Number() vs new Number()? [duplicate]

Question

I'm trying to understand the difference between writing m = Number() (which causes typeof m to evaluate as "number") vs m = new Number() (which causes typeof m to evaluate as "object").

I would have expected it to be an object either way. I was just messing around, and I added a .helloWorld() method to the Number prototype, and I was able to access it on m regardless of which method I used to instantiate it.

What's the difference here? What am I doing differently between writing Number() and new Number()? Why is one an object while the other is a number?

Answer 1

Number() by itself returns a number primitive. When you call new Number() you receive a new instance of an Object which represents Number's (relevant ES5 spec).

When you call a property on a primitive, the primitive is auto-boxed (like in Java) to an instance of that object, which is what lets you call helloWorld() on either the object or number.

However, try this;

var x = Number(5);
x.bar = function (x) { alert(x); };
x.bar("hello");

var y = new Number(5);
y.bar = function (x) { alert(x); };
y.bar("hello");

You'll see the latter works whilst the former does not; in the first, the number is autoboxed to a number, and the bar method is added to it (the object). When you call x.bar() you're creating a new auto-boxed Number, which bar doesn't exist on.

In the latter, you're adding a bar method to that Number instance, which behaves like any other Object instance, and therefore it persists throughout the lifetime of the object.