Annotating JavaScript for the Closure Compiler

Note: This page is out of date. The complete list is maintained at https://github.com/google/closure-compiler/wiki/Annotating-JavaScript-for-the-Closure-Compiler

Overview

The Closure Compiler can use data type information about JavaScript variables to provide enhanced optimization and warnings. JavaScript, however, has no way to declare types.

Because JavaScript has no syntax for declaring the type of a variable, you must use comments in the code to specify the data type.

The Closure Compiler's type language derives from the annotations used by the JSDoc document-generation tool, though it has since diverged. It now includes several annotations that JSDoc does not support, and vice versa. This document describes the set of annotations and type expressions that the Closure Compiler understands.

  1. JSDoc Tags
  2. Type Expressions
  3. Generic Types

JSDoc Tags

The Closure Compiler looks for type information in JSDoc tags. Use the JSDoc tags described in the reference table below to help the compiler optimize your code and check it for possible type errors and other mistakes.

This table includes only tags that affect on the behavior of the Closure Compiler. For information about other JSDoc tags see the JSDoc Toolkit documentation.

Tag Description
@abstract

Marks a method as abstract. Similar to setting a method to goog.abstractMethod, the compiler can prune methods annotated with @abstract to reduce code size.

The compiler produces a warning if a method marked with @abstract has a non-empty implementation.

For example: 
/** @abstract */
foo.MyClass.prototype.abstractMethod = function() {};
@const

Marks a variable as read-only. The compiler can inline @const variables, which optimizes the JavaScript code.

The type declaration is optional.

The compiler produces a warning if a variable marked with @const is assigned a value more than once. If the variable is an object, note that the compiler does not prohibit changes to the properties of the object.

For example: 
/** @const */ var MY_BEER = 'stout';

/**
 * My namespace's favorite kind of beer.
 * @const {string}
 */
mynamespace.MY_BEER = 'stout';

/** @const */ MyClass.MY_BEER = 'stout';
@constructor

Marks a function as a constructor. The compiler requires a @constructor annotation for any function that is used with the new keyword

For example:

/**
 * A rectangle.
 * @constructor
 */
function GM_Rect() {
  ...
}
@define Indicates a constant that can be overridden by the compiler at compile-time. With the example on the left, you can pass the flag --define='ENABLE_DEBUG=false' to the compiler to change the value of ENABLE_DEBUG to false. The type of a defined constant can be number, string or boolean. Defines are only allowed in the global scope.

For example:

/** @define {boolean} */
var ENABLE_DEBUG = true;

/** @define {boolean} */
goog.userAgent.ASSUME_IE = false;
@deprecated

Marks a function, method, or property so that using it will produce a compiler warning indicating that it should no longer be used.

For example:

/**
 * Determines whether a node is a field.
 * @return {boolean} True if the contents of
 *     the element are editable, but the element
 *     itself is not.
 * @deprecated Use isField().
 */
BN_EditUtil.isTopEditableField = function(node) {
  ...
};
@dict

@dict is used to create objects with a variable number of properties. When a constructor (Foo in the example) is annotated with @dict, you can only use the bracket notation to access the properties of Foo objects. The annotation can also be used directly on object literals.

For example:

/**
 * @constructor
 * @dict
 */
function Foo() {}
var obj1 = new Foo();
obj1['x'] = 123;
obj1.x = 234;  // warning

var obj2 = /** @dict */ { 'x': 321 };
obj2.x = 123;  // warning
@enum

Specifies the type of an enum. An enum is an object whose properties constitute a set of related constants. The @enum tag must be followed by a type expression.

The type label of an enum applies to each property of the enum. For example if an enum has type number, each of its enumerated properties must be a number. If the type of an enum is omitted, number is assumed.

