any other goog.module (for example, goog.module('parent'); and goog.module('parent.child'); cannot both exist safely, nor can goog.module('parent'); and goog.module('parent.child.grandchild');).

3.3.4 ES6 Modules

Do not use ES6 modules yet (i.e. the export and import keywords), as their semantics are not yet finalized. Note that this policy will be revisited once the semantics are fully-standard.

3.4 goog.require statements

Imports are done with goog.require statements, grouped together immediately following the module declaration. Each goog.require 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 required dependency, whether in code or in type annotations: the fully qualified name is never used except as the argument to goog.require. 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. goog.require statements may not appear anywhere else in the file.

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.

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 goog.require calls that are standalone (generally these are for modules imported just for their side effects).

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.

If a long alias or module name would cause a line to exceed the 80-column limit, it must not be wrapped: goog.require lines are an exception to the 80-column limit.

Example:

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');

Illegal:

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
  // …
}

3.4.1 goog.forwardDeclare

goog.forwardDeclare 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 goog.require statements. A goog.forwardDeclare statement must follow the same style rules as a goog.require statement.

3.5 The file’s implementation

The actual implementation follows after all dependency information is declared (separated by at least one blank line).

This may consist of any module-local declarations (constants, variables, classes, functions, etc), as well as any exported symbols.

4 Formatting

Terminology Note: block-like construct refers to the body of a class, function, method, or brace-delimited block of code. Note that, by ?? and ??, any array or object literal may optionally be treated as if it were a block-like construct.

Tip: Use clang-format. The JavaScript community has invested effort to make sure clang-format does the right thing on JavaScript files. clang-format has integration with several popular editors.

4.1 Braces

4.1.1 Braces are used for all control structures

Braces are required for all control structures (i.e. if, else, for, do, while, 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.

Illegal:

if (someVeryLongCondition())
  doSomething();

for (let i = 0; i < foo.length; i++) bar(foo[i]);

Exception: 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.

if (shortCondition()) return;

4.1.2 Nonempty blocks: K&R style

Braces follow the Kernighan and Ritchie style (Egyptian brackets) for nonempty blocks and block-like constructs:

• No line break before the opening brace.
• Line break after the opening brace.
• Line break before the closing brace.
• Line break after the closing brace if that brace terminates a statement or the body of a function or class statement, or a class method. Specifically, there is no line break after the brace if it is followed by else, catch, while, or a comma, semicolon, or right-parenthesis.

Example:

class InnerClass {
  constructor() {}

  /** @param {number} foo */
  method(foo) {
    if (condition(foo)) {
      try {
        // Note: this might fail.
        something();
      } catch (err) {
        recover();
      }
    }
  }
}

4.1.3 Empty blocks: may be concise

An empty block or block-like construct may be closed immediately after it is opened, with no characters, space, or line break in between (i.e. {}), unless it is a part of a multi-block statement (one that directly contains multiple blocks: if/else or try/catch/finally).

Example:

function doNothing() {}

Illegal:

if (condition) {
  // …
} else if (otherCondition) {} else {
  // …
}

try {
  // …
} catch (e) {}

4.2 Block indentation: +2 spaces

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 ??).

4.2.1 Array literals: optionally block-like

Any array literal may optionally be formatted as if it were a “block-like construct.” For example, the following are all valid (not an exhaustive list):

const a = [
  0,
  1,
  2,
];

const b =
    [0, 1, 2];

const c = [0, 1, 2];

someMethod(foo, [
  0, 1, 2,
], bar);

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.

4.2.2 Object literals: optionally block-like

Any object literal may optionally be formatted as if it were a “block-like construct.” The same examples apply as ??. For example, the following are all valid (not an exhaustive list):

const a = {
  a: 0,
  b: 1,
};

const b =
    {a: 0, b: 1};

const c = {a: 0, b: 1};

someMethod(foo, {
  a: 0, b: 1,
}, bar);

4.2.3 Class literals

Class literals (whether declarations or expressions) are indented as blocks. Do not add semicolons after methods, or after the closing brace of a class declaration (statements—such as assignments—that contain class expressions are still terminated with a semicolon). Use the extends keyword, but not the @extends JSDoc annotation unless the class extends a templatized type.

Example:

class Foo {
  constructor() {
    /** @type {number} */
    this.x = 42;
  }

  /** @return {number} */
  method() {
    return this.x;
  }
}
Foo.Empty = class {};

/** @extends {Foo<string>} */
foo.Bar = class extends Foo {
  /** @override */
  method() {
    return super.method() / 2;
  }
};

/** @interface */
class Frobnicator {
  /** @param {string} message */
  frobnicate(message) {}
}

4.2.4 Function expressions

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.

Example:

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();
      }
    });

4.2.5 Switch statements

As with any other block, the contents of a switch block are indented +2.

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.

A blank line is optional between a break and the following case.

Example:

switch (animal) {
  case Animal.BANDERSNATCH:
    handleBandersnatch();
    break;

  case Animal.JABBERWOCK:
    handleJabberwock();
    break;

  default:
    throw new Error('Unknown animal');
}

4.3 Statements

4.3.1 One statement per line

Each statement is followed by a line-break.

4.3.2 Semicolons are required

Every statement must be terminated with a semicolon. Relying on automatic semicolon insertion is forbidden.

4.4 Column limit: 80

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 ??.

Exceptions:

1. 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).
2. goog.module and goog.require statements (see ?? and ??).

4.5 Line-wrapping

Terminology Note: Line-wrapping is defined as breaking a single expression into multiple lines.

There is no comprehensive, deterministic formula showing exactly how to line-wrap in every situation. Very often there are several valid ways to line-wrap the same piece of code.

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.

Tip: Extracting a method or local variable may solve the problem without the need to line-wrap.

4.5.1 Where to break

The prime directive of line-wrapping is: prefer to break at a higher syntactic level.

Preferred:

currentEstimate =
    calc(currentEstimate + x * currentEstimate) /
        2.0f;

Discouraged:

currentEstimate = calc(currentEstimate + x *
    currentEstimate) / 2.0f;

In the preceding example, the syntactic levels from highest to lowest are as follows: assignment, division, function call, parameters, number constant.

Operators are wrapped as follows:

1. 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.)
   1. This does not apply to the dot (.), which is not actually an operator.
2. A method or constructor name stays attached to the open parenthesis (() that follows it.
3. A comma (,) stays attached to the token that precedes it.

Note: The primary goal for line wrapping is to have clear code, not necessarily code that fits in the smallest number of lines.

4.5.2 Indent continuation lines at least +4 spaces

When line-wrapping, each line after the first (each continuation line) is indented at least +4 from the original line, unless it falls under the rules of block indentation.

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.

?? addresses the discouraged practice of using a variable number of spaces to align certain tokens with previous lines.

4.6 Whitespace

4.6.1 Vertical whitespace

A single blank line appears:

1. Between consecutive methods in a class or object literal
   1. 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 logical groupings of fields.
2. Within method bodies, sparingly to create logical groupings of statements. Blank lines at the start or end of a function body are not allowed.
3. Optionally before the first or after the last method in a class or object literal (neither encouraged nor discouraged).
4. As required by other sections of this document (e.g. ??).

Multiple consecutive blank lines are permitted, but never required (nor encouraged).

