Generate a random number

The built in Math.random() method generates a random number between 0 and 0.9999999.

Typically this floating point number is not what you want. Here are a couple of ways to generate whole numbers:

MDN method

Using the random number from the MDN beginner guide which generates a number between 1 and 100.

let randomNumber = Math.floor(Math.random() * 100) + 1;

So Math.random() generates a number between 0 and less than 1.

Multiplying by 100 means you get a number between 0 and 99.

The Math.floor() function rounds the result DOWN to the nearest whole number.

Adding 1 means the numbers are now between 1 and 100.

Josh Comeau function

Another function is this one by Josh Comeau.

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

This takes two arguments, the lowest and highest numbers. So for say a straight percentage you’d pass 0 and 101.

Why 101? Josh says:

This random function includes the lower bound, but excludes the upper bound. For example, randomNo(10, 12) will grant either 10 or 11, but never 12.

This was done intentionally, to match the behaviour of Math.random, as well as JavaScript methods like slice.

Josh Comeau