Thursday, July 18

Animating Number Counters

Number animation, as in, imagine a number changing from 1 to 2, then 2 to 3, then 3 to 4, etc. over a specified time. Like a counter, except controlled by the same kind of animation that we use for other design animation on the web. This could be useful when designing something like a dashboard, to bring a little pizazz to the numbers. Amazingly, this can now be done in CSS without much trickery. You can jump right to the new solution if you like, but first let’s look at how we used to do it.

One fairly logical way to do number animation is by changing the number in JavaScript. We could do a rather simple setInterval, but here’s a fancier answer with a function that accepts a start, end, and duration, so you can treat it more like an animation:

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Keeping it to CSS, we could use CSS counters to animate a number by adjusting the count at different keyframes:

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Another way would be to line up all the numbers in a row and animate the position of them only showing one at a time:

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Some of the repetitive code in these examples could use a preprocessor like Pug for HTML or SCSS for CSS that offer loops to make them perhaps easier to manage, but use vanilla on purpose so you can see the fundamental ideas.

The New School CSS Solution

With recent support for CSS.registerProperty and @property, we can animate CSS variables. The trick is to declare the CSS custom property as an integer; that way it can be interpolated (like within a transition) just like any other integer.

@property --num {
  syntax: '<integer>';
  initial-value: 0;
  inherits: false;

div {
  transition: --num 1s;
  counter-reset: num var(--num);
div:hover {
  --num: 10000;
div::after {
  content: counter(num);

Important Note: At the time of this writing, this @property syntax is only supported in Chrome ( and other Chromium core browsers like Edge and Opera), so this isn’t cross-browser friendly. If you’re building something Chrome-only (e.g. an Electron app) it’s useful right away, if not, wait. The demos from above are more widely supported.

The CSS content property can be used to display the number, but we still need to use counter to convert the number to a string because content can only output <string> values.

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See how we can ease the animations just like any other animation? Super cool!

Typed CSS variables can also be used in @keyframes:

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One downside? Counters only support integers. That means decimals and fractions are out of the question. We’d have to display the integer part and fractional part separately somehow.

Can we animate decimals?

It’s possible to convert a decimal (e.g. --number) to an integer. Here’s how it works:

  1. Register an <integer> CSS variable ( e.g. --integer ), with the initial-value specified
  2. Then use calc() to round the value: --integer: calc(var(--number))

In this case, --number will be rounded to the nearest integer and store the result into --integer.

@property --integer {
  syntax: "<integer>";
  initial-value: 0;
  inherits: false;
--number: 1234.5678;
--integer: calc(var(--number)); /* 1235 */

Sometimes we just need the integer part. There is a tricky way to do it: --integer: max(var(--number) - 0.5, 0). This works for positive numbers. calc() isn’t even required this way.

/* @property --integer */
--number: 1234.5678;
--integer: max(var(--number) - 0.5, 0); /* 1234 */

We can extract the fractional part in a similar way, then convert it into string with counter (but remember that content values must be strings). To display concatenated strings, use following syntax:

content: "string1" var(--string2) counter(--integer) ...

Here’s a full example that animates percentages with decimals:

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Other tips

Because we’re using CSS counters, the format of those counters can be in other formats besides numbers. Here’s an example of animating the letters “CSS” to “YES”!

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Oh and here’s another tip: we can debug the values grabbing the computed value of the custom property with JavaScript:


That’s it! It’s amazing what CSS can do these days.


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