4.6.2 Horizontal whitespace

Use of horizontal whitespace depends on location, and falls into three broad categories: leading (at the start of a line), trailing (at the end of a line), and internal. Leading whitespace (i.e., indentation) is addressed elsewhere. Trailing whitespace is forbidden.

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 only.

1. Separating any reserved word (such as if, for, or catch) from an open parenthesis (() that follows it on that line.
2. Separating any reserved word (such as else or catch) from a closing curly brace (}) that precedes it on that line.
3. Before any open curly brace ({), with two exceptions:
   1. Before an object literal that is the first argument of a function or the first element in an array literal (e.g. foo({a: [{c: d}]})).
   2. In a template expansion, as it is forbidden by the language (e.g. abc${1 + 2}def).
4. On both sides of any binary or ternary operator.
5. After a comma (,) or semicolon (;). Note that spaces are never allowed before these characters.
6. After the colon (:) in an object literal.
7. On both sides of the double slash (//) that begins an end-of-line comment. Here, multiple spaces are allowed, but not required.
8. After an open-JSDoc comment character and on both sides of close characters (e.g. for short-form type declarations or casts: this.foo = /** @type {number} */ (bar); or function(/** string */ foo) {).

4.6.3 Horizontal alignment: discouraged

Terminology Note: Horizontal alignment 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.

This practice is permitted, but it is generally discouraged by Google Style. It is not even required to maintain horizontal alignment in places where it was already used.

Here is an example without alignment, followed by one with alignment. Both are allowed, but the latter is discouraged:

{
  tiny: 42, // this is great
  longer: 435, // this too
};

{
  tiny:   42,  // permitted, but future edits
  longer: 435, // may leave it unaligned
};

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 blast radius. This can at worst result in pointless busywork, but at best it still corrupts version history information, slows down reviewers and exacerbates merge conflicts.

4.6.4 Function arguments

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:

// 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) {
  // …
}

4.7 Grouping parentheses: recommended

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 not reasonable to assume that every reader has the entire operator precedence table memorized.

Do not use unnecessary parentheses around the entire expression following delete, typeof, void, return, throw, case, in, of, or yield.

Parentheses are required for type casts: /** @type {!Foo} */ (foo).

4.8 Comments

This section addresses implementation comments. JSDoc is addressed separately in ??.

4.8.1 Block comment style

Block comments are indented at the same level as the surrounding code. They may be in /* … */ or //-style. For multi-line /* … */ comments, subsequent lines must start with * aligned with the * 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.

/*
 * This is
 * okay.
 */

// And so
// is this.

/* This is fine, too. */

someFunction(obviousParam, true /* shouldRender */, 'hello' /* name */);

Comments are not enclosed in boxes drawn with asterisks or other characters.

Do not use JSDoc (/** … */) for any of the above implementation comments.

5 Language features

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.

5.1 Local variable declarations

5.1.1 Use const and let

Declare all local variables with either const or let. Use const by default, unless a variable needs to be reassigned. The var keyword must not be used.

5.1.2 One variable per declaration

Every local variable declaration declares only one variable: declarations such as let a = 1, b = 2; are not used.

5.1.3 Declared when needed, initialized as soon as possible

Local variables are not 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.

5.1.4 Declare types as needed

JSDoc type annotations may be added either on the line above the declaration, or else inline before the variable name.

Example:

const /** !Array<number> */ data = [];

/** @type {!Array<number>} */
const data = [];

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, Maps, or Sets), 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.

5.2 Array literals

5.2.1 Use trailing commas

Include a trailing comma whenever there is a line break between the final element and the closing bracket.

Example:

const values = [
  'first value',
  'second value',
];

5.2.2 Do not use the variadic Array constructor

The constructor is error-prone if arguments are added or removed. Use a literal instead.

Illegal:

const a1 = new Array(x1, x2, x3);
const a2 = new Array(x1, x2);
const a3 = new Array(x1);
const a4 = new Array();

This works as expected except for the third case: if x1 is a whole number then a3 is an array of size x1 where all elements are undefined. If x1 is any other number, then an exception will be thrown, and if it is anything else then it will be a single-element array.

Instead, write

const a1 = [x1, x2, x3];
const a2 = [x1, x2];
const a3 = [x1];
const a4 = [];

Explicitly allocating an array of a given length using new Array(length) is allowed when appropriate.

5.2.3 Non-numeric properties

Do not define or use non-numeric properties on an array (other than length). Use a Map (or Object) instead.

5.2.4 Destructuring

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 rest element may be included (with no space between the ... and the variable name). Elements should be omitted if they are unused.

const [a, b, c, ...rest] = generateResults();
let [, b,, d] = someArray;

Destructuring may also be used for function parameters (note that a parameter name is required but ignored). Always specify [] as the default value if a destructured array parameter is optional, and provide default values on the left hand side:

/** @param {!Array<number>=} param1 */
function optionalDestructuring([a = 4, b = 2] = []) { … };

Illegal:

function badDestructuring([a, b] = [4, 2]) { … };

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.*

5.2.5 Spread operator

Array literals may include the spread operator (...) to flatten elements out of one or more other iterables. The spread operator should be used instead of more awkward constructs with Array.prototype. There is no space after the ....

Example:

[...foo]   // preferred over Array.prototype.slice.call(foo)
[...foo, ...bar]   // preferred over foo.concat(bar)

5.3 Object literals

5.3.1 Use trailing commas

Include a trailing comma whenever there is a line break between the final property and the closing brace.

5.3.2 Do not use the Object constructor

While Object does not have the same problems as Array, it is still disallowed for consistency. Use an object literal ({} or {a: 0, b: 1, c: 2}) instead.

5.3.3 Do not mix quoted and unquoted keys

Object literals may represent either structs (with unquoted keys and/or symbols) or dicts (with quoted and/or computed keys). Do not mix these key types in a single object literal.

Illegal:

{
  a: 42, // struct-style unquoted key
  'b': 43, // dict-style quoted key
}

5.3.4 Computed property names

Computed property names (e.g., {['key' + foo()]: 42}) 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., [Symbol.iterator]). Enum values may also be used for computed keys, but should not be mixed with non-enum keys in the same literal.

5.3.5 Method shorthand

Methods can be defined on object literals using the method shorthand ({method() {… }}) in place of a colon immediately followed by a function or arrow function literal.

Example:

return {
  stuff: 'candy',
  method() {
    return this.stuff;  // Returns 'candy'
  },
};

Note that this in a method shorthand or function refers to the object literal itself whereas this in an arrow function refers to the scope outside the object literal.

Example:

class {
  getObjectLiteral() {
    this.stuff = 'fruit';
    return {
      stuff: 'candy',
      method: () => this.stuff,  // Returns 'fruit'
    };
  }
}

5.3.6 Shorthand properties

Shorthand properties are allowed on object literals.

Example:

const foo = 1;
const bar = 2;
const obj = {
  foo,
  bar,
  method() { return this.foo + this.bar; },
};
assertEquals(3, obj.method());

5.3.7 Destructuring

Object destructuring patterns may be used on the left-hand side of an assignment to perform destructuring and unpack multiple values from a single object.

Destructured objects may also be used as function parameters, but should be kept as simple as possible: a single level of unquoted shorthand properties. Deeper levels of nesting and computed properties may not be used in parameter destructuring. Specify any default values in the left-hand-side of the destructured parameter ({str = 'some default'} = {}, rather than {str} = {str: 'some default'}), and if a destructured object is itself optional, it must default to {}. The JSDoc for the destructured parameter may be given any name (the name is unused but is required by the compiler).

