问题
In Javascript, one of the reliable ways to convert a string to a number is the Number constructor:
var x = Number('09'); // 9, because it defaults to decimal
Inspired by this question, I started wondering — what is the difference between the above and:
var x =new Number('09');
Number certainly looks better, but it seems like a slightly inappropriate use of a constructor. Are there any side effects or any difference to using it without the new? If there is no difference, why not, and what is the purpose of new?
回答1:
In the first case, you are using the Number Constructor Called as a Function, as described in the Specification, that will simply perform a type conversion, returning you a Number primitive.
In the second case, you are using the Number Constructor to make a Number object:
var x = Number('09');
typeof x; // 'number'
var x = new Number('09');
typeof x; // 'object'
Number('1') === new Number('1'); // false
The difference may be subtle, but I think it's important to notice how wrapper objects act on primitive values.
回答2:
Number returns a primitive number value. Yeah, it's a bit odd that you can use a constructor function as a plain function too, but that's just how JavaScript is defined. Most of the language built-in types have weird and inconsistent extra features like this thrown in.
new Number constructs an explicit boxed Number object. The difference:
typeof Number(1) // number
typeof new Number(1) // object
In contrast to Java's boxed primitive classes, in JavaScript explicit Number objects are of absolutely no use.
I wouldn't bother with either use of Number. If you want to be explicit, use parseFloat('09'); if you want to be terse, use +'09'; if you want to allow only integers, use parseInt('09', 10).
回答3:
SpiderMonkey-1.7.0:
js> typeof new Number('09');
object
js> typeof Number('09');
number
回答4:
Number (without new) doesn't seem to result exactly in a primitive. In the following example the anyMethod() is called (if in the Number prototype).
Number(3).anyMethod()
Whereas
3.anyMethod()
will not work.
来源：https://stackoverflow.com/questions/2381399/what-is-the-difference-between-new-number-and-number-in-javascript