What should a JavaScript constructor return if it fails?

Question

If I have a javascript class which cannot be instantiated what should the constructor return that I can test for. The constructor always returns an object so I cannot return null if the constructor fails.

function SomeClass(id) {   if(typeof(id) === 'number' {     // This is good     this.id = id;   } else {     // This is bad     // This return is ignored and an empty object is returned     return null;   } }  var a = new SomeClass('badParam'); if(a){   // is true even though the class expects a number. }  // Could use this check if(a.id !== undefined){   // Do some stuff } 

but it seems there should be a better way.

Answer 1

It is probably best to throw an exception to notify the caller that the initialization failed and to take appropriate action.

Return codes are fine, but for the most part there is no motivation for the caller to implement the checks on the return code.

My advice is to break hard and break soon. This will make contract violations very evident during testing.

Answer 2

Returning any non-object from the constructor is practically the same as exiting the constructor. (The constructor will return a new object, with a prototype if one was specified.)

So, returning null, undefined, or 42 from the constructor are equivalent.

Check out section 13.2.2 of the ECMAScript spec (pdf) for more info.