For example:

/**
 * Enum for tri-state values.
 * @enum {number}
 */
project.TriState = {
  TRUE: 1,
  FALSE: -1,
  MAYBE: 0
};
@export

Given this code

/** @export */
foo.MyPublicClass.prototype.myPublicMethod = function() {
  // ...
};

when the compiler is run with the --generate_exports flag, it will generate the code:

goog.exportProperty(foo.MyPublicClass.prototype, 'myPublicMethod',
  foo.MyPublicClass.prototype.myPublicMethod);

which will export the symbols to uncompiled code. You can write /** @export {SomeType} */ as a shorthand for 

/**
 * @export
 * @type {SomeType}
 */

Code that uses the @export annotation must either

  1. include closure/base.js, or
  2. define both goog.exportSymbol and goog.exportProperty with the same method signature in their own codebase.
@extends

Marks a class or interface as inheriting from another class. A class marked with @extends must also be marked with either @constructor or @interface.

Note: @extends does not cause a class to inherit from another class. The annotation simply tells the compiler that it can treat one class as a subclass of another during type-checking.

For an example implementation of inheritance, see the Closure Library function goog.inherits().

For example:

/**
 * Immutable empty node list.
 * @constructor
 * @extends {goog.ds.BasicNodeList}
 */
goog.ds.EmptyNodeList = function() {
  ...
};
@final

Indicates that this class is not allowed to be extended. For methods, indicates that no subclass is allowed to override that method.

For example:

/**
 * A class that cannot be extended.
 * @final
 * @constructor
 */
sloth.MyFinalClass = function() { ... }

/**
 * A method that cannot be overridden.
 * @final
 */
sloth.MyFinalClass.prototype.method = function() { ... };
@implements

Used with @constructor to indicate that a class implements an interface.

The compiler produces a warning if you tag a constructor with @implements and then fail to implement all of the methods and properties defined by the interface.

For example:

/**
 * A shape.
 * @interface
 */
function Shape() {};
Shape.prototype.draw = function() {};

/**
 * @constructor
 * @implements {Shape}
 */
function Square() {};
Square.prototype.draw = function() {
  ...
};
@implicitCast

This annotation can only appear in externs property declarations. The property has a declared type, but you can assign any type to it without a warning. When accessing the property, you get back a value of the declared type. For example, element.innerHTML can be assigned any type, but will always return a string. 

/**
 * @type {string}
 * @implicitCast
 */
Element.prototype.innerHTML;
@inheritDoc

Indicates that a method or property of a subclass intentionally hides a method or property of the superclass, and has exactly the same documentation. Note that the @inheritDoc tag implies the @override tag.

For example:

/** @inheritDoc */
project.SubClass.prototype.toString = function() {
  ...
};
@interface

Marks a function as an interface. An interface specifies the required members of a type. Any class that implements an interface must implement all of the methods and properties defined on the interface's prototype. See @implements.

The compiler verifies that interfaces are not instantiated. If the new keyword is used with an interface function, the compiler produces a warning.

For example:

/**
 * A shape.
 * @interface
 */
function Shape() {};
Shape.prototype.draw = function() {};

/**
 * A polygon.
 * @interface
 * @extends {Shape}
 */
function Polygon() {};
Polygon.prototype.getSides = function() {};
@lends

Indicates that the keys of an object literal should be treated as properties of some other object. This annotation should only appear on object literals.

Notice that the name in braces is not a type name like in other annotations. It's an object name. It names the object to which the properties are lent. For example, @type {Foo} means "an instance of Foo", but @lends {Foo} means "the constructor Foo".

The JSDoc Toolkit docs have more information on this annotation.

For example:

goog.object.extend(
    Button.prototype,
    /** @lends {Button.prototype} */ ({
      isButton: function() { return true; }
    }));
@license or @preserve

Tells the compiler to insert the associated comment before the compiled code for the marked file. This annotation allows important notices (such as legal licenses or copyright text) to survive compilation unchanged. Line breaks are preserved.

For example:

/**
 * @preserve Copyright 2009 SomeThirdParty.
 * Here is the full license text and copyright
 * notice for this file. Note that the notice can span several
 * lines and is only terminated by the closing star and slash:
 */
@nocollapse

Denotes a property that should not be collapsed by the compiler into a variable. The primary use for @nocollapse is to allow exporting of mutable properties. Note that non-collapsed properties can still be renamed by the compiler. If you annotate a property that is an object with @nocollapse, all its properties will also remain uncollapsed.

