To Stop a Constructor

Question

I'm trying to implement a sort of Constructor Pattern into a library for a project. But, I want to check for certain conditions being passed before returning an object. If those conditions aren't met, then I want to stop the constructor and return false.

But, I noticed that no matter what I set as the return value, an object is always returned!

Even if I do this:

new function () { return false; }

The result is still an Object object:

In Chrome:

In Firefox:

Is there any way to have a constructor fail in Javascript?

Answer 1

If those conditions aren't met, then I want to stop the constructor and return false.

You can't. The return value of a constructor function is ignored unless it's a non-null object. Any primitive value (or null) is just ignored by the new expression.

If you really want to have a flag value you return, you can do that, but it has to be an object. (But it's a bad idea, more below.) E.g.:

// The constructor
function Foo(num) {
    if (num < 27) {
        return Foo.BAD_ARG; // This is a bad idea, see the right way below
    }
    this.num = num;
}
Foo.BAD_ARG = {}; // Our special "bad argument" object

Is there any way to have a constructor fail in Javascript?

Yes: Throw an exception:

function Foo(num) {
    if (num < 27) {
        throw "`num` must be >= 27";
    }
    this.num = num;
}

Answer 2

Only way to return a non-object will be to wrap your constructor in a function and have it call (or not call) using new for you.

To do this, new on the outer function will need to be prohibited.

---

You can even use the same function, as long as you never call it with new.

function Foo() {
   if (!(this instanceof Foo)) {
       if (some_condition)
           return false;
       else
           return new Foo()
   }
   // your constructor code here
}

var x = Foo();

The danger is if you forget and accidentally use new. To remedy this, you can use a separate function.

function Foo() {
    // constructor code
}

function Bar() {
    if (this instanceof Bar)
        throw "Not a constructor"

    if (some_condition)
         return false;
    else
        return new Foo();
}