JavaScript - Function constructor works without the 'new' keyword

Question

We all know that it's bad to call a JavaScript constructor function without the 'new' keyword. So why does this:

Function("a", "b", "return a + b")(1, 1); // returns "2"

Return the same value as this?:

new Function("a", "b", "return a + b")(1, 1); // Also returns "2"

And is there any harm (or benefit) in omitting the 'new' key word in this instance?

Answer 1

The Function constructor creates a new function whether you call it with new or not. That's just how it is written. Constructors can be written that way, if desired.

From the MDN page on the Function constructor:

Invoking the Function constructor as a function (without using the new operator) has the same effect as invoking it as a constructor.

There is no harm or benefit or even any difference in using or not using the new operator with the Function constructor.

Answer 2

No. There is indeed no harm of calling Function without the new keyword. In fact, I would advise against using new in your programs as much as possible: stop using the new keyword.

It would be much better to create a new function instead:

Function.prototype.new = function () {
    Factory.prototype = this.prototype;
    return new Factory(this, arguments);
};

function Factory(constructor, args) {
    return constructor.apply(this, args);
}

The advantage of doing so it that now you can create instances as follows:

function Foo(a, b) {
    this.a = a;
    this.b = b;
}

var foo = Foo.new(1, 2);

// instead of

var foo = new Foo(1, 2);

What's the point? The point is that you can now do things like:

var foo = Foo.new.apply(Foo, [1, 2]);

Because new is a function you can do things to it like apply it to an array of arguments, etc. You can't do the same thing with the new keyword.

Anyway, getting back to the topic at hand the Function constructor can be called with or without the new keyword. It doesn't make any difference. Hence I would advise you not to use the new keyword at all. Not only does it save you a few keystrokes but also it can be used for things like:

Function.apply(null, ["a", "b", "return a + b"]);

This is especially useful if you don't know how many arguments you want the function to have beforehand.

Hope that helps.