Object.prototype.lookupSetter()

Deprecated: This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.

Note: This feature is deprecated in favor of the Object.getOwnPropertyDescriptor() API. This method's behavior is only specified for web compatibility, and is not required to be implemented in any platform. It may not work everywhere.

The __lookupSetter__() method of Object instances returns the function bound as a setter to the specified property.

Syntax

__lookupSetter__(prop)

Parameters

prop: A string containing the name of the property whose setter should be returned.

Return value

The function bound as a setter to the specified property. Returns undefined if no such property is found, or the property is a setter has been defined for an object's property, it's not possible to reference the setter function through that property, because that property only calls the function when it's being set. __lookupSetter__() can be used to obtain a reference to the setter function.

__lookupSetter__() walks up the property descriptor for that property is returned. If that property is a data property, undefined is returned. If the property is not found along the entire prototype chain, undefined is also returned.

__lookupSetter__() is defined in the spec as "normative optional", which means no implementation is required to implement this. However, all major browsers implement it, and due to its continued usage, it's unlikely to be removed. If a browser implements __lookupSetter__(), it also needs to implement the __defineSetter__() methods.

Examples

Using lookupSetter()

const obj = {
  set foo(value) {
    this.bar = value;
  },
};

obj.__lookupSetter__("foo");
/ [Function: set foo]

Looking up a property's setter in the standard way

You should use the Object.getPrototypeOf().

const obj = {
  set foo(value) {
    this.bar = value;
  },
};

Object.getOwnPropertyDescriptor(obj, "foo").set;
/ [Function: set foo]

const obj2 = {
  __proto__: {
    set foo(value) {
      this.bar = value;
    },
  },
};

function findSetter(obj, prop) {
  while (obj) {
    const desc = Object.getOwnPropertyDescriptor(obj, prop);
    if (desc) {
      return desc.set;
    }
    obj = Object.getPrototypeOf(obj);
  }
}

console.log(findSetter(obj2, "foo")); / [Function: set foo]

Specifications

Specification
ECMAScript® 2026 Language Specification # sec-object.prototype.__lookupSetter__