setter

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:

• It can have an identifier which is either a number or a string;
• It must have exactly one parameter (see Incompatible ES5 change: literal getter and setter functions must now have exactly zero or one arguments for more information);
• It must not appear in an object literal with another set or with a data entry for the same property. ( { set x(v) { }, set x(v) { } } and { x: ..., set x(v) { } } are forbidden )

Examples

Defining a setter on new objects in object initializers

The following example define a pseudo-property current of object language. When current is assigned a value, it updates log with that value:

const 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 language.current;

Defining a setter on existing objects using defineProperty

To append a setter to an existing object, use Object.defineProperty().

const 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

const expr = 'foo';

const 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

Browser compatibility

See also

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