How to return null from a constructor called with new in Javascript?

Question

I'm trying to make the constructor abort the object construction if something fails, for example it can't get a hold of a canvas.

But as I'm using new I see that klass() always returns this regardless of any return null or any other value, can I work around this to return null?

Now that I think of, a solution may be to create the new instance inside klass() and return that instance or null, and not use new, is there a better solution?

function klass( canvas_id ) {
    var canvas = document.getElementById( canvas_id );

    if( ! ( canvas && canvas.getContext ) ) {
        return null;
    }   
}
var instance = new klass( 'wrong_id' );
console.log( instance, typeof instance );

Answer 1

You could make a "factory function" or "static factory method" instead:

Foo.CreateFoo = function() {
  // not to confuse with Foo.prototype. ...
  if (something) {
    return null;
  }
  return new Foo();
};

// then instead of new Foo():
var obj = Foo.CreateFoo();

Same thing using the newer class syntax:

class Foo {
  static CreateFoo() {
    if (something) {
      return null;
    }
    return new Foo();
  }
}

Answer 2

The better solution would be to throw an error:

function klass(canvas_id) {
    var canvas = document.getElementById( canvas_id );

    if( ! ( canvas && canvas.getContext ) ) {
        throw new Error('Not a canvas');
    }  
}

// later...

try {
    var c = new klass("canvas_id");
} catch(E) {
    // error caught
}

---

EDIT: Constructors can be "forced" to not return an instance:

function Foo() {
   var canvas = ...;

   if ('undefined' == '' + Foo.CANVAS_CHECK)
      Foo.CANVAS_CHECK = ( canvas && canvas.getContext );

   if (!Foo.CANVAS_CHECK)
      return []; // the constructor will actually return an empty array

   // passed; initialize instance here
}


// later on...

var foo;

if (!((foo = new Foo()) instanceof Foo)) {
   // Failed. Canvas is unsupported.
}

// You happy now, am not i am?  ;-)

The odd thing is, however, that if a "constructor" returns a number, string, true, false, etc., it actually does return an instance. The second solution only works when the constructor returns an empty array [] or an empty object {}.