prop

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

The function bound as a getter 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 __lookupGetter__() method. If a getter has been defined for an object's property, it's not possible to reference the getter function through that property, because that property refers to the return value of that function. __lookupGetter__() can be used to obtain a reference to the getter function.

__lookupGetter__() walks up the prototype chain to find the specified property. If any object along the prototype chain has the specified own property, the get 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.

__lookupGetter__() 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 __lookupGetter__(), it also needs to implement the __lookupSetter__(), __defineGetter__(), and __defineSetter__() methods.

You should use the Object.getOwnPropertyDescriptor() API to look up a property's getter. Compared to __lookupGetter__(), 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 __lookupGetter__() method. If __lookupGetter__()'s behavior of walking up the prototype chain is important, you may implement it yourself with Object.getPrototypeOf().

const obj = {
  get foo() {
    return Math.random() > 0.5 ? "foo" : "bar";
  },
};

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

const obj2 = {
  __proto__: {
    get foo() {
      return Math.random() > 0.5 ? "foo" : "bar";
    },
  },
};

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

console.log(findGetter(obj2, "foo")); // [Function: get foo]

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