set

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

Syntax

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

Parameters

prop

The name of the property to bind to the given function.

val

An alias for the variable that holds the value attempted to be assigned to prop.

expression

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

Description

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.

Examples

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 language = {
  set current(name) {
    this.log.push(name);
  },
  log: []
}

language.current = 'EN';
console.log(language.log); // ['EN']

language.current = 'FA';
console.log(language.log); // ['EN', 'FA']

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

var expr = 'foo';

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

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

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

Browser compatibility

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 ES2015 specification.

See also

• getter
• delete
• Object.defineProperty()
• __defineGetter__
• __defineSetter__
• Defining Getters and Setters in JavaScript Guide