how does new Number(value) work in js?

Question

<script>
var num = new Number(43);
console.log(num);
</script>

According to the tutorial here: http://www.w3schools.com/jsref/jsref_obj_number.asp

Syntax
var num = new Number(value);
Note: If the value parameter cannot be converted into a number, it returns NaN (Not-a-Number).

Question:

In firebug->console, it shows: Number {} , not what I expect Number {43}, no matter what number i put in new Number(value); it always shows:Number {} so what is the purpose to use new Number(value)? how does it work? BTW, I googled on line, but did not find a good explanation.

http://www.w3fools.com/ lol @zzzzBov looks like you won the "race". — Hogan, Jun 17 '13 at 03:06
Its just an object wrapper for a number, if you want to know the value of the wrapped number just call `valuOf`. — Musa, Jun 17 '13 at 03:08
Here is some good documentation https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number — Hogan, Jun 17 '13 at 03:10

score 1 · Answer 1 · edited May 23 '17 at 10:29

Number(43) is a number primitive but new Number(43) will create an object from that number primitive. The display Number {} is just how the Chrome console displays an object like that.

new Number(43) will still "behave" like a number. In the console:

var num = new Number(43); // Number {}
num == 43 // true
num + 5 // 48
num === 43 // false, since the types don't match.

For more info on how new works: How does the new operator work in JavaScript?. If you just want a number primitive, don't use new.

score 0 · Accepted Answer · answered Jun 17 '13 at 03:11

0

There's a difference between a number primitive and a number object. When you use the new keyword, you create a new number object, which is what you see. You can get the value of the number object with valueOf, which returns the primitive:

> var n = new Number(42);
> n.valueOf()
42

answered Jun 17 '13 at 03:11

Blender

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so is '42'stored as a property in number object? – user2294256 Jun 17 '13 at 03:20
@user2294256: Not as a property like `n.something = 12` is, but yes, sort of. – Blender Jun 17 '13 at 03:31
after this line runs: var n = new Number(42);is there a way that I can see what is the content inside object Number? I want to see how '42' stored in object Number? – user2294256 Jun 17 '13 at 03:34
@user2294256: You can't. `Number` is implemented in native code. – Blender Jun 17 '13 at 03:35

how does new Number(value) work in js?

2 Answers2