Example:

/**
 * @param {string} ordinary
 * @param {{num: (number|undefined), str: (string|undefined)}=} param1
 *     num: The number of times to do something.
 *     str: A string to do stuff to.
 */
function destructured(ordinary, {num, str = 'some default'} = {})

Illegal:

/** @param {{x: {num: (number|undefined), str: (string|undefined)}}} param1 */
function nestedTooDeeply({x: {num, str}}) {};
/** @param {{num: (number|undefined), str: (string|undefined)}=} param1 */
function nonShorthandProperty({num: a, str: b} = {}) {};
/** @param {{a: number, b: number}} param1 */
function computedKey({a, b, [a + b]: c}) {};
/** @param {{a: number, b: string}=} param1 */
function nontrivialDefault({a, b} = {a: 2, b: 4}) {};

Destructuring may also be used for goog.require statements, and in this case must not be wrapped: the entire statement occupies one line, regardless of how long it is (see ??).

5.3.8 Enums

Enumerations are defined by adding the @enum annotation to an object literal. Additional properties may not be added to an enum after it is defined. Enums must be constant, and all enum values must be deeply immutable.

/**
 * Supported temperature scales.
 * @enum {string}
 */
const TemperatureScale = {
  CELSIUS: 'celsius',
  FAHRENHEIT: 'fahrenheit',
};

/**
 * An enum with two options.
 * @enum {number}
 */
const Option = {
  /** The option used shall have been the first. */
  FIRST_OPTION: 1,
  /** The second among two options. */
  SECOND_OPTION: 2,
};

5.4 Classes

5.4.1 Constructors

Constructors are optional for concrete classes. Subclass constructors must call super() before setting any fields or otherwise accessing this. Interfaces must not define a constructor.

5.4.2 Fields

Set all of a concrete object’s fields (i.e. all properties other than methods) in the constructor. Annotate fields that are never reassigned with @const (these need not be deeply immutable). Private fields must be annotated with @private and their names must end with a trailing underscore. Fields are never set on a concrete class' prototype.

Example:

class Foo {
  constructor() {
    /** @private @const {!Bar} */
    this.bar_ = computeBar();
  }
}

Tip: Properties should never be added to or removed from an instance after the constructor is finished, since it significantly hinders VMs’ ability to optimize. If necessary, fields that are initialized later should be explicitly set to undefined in the constructor to prevent later shape changes. Adding @struct to an object will check that undeclared properties are not added/accessed. Classes have this added by default.

5.4.3 Computed properties

Computed properties may only be used in classes when the property is a symbol. Dict-style properties (that is, quoted or computed non-symbol keys, as defined in ??) are not allowed. A [Symbol.iterator] method should be defined for any classes that are logically iterable. Beyond this, Symbol should be used sparingly.

Tip: be careful of using any other built-in symbols (e.g., Symbol.isConcatSpreadable) as they are not polyfilled by the compiler and will therefore not work in older browsers.

5.4.4 Static methods

Where it does not interfere with readability, prefer module-local functions over private static methods.

Static methods should only be called on the base class itself. Static methods should not be called on variables containing a dynamic instance that may be either the constructor or a subclass constructor (and must be defined with @nocollapse if this is done), and must not be called directly on a subclass that doesn’t define the method itself.

Illegal:

class Base { /** @nocollapse */ static foo() {} }
class Sub extends Base {}
function callFoo(cls) { cls.foo(); }  // discouraged: don't call static methods dynamically
Sub.foo();  // illegal: don't call static methods on subclasses that don't define it themselves

5.4.5 Old-style class declarations

While ES6 classes are preferred, there are cases where ES6 classes may not be feasible. For example:

1. If there exist or will exist subclasses, including frameworks that create subclasses, that cannot be immediately changed to use ES6 class syntax. If such a class were to use ES6 syntax, all downstream subclasses not using ES6 class syntax would need to be modified.

2. Frameworks that require a known this value before calling the superclass constructor, since constructors with ES6 super classes do not have access to the instance this value until the call to super returns.

In all other ways the style guide still applies to this code: let, const, default parameters, rest, and arrow functions should all be used when appropriate.

goog.defineClass allows for a class-like definition similar to ES6 class syntax:

let C = goog.defineClass(S, {
  /**
   * @param {string} value
   */
  constructor(value) {
    S.call(this, 2);
    /** @const */
    this.prop = value;
  },

  /**
   * @param {string} param
   * @return {number}
   */
  method(param) {
    return 0;
  },
});

Alternatively, while goog.defineClass should be preferred for all new code, more traditional syntax is also allowed.

/**
  * @constructor @extends {S}
  * @param {string} value
  */
function C(value) {
  S.call(this, 2);
  /** @const */
  this.prop = value;
}
goog.inherits(C, S);

/**
 * @param {string} param
 * @return {number}
 */
C.prototype.method = function(param) {
  return 0;
};

Per-instance properties should be defined in the constructor after the call to the super class constructor, if there is a super class. Methods should be defined on the prototype of the constructor.

Defining constructor prototype hierarchies correctly is harder than it first appears! For that reason, it is best to use goog.inherits from the Closure Library .

5.4.6 Do not manipulate prototypes directly

The class keyword allows clearer and more readable class definitions than defining prototype properties. Ordinary implementation code has no business manipulating these objects, though they are still useful for defining @record interfaces and classes as defined in ??. Mixins and modifying the prototypes of builtin objects are explicitly forbidden.

Exception: Framework code (such as Polymer, or Angular) may need to use prototypes, and should not resort to even-worse workarounds to avoid doing so.

Exception: Defining fields in interfaces (see ??).

5.4.7 Getters and Setters

Do not use JavaScript getter and setter properties. They are potentially surprising and difficult to reason about, and have limited support in the compiler. Provide ordinary methods instead.

Exception: when working with data binding frameworks (such as Angular and Polymer), getters and setters may be used sparingly. Note, however, that compiler support is limited. When they are used, they must be defined either with get foo() and set foo(value) in the class or object literal, or if that is not possible, with Object.defineProperties. Do not use Object.defineProperty, which interferes with property renaming. Getters must not change observable state.

Illegal:

class Foo {
  get next() { return this.nextId++; }
}

5.4.8 Overriding toString

The toString method may be overridden, but must always succeed and never have visible side effects.

Tip: Beware, in particular, of calling other methods from toString, since exceptional conditions could lead to infinite loops.

5.4.9 Interfaces

Interfaces may be declared with @interface or @record. Interfaces declared with @record can be explicitly (i.e. via @implements) or implicitly implemented by a class or object literal.

All non-static method bodies on an interface must be empty blocks. Fields must be defined after the interface body as stubs on the prototype.

Example:

/**
 * Something that can frobnicate.
 * @record
 */
class Frobnicator {
  /**
   * Performs the frobnication according to the given strategy.
   * @param {!FrobnicationStrategy} strategy
   */
  frobnicate(strategy) {}
}

/** @type {number} The number of attempts before giving up. */
Frobnicator.prototype.attempts;

5.5 Functions

5.5.1 Top-level functions

Exported functions may be defined directly on the exports object, or else declared locally and exported separately. Non-exported functions are encouraged and should not be declared @private.