For example:

/**
 * A namespace.
 * @const
 */
var foo = {};

/**
 * @nocollapse
 */
foo.bar = 42;

window['foobar'] = foo.bar;
@nosideeffects

Indicates that a call to the declared external function has no side effects. This annotation allows the compiler to remove calls to the function if the return value is not used. The annotation is only allowed in extern files.

For example:

/** @nosideeffects */
function noSideEffectsFn1() {}

/** @nosideeffects */
var noSideEffectsFn2 = function() {};

/** @nosideeffects */
a.prototype.noSideEffectsFn3 = function() {};
@override

Indicates that a method or property of a subclass intentionally hides a method or property of the superclass. If no other annotations are included, the method or property automatically inherits annotations from its superclass.

For example:

/**
 * @return {string} Human-readable representation of
 *     project.SubClass.
 * @override
 */
project.SubClass.prototype.toString = function() {
  ...
};
@package

Marks a member or property as package private. Only code in the same directory can access names marked @package. In particular, code in parent and child directories cannot access names marked @package.

Public constructors can have @package properties to restrict the methods that callers outside the directory can use. On the other hand, @package constructors can have public properties to prevent callers outside the directory from directly instantiating a type.

For example:

/**
 * Returns the window object the foreign document resides in.
 *
 * @return {Object} The window object of the peer.
 * @package
 */
goog.net.xpc.CrossPageChannel.prototype.getPeerWindowObject = function() {
  // ...
};
@param

Used with method, function and constructor definitions to specify the types of function arguments. @param tags must be in same order as the parameters in the function definition.

The @param tag must be followed by a type expression.

Alternatively, you can annotate the types of the parameters inline (see function foo in the example).

For example:

/**
 * Queries a Baz for items.
 * @param {number} groupNum Subgroup id to query.
 * @param {string|number|null} term An itemName,
 *     or itemId, or null to search everything.
 */
goog.Baz.prototype.query = function(groupNum, term) {
  ...
};

function foo(/** number */ a, /** number */ b) {
  return a - b + 1;
}
 For parameters that are a destructuring pattern, you can use any name that is a valid JS identifier, after the type annotation. 
/**
 * @param {{name: string, age: number}} person
 */
function logPerson({name, age}) {
  console.log(`${name} is ${age} years old`);
}
@private

Marks a member as private. Only code in the same file can access global variables and functions marked @private. Constructors marked @private can only be instantiated by code in the same file and by their static and instance members.

The public static properties of constructors marked @private may also be accessed anywhere, and the instanceof operator can always access @private members.

For example:

/**
 * Handlers that are listening to this logger.
 * @private {Array<Function>}
 */
this.handlers_ = [];
@protected

Indicates that a member or property is protected.

A property marked @protected is accessible to:

  • all code in the same file
  • static methods and instance methods of any subclass of the class on which the property is defined.

For example:

/**
 * Sets the component's root element to the given element.
 * Considered protected and final.
 * @param {Element} element Root element for the component.
 * @protected
 */
goog.ui.Component.prototype.setElementInternal = function(element) {
  // ...
};
@record

Marks a function as a structural interface. A structural interface is similar to a nominal @interface, but allows implicit implementations. This means that any class that includes the methods and properties defined on the structural interface's prototoype implements the structural interface, whether or not it uses the @implements tag. Record types and object literals also implicitly implement a structural interface if they contain the required properties.

For example:

/**
 * Anything with a draw() method.
 * @record
 */
function Drawable() {};
Drawable.prototype.draw = function() {};

/**
 * A polygon.
 * @param {!Drawable} x
 */
function render(x) { x.draw(); };

var o = { draw() { /* ... */ } };
render(o);
@return

Specifies the return types of method and function definitions. The @return tag must be followed by a type expression.

Alternatively, you can annotate the return type inline (see function foo in the example).

If a function that is not in externs has no return value, you can omit the @return tag, and the compiler will assume that the function returns undefined.

