The set syntax binds an object property to a function to be called when there is an attempt to set that property.


{set prop(val) { . . . }}
{set [expression](val) { . . . }}


The name of the property to bind to the given function.
An alias for the variable that holds the value attempted to be assigned to prop.
Starting with ECMAScript 6, you can also use expressions for a computed property name to bind to the given function.


In JavaScript, a setter can be used to execute a function whenever a specified property is attempted to be changed. Setters are most often used in conjunction with getters to create a type of pseudo-property. It is not possible to simultaneously have a setter on a property that holds an actual value.

Note the following when working with the set syntax:

A setter can be removed using the delete operator.


Defining a setter on new objects in object initializers

This will define a pseudo-property current of object o that, when assigned a value, will update log with that value:

var o = {
  set current (str) {
    this.log[this.log.length] = str;
  log: []

Note that current is not defined and any attempts to access it will result in undefined.

Removing a setter with the delete operator

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

delete o.current;

Defining a setter on existing objects using defineProperty

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

var o = { a:0 };

Object.defineProperty(o, "b", { set: function (x) { this.a = x / 2; } });

o.b = 10; // Runs the setter, which assigns 10 / 2 (5) to the 'a' property
console.log(o.a) // 5

Using a computed property name

Note: Computed properties are experimental technology, part of the ECMAScript 6 proposal, and are not widely supported by browsers yet. This will trigger a syntax error in non-supporting environments.

var expr = "foo";

var obj = {
  baz: "bar",
  set [expr](v) { this.baz = v; }

console.log(obj.baz); // "bar" = "baz";      // run the setter
console.log(obj.baz); // "baz"


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 2017 Draft (ECMA-262)
The definition of 'Method definitions' in that specification.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 1 2.0 (1.8.1) 9 9.5 3
Computed property names No support 34 (34) No support No support No support
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) 1.0 (1.8.1) (Yes) (Yes) (Yes)
Computed property names No support No support 34.0 (34.0) No support No support No support

SpiderMonkey-specific notes

  • Starting with JavaScript 1.8.1, setters are no longer called when setting properties in object and array initializers.
  • From SpiderMonkey 38 on, a setter with a rest parameter is a SyntaxError as per the ES6 specification.

See also


© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.

ECMAScript5 Functions JavaScript