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.

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

Specification
ECMAScript Language Specification # sec-method-definitions

Browser compatibility

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