Javascript timestamp number is not unique

Question

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 1

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.

Answer 2

On Windows the resolution of the timer is about 10.5 ms. So you have chances of getting the same value even few milliseconds later. There are better timers of course, but AFAIK they are not available to JavaScript.