Javascript timestamp number is not unique

I need a unique number to be generated to be used in my code.I use

var id = new Date().valueOf()

I know the above returns the number of milliseconds. But the values are not unique.For example :1385035174752.This number is generated twice or more than that.

My question is Why is it not unique? and how do i get unique number from current date/time?

Answer

If you need uniqueness, use Math.random(), and not any of the Date() APIs.

Math.random returns an integer between and including 0 and 1. If you really want an semi-unique number based on the current time, you can combine the Date API with Math.random. For example:

var id = new Date().getTime() + Math.random();

In modern browsers, you can also use performance.now(). This API guarantees that every new call’s return value is unique.