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 Path: blob/main/files/en-us/web/javascript/reference/functions/set/index.md
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---
title: setter
slug: Web/JavaScript/Reference/Functions/set
page-type: javascript-language-feature
browser-compat: javascript.functions.set
---

{{jsSidebar("Functions")}}

The set syntax binds an object property to a function to be called when there is an attempt to set that property. It can also be used in classes.

{{EmbedInteractiveExample("pages/js/functions-setter.html")}}

Syntax

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

There are some additional syntax restrictions:

  • A setter must have exactly one parameter.

Parameters

  • 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.

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.

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.

Using setters in classes

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 class features pages.

Removing a setter with the delete operator

If you want to remove the setter, you can just {{jsxref("Operators/delete", "delete")}} it:

delete language.current;

Defining a setter on existing objects using defineProperty

To append a setter to an existing object, use {{jsxref("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

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

{{Specifications}}

Browser compatibility

{{Compat}}

See also