Examples:

/** @return {number} */
function helperFunction() {
  return 42;
}
/** @return {number} */
function exportedFunction() {
  return helperFunction() * 2;
}
/**
 * @param {string} arg
 * @return {number}
 */
function anotherExportedFunction(arg) {
  return helperFunction() / arg.length;
}
/** @const */
exports = {exportedFunction, anotherExportedFunction};

/** @param {string} arg */
exports.foo = (arg) => {
  // do some stuff ...
};

5.5.2 Nested functions and closures

Functions may contain nested function definitions. If it is useful to give the function a name, it should be assigned to a local const.

5.5.3 Arrow functions

Arrow functions provide a concise syntax and fix a number of difficulties with this. Prefer arrow functions over the function keyword, particularly for nested functions (but see ??).

Prefer using arrow functions over f.bind(this), and especially over goog.bind(f, this). Avoid writing const self = this. Arrow functions are particularly useful for callbacks, which sometimes pass unexpected additional arguments.

The right-hand side of the arrow may be a single expression or a block. Parentheses around the arguments are optional if there is only a single non-destructured argument.

Tip: It is a good practice to use parentheses even for single-argument arrows, since the code may still parse reasonably (but incorrectly) if the parentheses are forgotten when an additional argument is added.

5.5.4 Generators

Generators enable a number of useful abstractions and may be used as needed.

When defining generator functions, attach the * to the function keyword when present, and separate it with a space from the name of the function. When using delegating yields, attach the * to the yield keyword.

Example:

/** @return {!Iterator<number>} */
function* gen1() {
  yield 42;
}

/** @return {!Iterator<number>} */
const gen2 = function*() {
  yield* gen1();
}

class SomeClass {
  /** @return {!Iterator<number>} */
  * gen() {
    yield 42;
  }
}

5.5.5 Parameters

Function parameters must be typed with JSDoc annotations in the JSDoc preceding the function’s definition, except in the case of same-signature @overrides, where all types are omitted.

Parameter types may be specified inline, immediately before the parameter name (as in (/** number */ foo, /** string */ bar) => foo + bar). Inline and @param type annotations must not be mixed in the same function definition.

5.5.5.1 Default parameters

Optional parameters are permitted using the equals operator in the parameter list. Optional parameters must include spaces on both sides of the equals operator, be named exactly like required parameters (i.e., not prefixed with opt_), use the = suffix in their JSDoc type, come after required parameters, and not use initializers that produce observable side effects. All optional parameters must have a default value in the function declaration, even if that value is undefined.

Example:

/**
 * @param {string} required This parameter is always needed.
 * @param {string=} optional This parameter can be omitted.
 * @param {!Node=} node Another optional parameter.
 */
function maybeDoSomething(required, optional = '', node = undefined) {}

Use default parameters sparingly. Prefer destructuring (as in ??) to create readable APIs when there are more than a small handful of optional parameters that do not have a natural order.

Note: Unlike Python's default parameters, it is okay to use initializers that return new mutable objects (such as {} or []) because the initializer is evaluated each time the default value is used, so a single object won't be shared across invocations.

Tip: While arbitrary expressions including function calls may be used as initializers, these should be kept as simple as possible. Avoid initializers that expose shared mutable state, as that can easily introduce unintended coupling between function calls.

5.5.5.2 Rest parameters

Use a rest parameter instead of accessing arguments. Rest parameters are typed with a ... prefix in their JSDoc. The rest parameter must be the last parameter in the list. There is no space between the ... and the parameter name. Do not name the rest parameter var_args. Never name a local variable or parameter arguments, which confusingly shadows the built-in name.

Example:

/**
 * @param {!Array<string>} array This is an ordinary parameter.
 * @param {...number} numbers The remainder of arguments are all numbers.
 */
function variadic(array, ...numbers) {}

5.5.6 Returns

Function return types must be specified in the JSDoc directly above the function definition, except in the case of same-signature @overrides where all types are omitted.

5.5.7 Generics

Declare generic functions and methods when necessary with @template TYPE in the JSDoc above the class definition.

5.5.8 Spread operator

Function calls may use the spread operator (...). Prefer the spread operator to Function.prototype.apply when an array or iterable is unpacked into multiple parameters of a variadic function. There is no space after the ....

Example:

function myFunction(...elements) {}
myFunction(...array, ...iterable, ...generator());

5.6 String literals

5.6.1 Use single quotes

Ordinary string literals are delimited with single quotes ('), rather than double quotes (").

Tip: if a string contains a single quote character, consider using a template string to avoid having to escape the quote.

Ordinary string literals may not span multiple lines.

5.6.2 Template strings

Use template strings (delimited with `) over complex string concatenation, particularly if multiple string literals are involved. Template strings may span multiple lines.

If a template string spans multiple lines, it does not need to follow the indentation of the enclosing block, though it may if the added whitespace does not matter.

Example:

function arithmetic(a, b) {
  return `Here is a table of arithmetic operations:
${a} + ${b} = ${a + b}
${a} - ${b} = ${a - b}
${a} * ${b} = ${a * b}
${a} / ${b} = ${a / b}`;
}

5.6.3 No line continuations

Do not use line continuations (that is, ending a line inside a string literal with a backslash) in either ordinary or template string literals. Even though ES5 allows this, it can lead to tricky errors if any trailing whitespace comes after the slash, and is less obvious to readers.

Illegal:

const longString = 'This is a very long string that far exceeds the 80 \
    column limit. It unfortunately contains long stretches of spaces due \
    to how the continued lines are indented.';

Instead, write

const longString = 'This is a very long string that far exceeds the 80 ' +
    'column limit. It does not contain long stretches of spaces since ' +
    'the concatenated strings are cleaner.';

5.7 Number literals

Numbers may be specified in decimal, hex, octal, or binary. Use exactly 0x, 0o, and 0b prefixes, with lowercase letters, for hex, octal, and binary, respectively. Never include a leading zero unless it is immediately followed by x, o, or b.

5.8 Control structures

5.8.1 For loops

With ES6, the language now has three different kinds of for loops. All may be used, though for-of loops should be preferred when possible.

for-in loops may only be used on dict-style objects (see ??), and should not be used to iterate over an array. Object.prototype.hasOwnProperty should be used in for-in loops to exclude unwanted prototype properties. Prefer for-of and Object.keys over for-in when possible.

5.8.2 Exceptions

Exceptions are an important part of the language and should be used whenever exceptional cases occur. Always throw Errors or subclasses of Error: never throw string literals or other objects. Always use new when constructing an Error.

Custom exceptions provide a great way to convey additional error information from functions. They should be defined and used wherever the native Error type is insufficient.

Prefer throwing exceptions over ad-hoc error-handling approaches (such as passing an error container reference type, or returning an object with an error property).

5.8.2.1 Empty catch blocks

It is very rarely correct to do nothing in response to a caught exception. When it truly is appropriate to take no action whatsoever in a catch block, the reason this is justified is explained in a comment.

try {
  return handleNumericResponse(response);
} catch (ok) {
  // it's not numeric; that's fine, just continue
}
return handleTextResponse(response);

Illegal:

   try {
    shouldFail();
    fail('expected an error');
  }
  catch (expected) {}

Tip: Unlike in some other languages, patterns like the above simply don’t work since this will catch the error thrown by fail. Use assertThrows() instead.

