Composable value pipelines

Popmotion provides simple utility functions that can take a value and return it transformed. These are, unsurprisingly, called transformers.

import { transform } from 'popmotion';
// Or save your user's bytes!
import * as transform from 'popmotion/transformers';

These functions can perform a wide range of tasks. Something as simple as appending a unit:

const { appendUnit } = transform;
const px = appendUnit('px');
px(5); // '5px'

Make an infinite looping sequence:

wrap(0, 100)(150); // 50

Or converting a value from one range to another:

interpolate([0,1], [-500, 500])(0.5); // 0


As you can see, many of these transformers are curried. So we can make a function:

const restrictNormalised = clamp(0, 1);

And then reuse it elsewhere:

restrictNormalised(5); // 1


Because these functions are very simple and all carry the same signature, we can compose them. Popmotion provides a special transformer to do just that.

const { pipe } = transform;

pipe is named as such because it takes a list of functions and returns a new function that will run these functions from left to right. Essentially, creating a value pipeline.

As our functions are descriptively named, and as many of them are curried, our pipelines become very easy to read. They become less imperative, and more declarative.

For instance, let’s compose a function that will always return a valid RGB value. RGB values are simply integers between 0 and 255.

const rgbUnit = pipe(
  clamp(0, 255),

What’s cool about this example is that it shows that any function that 1) takes a number and 2) returns a number, can be composed. Math.round does exactly that, so we can compose it.

You don’t have to write this function yourself, because that exact code is already included as a Popmotion transformer, and used in the rgba value type.

The rgba transformer is itself composed. Here’s the exact code:

const rgba = pipe(
    red: rgbUnit,
    green: rgbUnit,
    blue: rgbUnit,

This is an example of composing functions which were themselves composed. This is a very clear way of expressing and reusing our logic.

Applying transformers to animations

Every Popmotion action and reaction has a native pipe function.

Providing a list of functions to pipe will return a new instance of the action or reaction, and whenever its update function is called, the value will be passed through these functions before being emitted.

Consider this tween:

tween({ from: 0, to: 255 });

We can use our rgbUnit transformer from before to ensure that whenever this tween is called, it always returns a valid RGB unit:

tween({ from: 0, to: 255 })

This is a versatile approach to adding functionality to any animation. For instance, many animation libraries offer an option to create stepped tweens, but with this kind of composition we can easily bring that same functionality to physics (or any other animation).

Here, we can easily create something that spins at a constant velocity, outputting an angle value that snaps to 45 degree intervals:

physics({ velocity: 100 })



These pure, simple functions are easily composed and reused. They can be used on their own, or with any action (not just tweens), making them extremely versatile.

We believe this functional approach gives developers the greatest flexibility and predictability.

We’ve covered just some of the many transformers here, but more are documented in our API docs. As they’re pure functions, not specific to Popmotion, you can easily have fun creating your own.