Object.prototype.__noSuchMethod__

Obsolete since Gecko 39 (Firefox 39 / Thunderbird 39 / SeaMonkey 2.36)
This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.

Non-standard
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.

The __noSuchMethod__ property references a function to be executed when a non-existent method is called on an object.

While __noSuchMethod__ is non-standard, the ECMAScript 2015 (ES6) specification has the Proxy object, with which you can achieve the below (and more).

Syntax

obj.__noSuchMethod__ = fun

Parameters

fun
A function that takes the form
function (id, args) { . . . }
id
The name of the non-existent method that was called
args
An array of the arguments passed to the method

Description

By default, an attempt to call a method that doesn't exist on an object results in a TypeError being thrown. This behavior can be circumvented by defining a function at that object's __noSuchMethod__ member. The function takes two arguments, the first is the name of the method attempted and the second is an array of the arguments that were passed in the method call. The second argument is an actual array (that is, it inherits through the Array.prototype chain) and not the array-like arguments object.

If this method cannot be called, either as if undefined by default, if deleted, or if manually set to a non-function, the JavaScript engine will revert to throwing TypeErrors.

Examples

Simple test of __noSuchMethod__

var o = {
  __noSuchMethod__: function(id, args) { 
                      console.log(id, '(' + args.join(', ') + ')'); 
                    }
};

o.foo(1, 2, 3);
o.bar(4, 5);
o.baz();

// Output
// foo (1, 2, 3)
// bar (4, 5)
// baz ()

Using __noSuchMethod__ to simulate multiple inheritance

An example of code that implements a primitive form of multiple inheritance is shown below.

// Doesn't work with multiple inheritance objects as parents
function noMethod(name, args) {
  var parents = this.__parents_;

  // Go through all parents
  for (var i = 0; i < parents.length; i++) {
    // If we find a function on the parent, we call it
    if (typeof parents[i][name] == "function") {
      return parents[i][name].apply(this, args);
    }
  }

  // If we get here, the method hasn't been found
  throw new TypeError;
}

// Used to add a parent for multiple inheritance
function addParent(obj, parent) {
  // If the object isn't initialized, initialize it
  if (!obj.__parents_) {
    obj.__parents_ = [];
    obj.__noSuchMethod__ = noMethod;
  }

  // Add the parent
  obj.__parents_.push(parent);
}

An example of using this idea is shown below.

// Example base class 1
function NamedThing(name){
  this.name=name;
}

NamedThing.prototype = {
  getName: function() { return this.name; },
  setName: function(newName) { this.name = newName; }
}

// Example base class 2
function AgedThing(age) {
  this.age = age;
}

AgedThing.prototype = {
  getAge: function() { return this.age; },
  setAge: function(age) { this.age = age; }
}

// Child class. inherits from NamedThing and AgedThing
// as well as defining address
function Person(name, age, address){
  addParent(this, NamedThing.prototype);
  NamedThing.call(this, name);
  addParent(this, AgedThing.prototype);
  AgedThing.call(this, age);
  this.address = address;
}

Person.prototype = {
  getAddr: function() { return this.address; },
  setAddr: function(addr) { this.address = addr; }
}

var bob = new Person("bob", 25, "New York");

console.log("getAge is " + (("getAge" in bob) ? "in" : "not in") + " bob");
// getAge is not in bob

console.log("bob's age is: " + bob.getAge());
// bob's age is: 25

console.log("getName is " + (("getName" in bob) ? "in" : "not in") + " bob");
// getName is not in bob

console.log("bob's name is: " + bob.getName());
// bob's name is: bob

console.log("getAddr is " + (("getAddr" in bob) ? "in" : "not in") + " bob");
// getAddr is in bob

console.log("bob's address is: " + bob.getAddr());
// bob's address is: New York

Specifications

Not part of any specifications. This feature is going to be removed in the future, see bug 683218.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support Not supported 1.0 (1.7 or earlier) Not supported Not supported Not supported
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support Not supported Not supported 1.0 (1.0) Not supported Not supported Not supported

Document Tags and Contributors

Last updated by: fscholz,