For example:

/**
 * Returns the ID of the last item.
 * @return {string} The hex ID.
 */
goog.Baz.prototype.getLastId = function() {
  ...
  return id;
};

function /** number */ foo(x) { return x - 1; }
@struct

@struct is used to create objects with a fixed number of properties. When a constructor (Foo in the example) is annotated with @struct, you can only use the dot notation to access the properties of Foo objects, not the bracket notation. Also, you cannot add a property to a Foo instance after it's constructed. The annotation can also be used directly on object literals.

For example:

/**
 * @constructor
 * @struct
 */
function Foo(x) {
  this.x = x;
}
var obj1 = new Foo(123);
var someVar = obj1.x;  // OK
obj1.x = "qwerty";  // OK
obj1['x'] = "asdf";  // warning
obj1.y = 5;  // warning

var obj2 = /** @struct */ { x: 321 };
obj2['x'] = 123;  // warning
@template

See Generic Types.

For example:

/**
 * @param {T} t
 * @constructor
 * @template T
 */
Container = function(t) { ... };
@this

Specifies the type of the object to which the keyword this refers within a function. The @this tag must be followed by a type expression.

To prevent compiler warnings, you must use a @this annotation whenever this appears in a function that is neither a prototype method nor a function marked as a @constructor.

For example:

chat.RosterWidget.extern('getRosterElement',
    /**
     * Returns the roster widget element.
     * @this {Widget}
     * @return {Element}
     */
    function() {
      return this.getComponent().getElement();
    });
@throws

Used to document the exceptions thrown by a function. The type checker does not currently use this information. It is only used to figure out if a function declared in an externs file has side effects.

For example:

/**
 * @throws {DOMException}
 */
DOMApplicationCache.prototype.swapCache = function() { ... };
@type

Identifies the type of a variable, property, or expression. The @type tag must be followed by a type expression.

When declaring a variable or function parameter, you can write the type annotation inline omitting the {} and @type, as in the second example. This shortcut can be done only where a variable or function parameter is declared. If you want to adjust the type later, you'll need a type cast.

For example:

/**
 * The message hex ID.
 * @type {string}
 */
var hexId = hexId;
 
var /** string */ name = 'Jamie';
function useSomething(/** (string|number|!Object) */ something) {
...
}
@typedef

Declares an alias for a more complex type. Currently, typedefs can only be defined at the top level, not inside functions. We have fixed this limitation in the new type inference.

For example:

/** @typedef {(string|number)} */
goog.NumberLike;

/** @param {goog.NumberLike} x A number or a string. */
goog.readNumber = function(x) {
  ...
}
@unrestricted

Indicates that a class is neither a @struct type, nor a @dict type. This is the default so it is generally not necessary to write it explicitly, unless you are using goog.defineClass, or the class keyword, which both produce classes which are @structs by default.

For example:

/**
 * @constructor
 * @unrestricted
 */
function Foo(x) {
  this.x = x;
}
var obj1 = new Foo(123);
var someVar = obj1.x;  // OK
obj1.x = "qwerty";  // OK
obj1['x'] = "asdf";  // OK
obj1.y = 5;  // OK

Type Expressions

You can specify the data type of any variable, property, expression or function parameter with a type expression. A type expression consists of curly braces ("{ }") containing some combination of the type operators described below.

Use a type expression with the @param tag to declare the type of a function parameter. Use a type expression with the @type tag to declare the type of a variable, property, or expression.

The more types you specify in your code, the more optimizations the compiler can make and the more mistakes it can catch.

The compiler uses these annotations to type-check your program. Note that the Closure Compiler does not make any promises that it will be able to figure out the type of every expression in your program. It makes a best effort by looking at how variables are used, and at the type annotations attached to their declarations. Then, it uses a number of type inference algorithms to figure out the type of as many expressions as possible. Some of these algorithms are straightforward ("if x is a number, and we see y = x;, then y is a number"). Some are more indirect ("if f's first parameter is documented as a callback that must take a number, and we see f(function(x) { /** ... */ });, then x must be a number").