5.8.3 Switch statements

Terminology Note: Inside the braces of a switch block are one or more statement groups. Each statement group consists of one or more switch labels (either case FOO: or default:), followed by one or more statements.

5.8.3.1 Fall-through: commented

Within a switch block, each statement group either terminates abruptly (with a break, return or thrown exception), or is marked with a comment to indicate that execution will or might continue into the next statement group. Any comment that communicates the idea of fall-through is sufficient (typically // fall through). This special comment is not required in the last statement group of the switch block.

Example:

switch (input) {
  case 1:
  case 2:
    prepareOneOrTwo();
  // fall through
  case 3:
    handleOneTwoOrThree();
    break;
  default:
    handleLargeNumber(input);
}

5.8.3.2 The default case is present

Each switch statement includes a default statement group, even if it contains no code.

5.9 this

Only use this in class constructors and methods, or in arrow functions defined within class constructors and methods. Any other uses of this must have an explicit @this declared in the immediately-enclosing function’s JSDoc.

Never use this to refer to the global object, the context of an eval, the target of an event, or unnecessarily call()ed or apply()ed functions.

5.10 Disallowed features

5.10.1 with

Do not use the with keyword. It makes your code harder to understand and has been banned in strict mode since ES5.

5.10.2 Dynamic code evaluation

Do not use eval or the Function(...string) constructor (except for code loaders). These features are potentially dangerous and simply do not work in CSP environments.

5.10.3 Automatic semicolon insertion

Always terminate statements with semicolons (except function and class declarations, as noted above).

5.10.4 Non-standard features

Do not use non-standard features. This includes old features that have been removed (e.g., WeakMap.clear), new features that are not yet standardized (e.g., the current TC39 working draft, proposals at any stage, or proposed but not-yet-complete web standards), or proprietary features that are only implemented in some browsers. Use only features defined in the current ECMA-262 or WHATWG standards. (Note that projects writing against specific APIs, such as Chrome extensions or Node.js, can obviously use those APIs). Non-standard language “extensions” (such as those provided by some external transpilers) are forbidden.

5.10.5 Wrapper objects for primitive types

Never use new on the primitive object wrappers (Boolean, Number, String, Symbol), nor include them in type annotations.

Illegal:

const /** Boolean */ x = new Boolean(false);
if (x) alert(typeof x);  // alerts 'object' - WAT?

The wrappers may be called as functions for coercing (which is preferred over using + or concatenating the empty string) or creating symbols.

Example:

const /** boolean */ x = Boolean(0);
if (!x) alert(typeof x);  // alerts 'boolean', as expected

5.10.6 Modifying builtin objects

Never modify builtin types, either by adding methods to their constructors or to their prototypes. Avoid depending on libraries that do this. Note that the JSCompiler’s runtime library will provide standards-compliant polyfills where possible; nothing else may modify builtin objects.

Do not add symbols to the global object unless absolutely necessary (e.g. required by a third-party API).

6 Naming

6.1 Rules common to all identifiers

Identifiers use only ASCII letters and digits, and, in a small number of cases noted below, underscores and very rarely (when required by frameworks like Angular) dollar signs.

Give as descriptive a name as possible, within reason. Do not worry about saving horizontal space as it is far more important to make your code immediately understandable by a new reader. Do not use abbreviations that are ambiguous or unfamiliar to readers outside your project, and do not abbreviate by deleting letters within a word.

priceCountReader      // No abbreviation.
numErrors             // "num" is a widespread convention.
numDnsConnections     // Most people know what "DNS" stands for.

Illegal:

n                     // Meaningless.
nErr                  // Ambiguous abbreviation.
nCompConns            // Ambiguous abbreviation.
wgcConnections        // Only your group knows what this stands for.
pcReader              // Lots of things can be abbreviated "pc".
cstmrId               // Deletes internal letters.
kSecondsPerDay        // Do not use Hungarian notation.

6.2 Rules by identifier type

6.2.1 Package names

Package names are all lowerCamelCase. For example, my.exampleCode.deepSpace, but not my.examplecode.deepspace or my.example_code.deep_space.

6.2.2 Class names

Class, interface, record, and typedef names are written in UpperCamelCase. Unexported classes are simply locals: they are not marked @private and therefore are not named with a trailing underscore.

Type names are typically nouns or noun phrases. For example, Request, ImmutableList, or VisibilityMode. Additionally, interface names may sometimes be adjectives or adjective phrases instead (for example, Readable).

6.2.3 Method names

Method names are written in lowerCamelCase. Private methods’ names must end with a trailing underscore.

Method names are typically verbs or verb phrases. For example, sendMessage or stop_. Getter and setter methods for properties are never required, but if they are used they should be named getFoo (or optionally isFoo or hasFoo for booleans), or setFoo(value) for setters.

Underscores may also appear in JsUnit test method names to separate logical components of the name. One typical pattern is test<MethodUnderTest>_<state>, for example testPop_emptyStack. There is no One Correct Way to name test methods.

6.2.4 Enum names

Enum names are written in UpperCamelCase, similar to classes, and should generally be singular nouns. Individual items within the enum are named in CONSTANT_CASE.

6.2.5 Constant names

Constant names use CONSTANT_CASE: all uppercase letters, with words separated by underscores. There is no reason for a constant to be named with a trailing underscore, since private static properties can be replaced by (implicitly private) module locals.

6.2.5.1 Definition of “constant”

Every constant is a @const static property or a module-local const declaration, but not all @const static properties and module-local consts are constants. Before choosing constant case, consider whether the field really feels like a deeply immutable constant. For example, if any of that instance's observable state can change, it is almost certainly not a constant. Merely intending to never mutate the object is generally not enough.

Examples:

// Constants
const NUMBER = 5;
/** @const */ exports.NAMES = ImmutableList.of('Ed', 'Ann');
/** @enum */ exports.SomeEnum = { ENUM_CONSTANT: 'value' };

// Not constants
let letVariable = 'non-const';
class MyClass { constructor() { /** @const */ this.nonStatic = 'non-static'; } };
/** @type {string} */ MyClass.staticButMutable = 'not @const, can be reassigned';
const /** Set<String> */ mutableCollection = new Set();
const /** ImmutableSet<SomeMutableType> */ mutableElements = ImmutableSet.of(mutable);
const Foo = goog.require('my.Foo');  // mirrors imported name
const logger = log.getLogger('loggers.are.not.immutable');

Constants’ names are typically nouns or noun phrases.

6.2.5.1 Local aliases

Local aliases should be used whenever they improve readability over fully-qualified names. Follow the same rules as goog.requires (??), maintaining the last part of the aliased name. Aliases may also be used within functions. Aliases must be const.

Examples:

const staticHelper = importedNamespace.staticHelper;
const CONSTANT_NAME = ImportedClass.CONSTANT_NAME;
const {assert, assertInstanceof} = asserts;

6.2.6 Non-constant field names

Non-constant field names (static or otherwise) are written in lowerCamelCase, with a trailing underscore for private fields.

These names are typically nouns or noun phrases. For example, computedValues or index_.

6.2.7 Parameter names

Parameter names are written in lowerCamelCase. Note that this applies even if the parameter expects a constructor.

One-character parameter names should not be used in public methods.

