How to get a random value from a JavaScript array

Feb 07, 2019 · Updated: Jul 17, 2021 · by Tim Kamanin

I was recently tasked to create a function that selects a random color from a predefined list of colors.

This is what my array of colors looks like:

const colors = ["#FFFFFF", "#F06B4F", "#F2AE52", "#B0CD6D", "#A33120"];

And here's the randomColor function I wrote. This function randomly chooses a color from a list:

function randomColor(colors) {
  return colors[Math.floor(Math.random() * colors.length)];

Of course, you can use this function to select any random value from an array, not just an array of colors.

We could stop here and call it a day, but let's take a closer look at the randomColor function and see what it does, bit by bit.

  1. Math.floor(Math.random() * colors.length) generates a random index for the colors array;
  2. Math.floor function returns the largest integer for a given number;
  3. Math.random function returns us a random number in the range of 0 - 1;
  4. Math.random() * colors.length, we get a random number between 0 - 1, let's say 0.5 and normalize it, multiplying it by the length of the array (so we don't get the index larger than our array's length);
  5. Math.floor(Math.random() * colors.length), gives us a random integer (array index) by rounding-down our normalized value to the nearest integer. Note that we don't use Math.ceil to round the number up because if we get something like 0.99 x 5 = 4.95, it'd be rounded up to 5, which gives us an index larger than the length of the array. Thus, we always round down that number: Math.floor(4.95) = 4, i.e. last element of the array;
  6. colors[Math.floor(Math.random() * colors.length)] returns us the array element by a random index, just like we wanted.

Below you can see the complete code with an example of how to use it:

const colors = ["#FFFFFF", "#F06B4F", "#F2AE52", "#B0CD6D", "#A33120"];

function randomColor(colors) {
  return colors[Math.floor(Math.random() * colors.length)];

randomColor(colors); // #B0CD6D

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