Operator Name Syntax Examples Description
Type Name {boolean}
{Window}
{goog.ui.Menu} 		Specifies the name of a type.
Type Application {Array<string>}
An array of strings.

{Object<string, number>}
An object in which the keys are strings and the values are numbers.

Parameterizes a type with a set of type arguments. Similar to Java generics.
Type Union {(number|boolean)}
A number or a boolean.

Note the parentheses, which are required. 		Indicates that a value might have type A OR type B.
Record Type {{myNum: number, myObject}}
An anonymous type with both a property named myNum that has a value of type number and a property named myObject that has a value of any type.

Indicates that the value has the specified members with values of the specified types.

Braces are part of the type syntax. For example, to denote an Array of objects that have a length property, you might write:
Array<{length}>. In the example on the left, the outer braces indicate that this is a type expression and the inner braces indicate that this is a record type.
Nullable type {?number}
A number or null.

Indicates that a value is type A or null.

All object types are nullable by default whether or not they are declared with the Nullable operator. An object type is defined as anything except a function, string, number, or boolean. To make an object type non-nullable, use the Non-nullable operator.
Non-nullable type {!Object}
An Object, but never the null value.

Indicates that a value is type A and not null.

Functions and all value types (boolean, number, and string) are non-nullable by default whether or not they are declared with the Non-nullable operator. To make a value or function type nullable, use the Nullable operator.
Function Type {function(string, boolean)}
A function that takes two parameters (a string and a boolean), and has an unknown return value.
 Specifies a function and the types of the function's parameters.
Function Return Type {function(): number}
A function that takes no parameters and returns a number.
 Specifies the type of a function's return value.
Function this Type {function(this:goog.ui.Menu, string)}
A function that takes one parameter (a string), and executes in the context of a goog.ui.Menu. 		Specifies the type of the value of this within the function.
Function new Type {function(new:goog.ui.Menu, string)}
A function that takes one parameter (a string), and creates a new instance of goog.ui.Menu when called with the 'new' keyword. 		Specifies the constructed type of a constructor.
Variable parameters {function(string, ...number): number}
A function that takes one parameter (a string), and then a variable number of parameters that must be numbers. 		Indicates that a function type takes a variable number of parameters, and specifies a type for the variable parameters.
Variable parameters (in @param annotations) @param {...number} var_args
A variable number of parameters to an annotated function. 		Indicates that the annotated function accepts a variable number of parameters, and specifies a type for the variable parameters.
Optional parameter in a @param annotation @param {number=} opt_argument
An optional parameter of type number.

Indicates that the argument described by a @param annotation is optional. A function call can omit an optional argument. An optional parameter cannot precede a non-optional parameter in the parameter list.

If a method call omits an optional parameter, that argument will have a value of undefined. Therefore if the method stores the parameter's value in a class property, the type declaration of that property must include a possible value of undefined, as in the following example: 

/**
 * Some class, initialized with an optional value.
 * @param {Object=} opt_value Some value (optional).
 * @constructor
 */
function MyClass(opt_value) {
  /**
   * Some value.
   * @type {Object|undefined}
   */
  this.myValue = opt_value;
}
Optional argument in a function type {function(?string=, number=)}
A function that takes one optional, nullable string and one optional number as arguments. 		Indicates that an argument in a function type is optional. An optional argument can be omitted from the function call. An optional argument cannot precede a non-optional argument in the argument list.
The ALL type {*} Indicates that the variable can take on any type.
The UNKNOWN type {?} Indicates that the variable can take on any type, and the compiler should not type-check any uses of it.

Type Casting

To cast a value to a specific type use this syntax 

/** @type {!MyType} */ (valueExpression)
 The parentheses around the expression are always required.

Generic Types

Much like Java, the Closure Compiler supports generic types, functions, and methods. Generics operate on objects of various types while preserving compile-time type safety.

You can use generics to implement generalized collections that hold references to objects of a particular type, and generalized algorithms that operate over objects of a particular type.

