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generate 4 digit random number using substring

I am trying to execute below code:

var a = Math.floor(100000 + Math.random() * 900000);
a = a.substring(-2);

I am getting error like undefined is not a function at line 2, but when I try to do alert(a), it has something. What is wrong here?


That’s because a is a number, not a string. What you probably want to do is something like this:

var val = Math.floor(1000 + Math.random() * 9000);
  • Math.random() will generate a floating point number in the range [0, 1) (this is not a typo, it is standard mathematical notation to show that 1 is excluded from the range).
  • Multiplying by 9000 results in a range of [0, 9000).
  • Adding 1000 results in a range of [1000, 10000).
  • Flooring chops off the decimal value to give you an integer. Note that it does not round.

General Case

If you want to generate an integer in the range [x, y), you can use the following code:

Math.floor(x + (y - x) * Math.random());