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Chris Bongers
Chris Bongers

Posted on • Originally published at daily-dev-tips.com

Vanilla JavaScript random colours

Ever wondered how you can create random colours using Vanilla JavaScript?

The other day we made a JavaScript data-attribute filter, and I wanted to give each block a random colour.

So today we are going to do just that.

The end result will be as this Codepen (Open or reload to see random new colours)

JavaScript random hex colour

In our example, we will be generating a random hex number. These can be any six-characters from 0-9 and A-F.

Luckily in JavaScript, it's even easier to create a random hex string.

Let's break it down.

Math.random()*10000000
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This will give us a random number which will look like this:

2773929.134011086
9192315.941572387
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The next step is to floor this number to we will only get the first part.

Math.floor(Math.random()*10000000)
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Which would result in the following for the above examples:

2773929
9192315
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Now we need to create strings else we would only have numbers and one too many.

We can use the toString() method and specify the radix parameter as 16.

This will convert 254 to fe, for instance.

Math.floor(Math.random()*10000000).toString(16)
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This will get results like:

'2a53a9'
'8c437b'
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Awesome, Perfect hex values!

Random colour blocks in JavaScript

Now let's give our blocks all a random colour.

<ul>
  <li data-rating="4"><span>item 1</span><i>rating 4</i></li>
  <li data-rating="2"><span>item 2</span><i>rating 2</i></li>
  <li data-rating="3"><span>item 3</span><i>rating 3</i></li>
  <li data-rating="1"><span>item 4</span><i>rating 1</i></li>
  <li data-rating="4"><span>item 5</span><i>rating 4</i></li>
  <li data-rating="1"><span>item 6</span><i>rating 1</i></li>
  <li data-rating="4"><span>item 7</span><i>rating 4</i></li>
  <li data-rating="4"><span>item 8</span><i>rating 4</i></li>
  <li data-rating="1"><span>item 9</span><i>rating 1</i></li>
  <li data-rating="5"><span>item 10</span><i>rating 5</i></li>
  <li data-rating="1"><span>item 11</span><i>rating 1</i></li>
  <li data-rating="2"><span>item 12</span><i>rating 2</i></li>
  <li data-rating="3"><span>item 13</span><i>rating 3</i></li>
  <li data-rating="1"><span>item 14</span><i>rating 1</i></li>
  <li data-rating="3"><span>item 15</span><i>rating 3</i></li>
  <li data-rating="5"><span>item 16</span><i>rating 5</i></li>
  <li data-rating="3"><span>item 17</span><i>rating 3</i></li>
  <li data-rating="5"><span>item 18</span><i>rating 5</i></li>
  <li data-rating="1"><span>item 19</span><i>rating 1</i></li>
  <li data-rating="2"><span>item 20</span><i>rating 2</i></li>
</ul>
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Now we need to get all our list items and loop over them.

const elements = document.querySelectorAll("li");

[...elements].forEach((element) => {
  element.style.backgroundColor = "#" + (Math.floor(Math.random() * 1e7).toString(16));
});
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It might not have the prettiest colours, but at least they're random!

You might have noted the 1e7 which is a shorthand Decimal Base Exponent.

It means one followed by seven zeroes.

Thank you for reading, and let's connect!

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Top comments (8)

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lionelrowe profile image
lionel-rowe • Edited on

You'll get a more complete distribution throughout the RGB color space using 0x1_00_00_00 (decimal 16,777,216) as a multiplier, rather than 1e7 (decimal 10,000,000). Math.random is inclusive of the lower bound but not the upper bound, so Math.floor(Math.random() * 0x1_00_00_00) gives a number in the range 0x00_00_00..0xff_ff_ff (pure black to pure white). 1e7 - 1 (the maximum value with 1e7 as upper bound), on the other hand, converts to 0x98_96_7f, which is a sort of beige color.

I think that's why the colors you're generating are skewing toward greens and purples, bearing in mind that contiguous 6-digit hex numbers don't always look like similar colors, seeing as they're split into red-green-blue segments.

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lexlohr profile image
Alex Lohr • Edited on

You'll still have a chance to hit a number low enough not to yield a 3- or 6-digit hex number, which will result in an invalid color (0x0-0xFF, 0x1000-0xfffff). To solve that, you can use

'#' + (0x1000000 + 0x1000000 * Math.random() | 0).toString(16).slice(-6)
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instead.

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lionelrowe profile image
lionel-rowe • Edited on

Good point, forgot about that. A simpler fix would probably be hexStr.padStart(6, '0').

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dailydevtips1 profile image
Chris Bongers Author

Good point, it's not a failsafe method indeed thanks for this addition Lionel!

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hellnar profile image
Stas Klymenko

Hey, Chris!
I've actually made a few things with random colors both with hex and RGB. I think the RGB approach is a bit easier. Especially for newbies.
Here is a simple example:
codepen.io/Hellnar/pen/JjKMzzm

But anyway thanks for sharing your option. Also very nice and clean.

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dailydevtips1 profile image
Chris Bongers Author

True RGB(a) might be easier to understand for most people because it's easier to calculate with, nice one Stan!

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waylonwalker profile image
Waylon Walker

This looks like a super fun one. Very innovative and elegant solution 👍

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dailydevtips1 profile image
Chris Bongers Author

Thanks Waylon, as mentioned there are many ways of doing it in the comments as well.

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