Declaring a Generic Type

A type can be made generic by adding a @template annotation to the type's constructor (for classes) or interface declaration (for interfaces). For example: 

/**
 * @constructor
 * @template T
 */
Foo = function() { ... };

The annotation @template T indicates that Foo is a generic type with one template type, T. The template type T can be used as a type within the scope of the definition of Foo. For example: 

/** @return {T} */
Foo.prototype.get = function() { ... };

/** @param {T} t */
Foo.prototype.set = function(t) { ... };

The method get will return an object of type T, and the method set will only accept objects of type T.

Instantiating a Generic Type

Reusing the example above, a templated instance of Foo can be created in several ways: 

/** @type {!Foo<string>} */ var foo = new Foo();
var foo = /** @type {!Foo<string>} */ (new Foo());

Both of the above constructor statements create a Foo instance whose template type T is string. The compiler will enforce that calls to foo's methods, and accesses to foo's properties, respect the templated type. For example: 

foo.set("hello");  // OK.
foo.set(3);        // Error - expected a string, found a number.
var x = foo.get(); // x is a string.

Instances can also be implicitly typed by their constructor arguments. Consider a different generic type, Bar: 

/**
 * @param {T} t
 * @constructor
 * @template T
 */
Bar = function(t) { ... };
var bar = new Bar("hello"); // bar is a Bar<string>

The type of the argument to the Bar constructor is inferred as string, and as a result, the created instance bar is inferred as Bar<string>.

Multiple Template Types

A generic can have any number of template types. The following map class has two template types: 

/**
 * @constructor
 * @template Key, Val
 */
MyMap = function() { ... };

All template types for a generic type must be specified in the same @template annotation, as a comma-separated list. The order of the template type names is important, since templated type annotations will use the ordering to pair template types with the values. For example: 

/** @type {MyMap<string, number>} */ var map; // Key = string, Val = number.

Invariance of Generic Types

The Closure Compiler enforces invariant generic typing. This means that if a context expects a type Foo<X>, you cannot pass a type Foo<Y> when X and Y are different types, even if one is a subtype of the other. For example: 

/**
 * @constructor
 */
X = function() { ... };

/**
 * @extends {X}
 * @constructor
 */
Y = function() { ... };

/** @type {Foo<X>} */ var fooX;
/** @type {Foo<Y>} */ var fooY;

fooX = fooY; // Error
fooY = fooX; // Error

/** @param {Foo<Y>} fooY */
takesFooY = function(fooY) { ... };

takesFooY(fooY); // OK.
takesFooY(fooX); // Error

Inheritance of Generic Types

Generic types can be inherited, and their template types can either be fixed or propagated to the inheriting type. Here is an example of an inheriting type fixing the template type of its supertype: 

/**
 * @constructor
 * @template T
 */
A = function() { ... };

/** @param {T} t */
A.prototype.method = function(t) { ... };

/**
 * @constructor
 * @extends {A<string>}
 */
B = function() { ... };

By extending A<string>, B will have a method method that takes a parameter of type string.

Here is an example of an inheriting type propagating the template type of its supertype: 

/**
 * @constructor
 * @template U
 * @extends {A<U>}
 */
C = function() { ... };

By extending A<U>, templated instances of C will have a method method that takes a parameter of the template type U.

Interfaces can be implemented and extended in a similar fashion, but a single type cannot implement the same interface multiple times with different template types. For example: 

/**
 * @interface
 * @template T
 */
Foo = function() {};

/** @return {T} */
Foo.prototype.get = function() {};

/**
 * @constructor
 * @implements {Foo<string>}
 * @implements {Foo<number>}
 */
FooImpl = function() { ... }; // Error - implements the same interface twice

Generic Functions and Methods

Similar to generic types, functions and methods can be made generic by adding a @template annotation to their definition. For example: 

/**
 * @param {T} a
 * @return {T}
 * @template T
 */
identity = function(a) { return a; };

/** @type {string} */ var msg = identity("hello") + identity("world"); // OK
/** @type {number} */ var sum = identity(2) + identity(2); // OK
/** @type {number} */ var sum = identity(2) + identity("2"); // Type mismatch