How to generate a random number between a range in JavaScript

In JavaScript, the Math.random function will generate a random floating point number between 0 and 1(0 is included 1 is excluded):

Math.random();

Generate a random number with a Max limit

Let’s say we need to generate a random number under 100. To do this, we would multiply the random number by 100. Math.random() will not return 1, so the value will be below 100.

Math.random() * 100; // 84.06448370345245

We can use Math.floor to round it down as shown here:

function generateRandom(maxLimit = 100){
  let rand = Math.random() * maxLimit;
  console.log(rand); // say 99.81321410836433
  rand = Math.floor(rand); // 99
  return rand;
}
generateRandom(); // 43
generateRandom(500); // 165

Generate a random number between two numbers

In this case, we will have a MIN and a MAX value and will need to include the MIN value and exclude the MAX value.

If the MIN value is always 0, then we can multiply the random number with the MAX value, but if the MIN value is other than 0, we need to find the difference between MAX and MIN. Then we will multiply the random number with that difference and add the MIN value.

  • Let’s say MIN = 3 & MAX = 5.
  • Difference is MAX - MIN = 2.
  • Upon multiplying a random number by 2, we will get a value between 0 and below 2. So, the possible value will be either 0 or 1.
  • When we add the MIN(3) value to the random number, the result is either 3 or 4. We can see this as follows:
function generateRandom(min = 0, max = 100) {
    // find diff
    let difference = max - min;
    // generate random number 
    let rand = Math.random();
    // multiply with difference 
    rand = Math.floor( rand * difference);
    // add with min value 
    rand = rand + min;
    return rand;
}
console.log(generateRandom());

The above can be also be written as:

const random = (min, max) => Math.floor(Math.random() * (max - min)) + min;

Free Resources

License: Creative Commons-Attribution-ShareAlike 4.0 (CC-BY-SA 4.0)