Revision 72091 of Object.prototype.__noSuchMethod__

  • Revision slug: JavaScript/Reference/Global_Objects/Object/noSuchMethod
  • Revision title: __noSuchMethod__
  • Revision id: 72091
  • Created:
  • Creator: tmim
  • Is current revision? No
  • Comment 444 words added, 336 words removed

Revision Content

{{ Non-standard_header() }}

Summary

Executes a function when a non-existent method is called on an object.

Method of Object
Implemented in JavaScript ?
ECMAScript Edition None

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.

Example

__noSuchMethod__ can be used to simulate multiple inheritance.

The code that can implement 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 object isn't initialised, 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");
console.log("bob's age is: "+bob.getAge());
console.log("getName is "+(("getName" in bob)?"in":"not in")+" bob");
console.log("bob's name is: "+bob.getName());
console.log("getAddr is "+(("getAddr" in bob)?"in":"not in")+" bob");
console.log("bob's address is: "+bob.getAddr());

{{ languages( { "fr": "fr/R\u00e9f\u00e9rence_de_JavaScript_1.5_Core/Objets_globaux/Object/noSuchMethod", "ja": "ja/Core_JavaScript_1.5_Reference/Global_Objects/Object/noSuchMethod" } ) }}

Revision Source

<p>{{ Non-standard_header() }}</p>
<h2 name="Summary">Summary</h2>
<p>Executes a function when a non-existent method is called on an object.</p>
<table class="standard-table"> <thead> <tr> <th class="header" colspan="2">Method of <a href="/en/JavaScript/Reference/Global_Objects/Object" title="en/JavaScript/Reference/Global_Objects/Object"><code>Object</code></a></th> </tr> </thead> <tbody> <tr> <td>Implemented in</td> <td>JavaScript ?</td> </tr> <tr> <td>ECMAScript Edition</td> <td>None</td> </tr> </tbody>
</table>
<h2 name="Syntax">Syntax</h2>
<p><code><em>obj</em>.__noSuchMethod__ = <em>fun</em></code></p>
<h2 name="Parameters">Parameters</h2>
<dl> <dt><code>fun</code></dt> <dd>a function that takes the form</dd> <dd><code>function (<em>id</em>, <em>args</em>) { . . . }</code> <dl> <dt><code>id</code></dt> <dd>the name of the non-existent method that was called</dd> <dt><code>args</code></dt> <dd>an array of the arguments passed to the method</dd> </dl> </dd>
</dl>
<h2 name="Description">Description</h2>
<p>By default, an attempt to call a method that doesn't exist on an object results in a <a href="/En/Core_JavaScript_1.5_Reference/Global_Objects/TypeError" title="en/Core_JavaScript_1.5_Reference/Global_Objects/TypeError">TypeError</a> being thrown. This behavior can be circumvented by defining a function at that object's <code>__noSuchMethod__</code> 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 <a href="/en/JavaScript/Reference/Global_Objects/Array/prototype" title="en/Core_JavaScript_1.5_Reference/Global_Objects/Array/prototype">Array prototype</a> chain) and not the array-like <a href="/en/JavaScript/Reference/Functions_and_function_scope/arguments" title="en/Core_JavaScript_1.5_Reference/Functions/arguments">arguments object</a>.</p>
<p>If this method cannot be called, either as if <code>undefined</code> by default, if deleted, or if manually set to a non-function, the JavaScript engine will revert to throwing <code>TypeError</code>s.</p>
<h2 name="Example">Example</h2>
<p>__noSuchMethod__ can be used to simulate multiple inheritance.</p>
<p>The code that can implement a primitive form of multiple inheritance is shown below.</p>
<pre class="eval">//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&lt;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 object isn't initialised, initialize it
    if(!obj.__parents_){
        obj.__parents_=[];
        obj.__noSuchMethod__=noMethod;
    }
    //Add the parent
    obj.__parents_.push(parent);
}
</pre>
<p>An example of using this idea is shown below.</p>
<pre class="eval">//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");
console.log("bob's age is: "+bob.getAge());
console.log("getName is "+(("getName" in bob)?"in":"not in")+" bob");
console.log("bob's name is: "+bob.getName());
console.log("getAddr is "+(("getAddr" in bob)?"in":"not in")+" bob");
console.log("bob's address is: "+bob.getAddr());
</pre>
<p>{{ languages( { "fr": "fr/R\u00e9f\u00e9rence_de_JavaScript_1.5_Core/Objets_globaux/Object/noSuchMethod", "ja": "ja/Core_JavaScript_1.5_Reference/Global_Objects/Object/noSuchMethod" } ) }}</p>
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