{ set prop(val) { /* … */ } }
{ set [expression](val) { /* … */ } }

There are some additional syntax restrictions:

• A setter must have exactly one parameter.

prop

The name of the property to bind to the given function. In the same way as other properties in object initializers, it can be a string literal, a number literal, or an identifier.

val

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

expression

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.

The following example defines 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.

You can use the exact same syntax to define public instance setters that are available on class instances. In classes, you don't need the comma separator between methods.

class ClassWithGetSet {
  #msg = "hello world";
  get msg() {
    return this.#msg;
  }
  set msg(x) {
    this.#msg = `hello ${x}`;
  }
}

const instance = new ClassWithGetSet();
console.log(instance.msg); // "hello world"

instance.msg = "cake";
console.log(instance.msg); // "hello cake"

Setter properties are defined on the prototype property of the class and are thus shared by all instances of the class. Unlike setter properties in object literals, setter properties in classes are not enumerable.

Static setters and private setters use similar syntaxes, which are described in the static and private properties pages.

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

delete language.current;

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

const o = { a: 0 };

Object.defineProperty(o, "b", {
  set(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

• Working with objects guide
• Functions
• get
• Object.defineProperty()
• Object initializer
• class
• Property accessors
• Incompatible ES5 change: literal getter and setter functions must now have exactly zero or one arguments by Jeff Walden (2010)
• More SpiderMonkey changes: ancient, esoteric, very rarely used syntax for creating getters and setters is being removed by Jeff Walden (2010)