Exception: When required by a third-party framework, parameter names may begin with a $. This exception does not apply to any other identifiers (e.g. local variables or properties).

6.2.8 Local variable names

Local variable names are written in lowerCamelCase, except for module-local (top-level) constants, as described above. Constants in function scopes are still named in lowerCamelCase. Note that lowerCamelCase applies even if the variable holds a constructor.

6.2.9 Template parameter names

Template parameter names should be concise, single-word or single-letter identifiers, and must be all-caps, such as TYPE or THIS.

6.3 Camel case: defined

Sometimes there is more than one reasonable way to convert an English phrase into camel case, such as when acronyms or unusual constructs like IPv6 or iOS are present. To improve predictability, Google Style specifies the following (nearly) deterministic scheme.

Beginning with the prose form of the name:

1. Convert the phrase to plain ASCII and remove any apostrophes. For example, Müller's algorithm might become Muellers algorithm.
2. Divide this result into words, splitting on spaces and any remaining punctuation (typically hyphens).
   1. Recommended: if any word already has a conventional camel case appearance in common usage, split this into its constituent parts (e.g., AdWords becomes ad words). Note that a word such as iOS is not really in camel case per se; it defies any convention, so this recommendation does not apply.
3. Now lowercase everything (including acronyms), then uppercase only the first character of:
   1. … each word, to yield upper camel case, or
   2. … each word except the first, to yield lower camel case
4. Finally, join all the words into a single identifier.

Note that the casing of the original words is almost entirely disregarded.

Examples:

*Acceptable, but not recommended.

Note: Some words are ambiguously hyphenated in the English language: for example nonempty and non-empty are both correct, so the method names checkNonempty and checkNonEmpty are likewise both correct.

7 JSDoc

JSDoc is used on all classes, fields, and methods.

7.1 General form

The basic formatting of JSDoc blocks is as seen in this example:

/**
 * Multiple lines of JSDoc text are written here,
 * wrapped normally.
 * @param {number} arg A number to do something to.
 */
function doSomething(arg) { … }

or in this single-line example:

/** @const @private {!Foo} A short bit of JSDoc. */
this.foo_ = foo;

If a single-line comment overflows into multiple lines, it must use the multi-line style with /** and */ on their own lines.

Many tools extract metadata from JSDoc comments to perform code validation and optimization. As such, these comments must be well-formed.

7.2 Markdown

JSDoc is written in Markdown, though it may include HTML when necessary.

Note that tools that automatically extract JSDoc (e.g. JsDossier) will often ignore plain text formatting, so if you did this:

/**
 * Computes weight based on three factors:
 *   items sent
 *   items received
 *   last timestamp
 */

it would come out like this:

Computes weight based on three factors: items sent items received last timestamp

Instead, write a Markdown list:

/**
 * Computes weight based on three factors:
 *  - items sent
 *  - items received
 *  - last timestamp
 */

7.3 JSDoc tags

Google style allows a subset of JSDoc tags. See ?? for the complete list. Most tags must occupy their own line, with the tag at the beginning of the line.

Illegal:

/**
 * The "param" tag must occupy its own line and may not be combined.
 * @param {number} left @param {number} right
 */
function add(left, right) { ... }

Simple tags that do not require any additional data (such as @private, @const, @final, @export) may be combined onto the same line, along with an optional type when appropriate.

/**
 * Place more complex annotations (like "implements" and "template")
 * on their own lines.  Multiple simple tags (like "export" and "final")
 * may be combined in one line.
 * @export @final
 * @implements {Iterable<TYPE>}
 * @template TYPE
 */
class MyClass {
  /**
   * @param {!ObjType} obj Some object.
   * @param {number=} num An optional number.
   */
  constructor(obj, num = 42) {
    /** @private @const {!Array<!ObjType|number>} */
    this.data_ = [obj, num];
  }
}

There is no hard rule for when to combine tags, or in which order, but be consistent.

For general information about annotating types in JavaScript see Annotating JavaScript for the Closure Compiler and Types in the Closure Type System.

7.4 Line wrapping

Line-wrapped block tags are indented four spaces. Wrapped description text may be lined up with the description on previous lines, but this horizontal alignment is discouraged.

/**
 * 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.
 */
exports.method = function(foo) {
  return 5;
};

Do not indent when wrapping a @fileoverview description.

7.5 Top/file-level comments

A file may have a top-level file overview. A copyright notice , author information, and default visibility level 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 may provide a description of the file's contents and any dependencies or compatibility information. Wrapped lines are not indented.

Example:

/**
 * @fileoverview Description of file, its uses and information
 * about its dependencies.
 * @package
 */

7.6 Class comments

Classes, interfaces and records must be documented with a description and any template parameters, implemented interfaces, visibility, or other appropriate tags. The class description should provide the reader with enough information to know how and when to use the class, as well as any additional considerations necessary to correctly use the class. Textual descriptions may be omitted on the constructor. @constructor and @extends annotations are not used with the class keyword unless the class is being used to declare an @interface or it extends a generic class.

/**
 * A fancier event target that does cool things.
 * @implements {Iterable<string>}
 */
class MyFancyTarget extends EventTarget {
  /**
   * @param {string} arg1 An argument that makes this more interesting.
   * @param {!Array<number>} arg2 List of numbers to be processed.
   */
  constructor(arg1, arg2) {
    // ...
  }
};

/**
 * Records are also helpful.
 * @extends {Iterator<TYPE>}
 * @record
 * @template TYPE
 */
class Listable {
  /** @return {TYPE} The next item in line to be returned. */
  next() {}
}

7.7 Enum and typedef comments

Enums and typedefs must be documented. Public enums and typedefs must have a non-empty description. Individual enum items may be documented with a JSDoc comment on the preceding line.

/**
 * A useful type union, which is reused often.
 * @typedef {!Bandersnatch|!BandersnatchType}
 */
let CoolUnionType;


/**
 * Types of bandersnatches.
 * @enum {string}
 */
const BandersnatchType = {
  /** This kind is really frumious. */
  FRUMIOUS: 'frumious',
  /** The less-frumious kind. */
  MANXOME: 'manxome',
};

Typedefs are useful for defining short record types, or aliases for unions, complex functions, or generic types. Typedefs should be avoided for record types with many fields, since they do not allow documenting individual fields, nor using templates or recursive references. For large record types, prefer @record.

7.8 Method and function comments

Parameter and return types must be documented. The this type should be documented when necessary. Method, parameter, and return descriptions (but not types) may be omitted if they are obvious from the rest of the method’s JSDoc or from its signature. Method descriptions should start with a sentence written in the third person declarative voice. If a method overrides a superclass method, it must include an @override annotation. Overridden methods must include all @param and @return annotations if any types are refined, but should omit them if the types are all the same.

/** This is a class. */
class SomeClass extends SomeBaseClass {
  /**
   * Operates on an instance of MyClass and returns something.
   * @param {!MyClass} obj An object that for some reason needs detailed
   *     explanation that spans multiple lines.
   * @param {!OtherClass} obviousOtherClass
   * @return {boolean} Whether something occurred.
   */
  someMethod(obj, obviousOtherClass) { ... }

  /** @override */
  overriddenMethod(param) { ... }
}

/**
 * Demonstrates how top-level functions follow the same rules.  This one
 * makes an array.
 * @param {TYPE} arg
 * @return {!Array<TYPE>}
 * @template TYPE
 */
function makeArray(arg) { ... }

Anonymous functions do not require JSDoc, though parameter types may be specified inline if the automatic type inference is insufficient.

promise.then(
    (/** !Array<number|string> */ items) => {
      doSomethingWith(items);
      return /** @type {string} */ (items[0]);
    });

7.9 Property comments

Property types must be documented. The description may be omitted for private properties, if name and type provide enough documentation for understanding the code.

