prop

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

The function bound as a setter to the specified property. Returns undefined if no such property is found, or the property is a data property.

All objects that inherit from Object.prototype (that is, all except null-prototype objects) inherit the __lookupSetter__() method. If 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 prototype chain to find the specified property. If any object along the prototype chain has the specified own property, the set attribute of 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 __lookupGetter__(), __defineGetter__(), and __defineSetter__() methods.

You should use the Object.getOwnPropertyDescriptor() API to look up a property's setter. Compared to __lookupSetter__(), this method allows looking up symbol properties. The Object.getOwnPropertyDescriptor() method also works with null-prototype objects, which don't inherit from Object.prototype and therefore don't have the __lookupSetter__() method. If __lookupSetter__()'s behavior of walking up the prototype chain is important, you may implement it yourself with 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]

• Polyfill of Object.prototype.__lookupSetter__ in core-js
• Object.prototype.__lookupGetter__()
• set
• Object.getOwnPropertyDescriptor()
• Object.prototype.__defineGetter__()
• Object.prototype.__defineSetter__()
• JS Guide: Defining Getters and Setters