Object.prototype.__noSuchMethod__

The __noSuchMethod__ property used to reference a function to be executed when a non-existent method is called on an object, but this function is no longer available.

While __noSuchMethod__ has been dropped, the ECMAScript 2015 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 has been removed, see bug 683218.

Browser compatibility

[1] This feature was implemented until version 43.