Can I construct a JavaScript object without using the new keyword?

Question

Here is what I'd like to do:

function a() {   // ... } function b() {   //  Some magic, return a new object. } var c = b();  c instanceof b // -> true c instanceof a // -> true b instanceof a // -> true 

Is it possible? I can make b be an instance of a easily by hooking a into its prototype chain but then I have to do new b(), which is what I'm trying to avoid. Is what I want possible?

Update: I feel that it might be possible with judicious use of b.__proto__ = a.prototype. I'm going to experiment more after work.

Update 2: Below is what seems to be the closest you can get, which is good enough for me. Thanks all for the interesting answers.

function a() {   // ... } function b() {   if (!(this instanceof arguments.callee)) {     return new arguments.callee();   } } b.__proto__ = a.prototype  var c = b(); c instanceof b // -> true c instanceof a // -> false b instanceof a // -> true 

Update 3: I found exactly what I wanted in a blog post on 'power constructors', once I added the essential b.__proto__ = a.prototype line:

var object = (function() {      function F() {}      return function(o) {          F.prototype = o;          return new F();      }; })();  function a(proto) {   var p = object(proto || a.prototype);   return p; }  function b(proto) {   var g = object(a(proto || b.prototype));   return g; } b.prototype = object(a.prototype); b.__proto__ = a.prototype;  var c = b(); c instanceof b // -> true c instanceof a // -> true b instanceof a // -> true a() instanceof a // -> true 

Answer 1

You can use this pattern:

function SomeConstructor(){    if (!(this instanceof SomeConstructor)){         return new SomeConstructor();    }    //the constructor properties and methods here } 

after which you can do:

var myObj = SomeConstructor(); 

In addition to this (rather old) answer: you can use a module pattern to create an object:

function Person(name, age, male) {   name = name || 'unknown';   age = age || 0;   function get() {     return ['This person is called ', name,             (!male ? ', her' : ', his'),' age is ',             age].join('');   }   function setAge(nwage) {      age = nwage;   }   return Object.freeze({get: get, setAge: setAge}); } // usage var jane =  Person('Jane', 23)    ,charles = Person('Charles', 32, 1)    ,mary = Person('Mary', 16);  console.log(jane.get()); //=> This person is called Jane, her age is 23 mary.setAge(17); console.log(mary.get()); //=> This person is called Mary, her age is 17 

Here's a jsFiddle for some Date functionallity I created using that pattern.

Answer 2

What's wrong with using the new keyword?

At any rate, it sounds like the best thing to do is read up on Javascript inheritance: http://javascript.crockford.com/inheritance.html