Question

194

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

rated 0 times [ 200] [ 6] / answers: 1 / hits: 17439 / 13 Years ago, fri, december 23, 2011, 12:00:00

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 );

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lizet

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3 Years ago

answered 13 Years ago morrismilom · Accepted Answer

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();

  }

}