Publicly exported constants are commented the same way as properties. Explicit types may be omitted for @const properties initialized from an expression with an obviously known type.

Tip: A @const property’s type can be considered “obviously known” if it is assigned directly from a constructor parameter with a declared type, or directly from a function call with a declared return type. Non-const properties and properties assigned from more complex expressions should have their types declared explicitly.

/** My class. */
class MyClass {
  /** @param {string=} someString */
  constructor(someString = 'default string') {
    /** @private @const */
    this.someString_ = someString;

    /** @private @const {!OtherType} */
    this.someOtherThing_ = functionThatReturnsAThing();

    /**
     * Maximum number of things per pane.
     * @type {number}
     */
    this.someProperty = 4;
  }
}

/**
 * The number of times we'll try before giving up.
 * @const
 */
MyClass.RETRY_COUNT = 33;

7.10 Type annotations

Type annotations are found on @param, @return, @this, and @type tags, and optionally on @const, @export, and any visibility tags. Type annotations attached to JSDoc tags must always be enclosed in braces.

7.10.1 Nullability

The type system defines modifiers ! and ? for non-null and nullable, respectively. Primitive types (undefined, string, number, boolean, symbol, and function(...): ...) and record literals ({foo: string, bar: number}) are non-null by default. Do not add an explicit ! to these types. Object types (Array, Element, MyClass, etc) are nullable by default, but cannot be immediately distinguished from a name that is @typedef’d to a non-null-by-default type. As such, all types except primitives and record literals must be annotated explicitly with either ? or ! to indicate whether they are nullable or not.

7.10.2 Type Casts

In cases where type checking doesn't accurately infer the type of an expression, it is possible to tighten the type by adding a type annotation comment and enclosing the expression in parentheses. Note that the parentheses are required.

/** @type {number} */ (x)

7.10.3 Template Parameter Types

Always specify template parameters. This way compiler can do a better job and it makes it easier for readers to understand what code does.

Bad:

const /** !Object */ users = {};
const /** !Array */ books = [];
const /** !Promise */ response = ...;

Good:

const /** !Object<string, !User> */ users = {};
const /** !Array<string> */ books = [];
const /** !Promise<!Response> */ response = ...;

const /** !Promise<undefined> */ thisPromiseReturnsNothingButParameterIsStillUseful = ...;
const /** !Object<string, *> */ mapOfEverything = {};

Cases when template parameters should not be used:

• Object is used for type hierarchy and not as map-like structure.

7.11 Visibility annotations

Visibility annotations (@private, @package, @protected) may be specified in a @fileoverview block, or on any exported symbol or property. Do not specify visibility for local variables, whether within a function or at the top level of a module. All @private names must end with an underscore.

8 Policies

8.1 Issues unspecified by Google Style: Be Consistent!

For any style question that isn't settled definitively by this specification, prefer to do what the other code in the same file is already doing. If that doesn't resolve the question, consider emulating the other files in the same package.

8.2 Compiler warnings

8.2.1 Use a standard warning set

As far as possible projects should use --warning_level=VERBOSE.

8.2.2 How to handle a warning

Before doing anything, make sure you understand exactly what the warning is telling you. If you're not positive why a warning is appearing, ask for help .

Once you understand the warning, attempt the following solutions in order:

1. First, fix it or work around it. Make a strong attempt to actually address the warning, or find another way to accomplish the task that avoids the situation entirely.
2. Otherwise, determine if it's a false alarm. If you are convinced that the warning is invalid and that the code is actually safe and correct, add a comment to convince the reader of this fact and apply the @suppress annotation.
3. Otherwise, leave a TODO comment. This is a last resort. If you do this, do not suppress the warning. The warning should be visible until it can be taken care of properly.

8.2.3 Suppress a warning at the narrowest reasonable scope

Warnings are suppressed at the narrowest reasonable scope, usually that of a single local variable or very small method. Often a variable or method is extracted for that reason alone.

Example

/** @suppress {uselessCode} Unrecognized 'use asm' declaration */
function fn() {
  'use asm';
  return 0;
}

Even a large number of suppressions in a class is still better than blinding the entire class to this type of warning.

8.3 Deprecation

Mark deprecated methods, classes or interfaces with @deprecated annotations. A deprecation comment must include simple, clear directions for people to fix their call sites.

8.4 Code not in Google Style

You will occasionally encounter files in your codebase that are not in proper Google Style. These may have come from an acquisition, or may have been written before Google Style took a position on some issue, or may be in non-Google Style for any other reason.

8.4.1 Reformatting existing code

When updating the style of existing code, follow these guidelines.

1. It is not required to change all existing code to meet current style guidelines. Reformatting existing code is a trade-off between code churn and consistency. Style rules evolve over time and these kinds of tweaks to maintain compliance would create unnecessary churn. However, if significant changes are being made to a file it is expected that the file will be in Google Style.
2. Be careful not to allow opportunistic style fixes to muddle the focus of a CL. If you find yourself making a lot of style changes that aren’t critical to the central focus of a CL, promote those changes to a separate CL.

8.4.2 Newly added code: use Google Style

Brand new files use Google Style, regardless of the style choices of other files in the same package.

When adding new code to a file that is not in Google Style, reformatting the existing code first is recommended, subject to the advice in ??.

If this reformatting is not done, then new code should be as consistent as possible with existing code in the same file, but must not violate the style guide.

8.5 Local style rules

Teams and projects may adopt additional style rules beyond those in this document, but must accept that cleanup changes may not abide by these additional rules, and must not block such cleanup changes due to violating any additional rules. Beware of excessive rules which serve no purpose. The style guide does not seek to define style in every possible scenario and neither should you.

8.6 Generated code: mostly exempt

Source code generated by the build process is not required to be in Google Style. However, any generated identifiers that will be referenced from hand-written source code must follow the naming requirements. As a special exception, such identifiers are allowed to contain underscores, which may help to avoid conflicts with hand-written identifiers.

9 Appendices

9.1 JSDoc tag reference

JSDoc serves multiple purposes in JavaScript. In addition to being used to generate documentation it is also used to control tooling. The best known are the Closure Compiler type annotations.

9.1.1 Type annotations and other Closure Compiler annotations

Documentation for JSDoc used by the Closure Compiler is described in Annotating JavaScript for the Closure Compiler and Types in the Closure Type System.

9.1.2 Documentation annotations

In addition to the JSDoc described in Annotating JavaScript for the Closure Compiler the following tags are common and well supported by various documentation generations tools (such as JsDossier) for purely documentation purposes.

