generate 4 digit random number using substring

Question

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?

Answer 1

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);
console.log(val);

• 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());

Answer 2

This will generate 4-digit random number (0000-9999) using substring:

var seq = (Math.floor(Math.random() * 10000) + 10000).toString().substring(1);
console.log(seq);

Answer 3

I adapted Balajis to make it immutable and functional.

Because this doesn't use math you can use alphanumeric, emojis, very long pins etc

const getRandomPin = (chars, len)=>[...Array(len)].map(
   (i)=>chars[Math.floor(Math.random()*chars.length)]
).join('');


//use it like this
getRandomPin('0123456789',4);

Answer 4

$( document ).ready(function() {
  
    var a = Math.floor(100000 + Math.random() * 900000);   
    a = String(a);
    a = a.substring(0,4);
    alert( "valor:" +a );
  
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>