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The get syntax binds an object property to a function that will be called when that property is looked up.

Syntax

{get prop() { ... } }
{get [expression]() { ... } }

Parameters

prop
The name of the property to bind to the given function.
expression
Starting with ECMAScript 2015, you can also use expressions for a computed property name to bind to the given function.

Description

Sometimes it is desirable to allow access to a property that returns a dynamically computed value, or you may want to reflect the status of an internal variable without requiring the use of explicit method calls. In JavaScript, this can be accomplished with the use of a getter. It is not possible to simultaneously have a getter bound to a property and have that property actually hold a value, although it is possible to use a getter and a setter in conjunction to create a type of pseudo-property.

Note the following when working with the get syntax:

A getter can be removed using the delete operator.

Examples

Defining a getter on new objects in object initializers

This will create a pseudo-property latest for object obj, which will return the last array item in log.

var obj = {
  log: ['example','test'],
  get latest() {
    if (this.log.length == 0) return undefined;
    return this.log[this.log.length - 1];
  }
}
console.log(obj.latest); // "test".

Note that attempting to assign a value to latest will not change it.

Deleting a getter using the delete operator

If you want to remove the getter, you can just delete it:

delete obj.latest;

Defining a getter on existing objects using defineProperty

To append a getter to an existing object later at any time, use Object.defineProperty().

var o = {a: 0};

Object.defineProperty(o, 'b', { get: function() { return this.a + 1; } });

console.log(o.b) // Runs the getter, which yields a + 1 (which is 1)

Using a computed property name

var expr = 'foo';

var obj = {
  get [expr]() { return 'bar'; }
};

console.log(obj.foo); // "bar"

Smart / self-overwriting / lazy getters

Getters give you a way to define a property of an object, but they do not calculate the property's value until it is accessed. A getter defers the cost of calculating the value until the value is needed, and if it is never needed, you never pay the cost.

An additional optimization technique to lazify or delay the calculation of a property value and cache it for later access are smart or memoized getters. The value is calculated the first time the getter is called, and is then cached so subsequent accesses return the cached value without recalculating it. This is useful in the following situations:

  • If the calculation of a property value is expensive (takes much RAM or CPU time, spawns worker thread, retrieves remote file, etc).
  • If the value isn't needed just now. It will be used later, or in some case it's not used at all.
  • If it's used, it will be accessed several times, and there is no need to re-calculate that value will never be changed, or shouldn't be re-calculated.

This means that you shouldn't use a lazy getter for a property whose value you expect to change, because the getter will not recalculate the value.

In the following example, the object has a getter as its own property. On getting the property, the property is removed from the object and re-added, but implicitly as a data property this time. Finally the value gets returned.

get notifier() {
  delete this.notifier;
  return this.notifier = document.getElementById('bookmarked-notification-anchor');
},

For Firefox code, see also the XPCOMUtils.jsm code module, which defines the defineLazyGetter() function.

get Vs. defineProperty

While using the get keyword and Object.defineProperty() have similar results there is a subtle difference between the two when used on classes.

When using get the property will be defined on the prototype of the object while using Object.defineProperty() the property will be defined on the instance it is applied to.

class Example {
  get hello() {
    return 'world';
  }
}

const obj = new Example();
console.log(obj.hello);
// "world"
console.log(Object.getOwnPropertyDescriptor(obj, 'hello'));
// undefined
console.log(Object.getOwnPropertyDescriptor(Object.getPrototypeOf(obj), 'hello'));
// { configurable: true, enumerable: false, get: function get hello() { return 'world'; }, set: undefined }

Specifications

Specification Status Comment
ECMAScript 5.1 (ECMA-262)
The definition of 'Object Initializer' in that specification.
Standard Initial definition.
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Method definitions' in that specification.
Standard Added computed property names.
ECMAScript Latest Draft (ECMA-262)
The definition of 'Method definitions' in that specification.
Draft  

Browser compatibility

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung InternetNode.js
Basic supportChrome Full support 1Edge Full support YesFirefox Full support 2IE Full support 9Opera Full support 9.5Safari Full support 3WebView Android Full support 1Chrome Android Full support 18Edge Mobile Full support YesFirefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support Yes
Computed property namesChrome Full support 46Edge Full support YesFirefox Full support 34IE No support NoOpera Full support 47Safari No support NoWebView Android Full support 46Chrome Android Full support 46Edge Mobile Full support YesFirefox Android Full support 34Opera Android Full support YesSafari iOS No support NoSamsung Internet Android Full support 5.0nodejs Full support Yes

Legend

Full support  
Full support
No support  
No support

See also