/**
 * @fileoverview Utilities for handling textareas.
 * @author kuth@google.com (Uthur Pendragon)
 */
 

/** @bug 1234567 */
function testSomething() {
  // …
}

/**
 * @bug 1234568
 * @bug 1234569
 */
function testTwoBugs() {
  // …
}

/**
 * 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() {
  // …
};

/**
 * Adds a single item, recklessly.
 * @see #addSafely
 * @see goog.Collect
 * @see goog.RecklessAdder#add
 */
 

/**
 * @fileoverview Event Manager
 * Provides an abstracted interface to the
 * browsers' event systems.
 * @supported IE10+, Chrome, Safari
 */

/** @desc Notifying a user that their account has been created. */
exports.MSG_ACCOUNT_CREATED = goog.getMsg(
    'Your account has been successfully created.');
 

You may also see other types of JSDoc annotations in third-party code. These annotations appear in the JSDoc Toolkit Tag Reference but are not considered part of valid Google style.

9.1.3 Framework specific annotations

The following annotations are specific to a particular framework.

9.1.4 Notes about standard Closure Compiler annotations

The following tags used to be standard but are now deprecated.

9.2 Commonly misunderstood style rules

Here is a collection of lesser-known or commonly misunderstood facts about Google Style for JavaScript. (The following are true statements; this is not a list of myths.)

• Neither a copyright statement nor @author credit is required in a source file. (Neither is explicitly recommended, either.)
• Aside from the constructor coming first (??), there is no hard and fast rule governing how to order the members of a class (??).
• Empty blocks can usually be represented concisely as {}, as detailed in (??).
• The prime directive of line-wrapping is: prefer to break at a higher syntactic level (??).
• Non-ASCII characters are allowed in string literals, comments and Javadoc, and in fact are recommended when they make the code easier to read than the equivalent Unicode escape would (??).

9.3 Style-related tools

The following tools exist to support various aspects of Google Style.

9.3.1 Closure Compiler

This program performs type checking and other checks, optimizations and other transformations (such as ECMAScript 6 to ECMAScript 5 code lowering).

9.3.2 clang-format

This program reformats JavaScript source code into Google Style, and also follows a number of non-required but frequently readability-enhancing formatting practices.

clang-format is not required. Authors are allowed to change its output, and reviewers are allowed to ask for such changes; disputes are worked out in the usual way. However, subtrees may choose to opt in to such enforcement locally.

9.3.3 Closure compiler linter

This program checks for a variety of missteps and anti-patterns.

9.3.4 Conformance framework

The JS Conformance Framework is a tool that is part of the Closure Compiler that provides developers a simple means to specify a set of additional checks to be run against their code base above the standard checks. Conformance checks can, for example, forbid access to a certain property, or calls to a certain function, or missing type information (unknowns).

These rules are commonly used to enforce critical restrictions (such as defining globals, which could break the codebase) and security patterns (such as using eval or assigning to innerHTML), or more loosely to improve code quality.

For additional information see the official documentation for the JS Conformance Framework.

9.4 Exceptions for legacy platforms

9.4.1 Overview

This section describes exceptions and additional rules to be followed when modern ECMAScript 6 syntax is not available to the code authors. Exceptions to the recommended style are required when ECMAScript 6 syntax is not possible and are outlined here:

• Use of var declarations is allowed
• Use of arguments is allowed
• Optional parameters without default values are allowed

9.4.2 Use var

9.4.2.1 var declarations are NOT block-scoped

var declarations are scoped to the beginning of the nearest enclosing function, script or module, which can cause unexpected behavior, especially with function closures that reference var declarations inside of loops. The following code gives an example:

for (var i = 0; i < 3; ++i) {
  var iteration = i;
  setTimeout(function() { console.log(iteration); }, i*1000);
}

// logs 2, 2, 2 -- NOT 0, 1, 2
// because `iteration` is function-scoped, not local to the loop.

9.4.2.2 Declare variables as close as possible to first use

Even though var declarations are scoped to the beginning of the enclosing function, var declarations should be as close as possible to their first use, for readability purposes. However, do not put a var declaration inside a block if that variable is referenced outside the block. For example:

function sillyFunction() {
  var count = 0;
  for (var x in y) {
    // "count" could be declared here, but don't do that.
    count++;
  }
  console.log(count + ' items in y');
}

9.4.2.3 Use @const for constants variables

For global declarations where the const keyword would be used, if it were available, annotate the var declaration with @const instead (this is optional for local variables).

9.4.3 Do not use block scoped functions declarations

Do not do this:

if (x) {
  function foo() {}
}

While most JavaScript VMs implemented before ECMAScript 6 support function declarations within blocks it was not standardized. Implementations were inconsistent with each other and with the now-standard ECMAScript 6 behavior for block scoped function declaration. ECMAScript 5 and prior only allow for function declarations in the root statement list of a script or function and explicitly ban them in block scopes in strict mode.

To get consistent behavior, instead use a var initialized with a function expression to define a function within a block:

if (x) {
  var foo = function() {};
}

9.4.4 Dependency management with goog.provide/goog.require

goog.provide is deprecated. All new files should use goog.module, even in projects with existing goog.provide usage. The following rules are for pre-existing goog.provide files, only.

9.4.4.1 Summary

• Place all goog.provides first, goog.requires second. Separate provides from requires with an empty line.
• Sort the entries alphabetically (uppercase first).
• Don't wrap goog.provide and goog.require statements. Exceed 80 columns if necessary.
• Only provide top-level symbols.

As of Oct 2016, goog.provide/goog.require dependency management is deprecated. All new files, even in projects using goog.provide for older files, should use goog.module.

goog.provide statements should be grouped together and placed first. All goog.require statements should follow. The two lists should be separated with an empty line.

Similar to import statements in other languages, goog.provide and goog.require statements should be written in a single line, even if they exceed the 80 column line length limit.

The lines should be sorted alphabetically, with uppercase letters coming first:

goog.provide('namespace.MyClass');
goog.provide('namespace.helperFoo');

goog.require('an.extremelyLongNamespace.thatSomeoneThought.wouldBeNice.andNowItIsLonger.Than80Columns');
goog.require('goog.dom');
goog.require('goog.dom.TagName');
goog.require('goog.dom.classes');
goog.require('goog.dominoes');

All members defined on a class should be in the same file. 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).

Do this:

goog.provide('namespace.MyClass');

Not this:

goog.provide('namespace.MyClass');
goog.provide('namespace.MyClass.CONSTANT');
goog.provide('namespace.MyClass.Enum');
goog.provide('namespace.MyClass.InnerClass');
goog.provide('namespace.MyClass.TypeDef');
goog.provide('namespace.MyClass.staticMethod');

Members on namespaces may also be provided:

goog.provide('foo.bar');
goog.provide('foo.bar.CONSTANT');
goog.provide('foo.bar.method');

9.4.4.2 Aliasing with goog.scope

goog.scope is deprecated. New files should not use goog.scope even in projects with existing goog.scope usage.

goog.scope may be used to shorten references to namespaced symbols in code using goog.provide/goog.require dependency management.

Only one goog.scope invocation may be added per file. Always place it in the global scope.

The opening goog.scope(function() { invocation must be preceded by exactly one blank line and follow any goog.provide statements, goog.require statements, or top-level comments. The invocation must be closed on the last line in the file. Append // goog.scope to the closing statement of the scope. Separate the comment from the semicolon by two spaces.

Similar to C++ namespaces, do not indent under goog.scope declarations. Instead, continue from the 0 column.

Only make aliases for 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):

goog.scope(function() {
var Button = goog.ui.Button;

Button = function() { ... };
...

Names must be the same as the last property of the global that they are aliasing.

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 after 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