Daniel Eden, Designer

Square Peg, Round Hole: Typed Programming For Designers

Think back to the last time that you cooked a meal. The recipe you followed might have called for a particular ingredient, like yellow onions, but upon raiding your kitchen, you found you only had white onions.

Depending on the recipe, this substitution is probably just fine—but your dinner guests might have been disappointed if you’d chosen to use peaches in place of an onion.

In software programming, there’s a very similar problem that’s solved by something called “typing.” If you were to “type” the recipe above, you might say that it requires yellow (or white) onions. If our recipe could talk back to us, it would kick up a fuss if we tried to swap onions for peaches.

Similar to how following a recipe and not making strange substitutions usually yields a more enjoyable meal, using typed programming languages can help create more resilient design systems with a better experience for the people using them.

A “typed” programming language means a programming language that has certain expectations and rules about the kinds (or types) of data being passed between functions and operations.

As an example, you might have a “sum” function that adds two numbers together and returns the result.

function sum(a, b) {
    return a + b

sum(1, 2)
// => 3

It doesn’t make sense to try and add together a number and a string. In order to prevent people from trying to make that operation happen, you would add types to the function to limit what arguments people can pass to it.

// These are “type signatures”, indicating the types
// of data the function can accept and return
//              👇         👇       👇
function sum(a: number, b: number): number {
    return a + b

sum("hello", 2)
// => Error because type 'string' is incompatible with 'number'

sum(1, 2)
// => 3

This tells our code compiler—as well as people wanting to use our function—that sum is a function that accepts two number arguments, and returns a number as a result.

Typing in Design Systems

How does the concept of typing help us create resilient Design Systems? Before even writing any code, typing can help us think about the limits of the components we’re creating.

Let’s start with a simple button component. Our Button component is essentially just a rectangle with some text inside it.


But what would happen if we were to put another Button inside this one?


This obviously shouldn’t be permitted: it’s unlikely that someone would intentionally do this, but it could happen by accident. If our Button component was strictly typed, we could prevent anything other than a string being rendered inside of it:

function Button(label: string) {

This probably seems like an obvious property for buttons: they shouldn’t contain other buttons. But what about other, more complex or generic kinds of components? For example, there are many kinds of components that are designed to house all sorts of information: generic card containers, tooltips, or lists. Let’s take a look at a list component as an example.

Our example list will be a list of cities, with each item in the list showing three pieces of information:

Before writing any code, or even designing what the list should look like, we have some information at hand about the types of data we’ll be dealing with: the city’s name and country should be strings of text, and the photo would be an image. This image might be represented just by a string itself, with a URL to the image on a server, or it may be another component itself.

Let’s look at those pieces of information again, but this time we’ll type it the same way that the engineer implementing the list might do:

In Flow or Typescript, type-checking systems for JavaScript, the type signature for our list item might look like this:

type CityListItemData = {
    name: string
    country: string
    photo: string | Image

Designing components and products with such types in mind has many benefits.

First, without even looking at or creating a visual design for this list component, just reading the information it requires and the data types for that information gives us a good picture of what that component might look like.

Second, it helps make it clear to the engineers you’re working with how they should approach building the component: they know, for instance, that we should be prepared to guard against people trying to use a number for the city’s photo, or an image for its name.

And third, it creates an abstract “shape” for the information associated with a city that you might be able to use in other places: perhaps instead of a list, you want to create a table, or show a particular city’s information in a single card. When the data has a shape, you can more easily think about other ways of representing the data.

Designers tend to do this naturally throughout their process of iteration: given a set of information, we explore different ways to display that information. Explicitly acknowledging what types of data to expect in a given design can lead us to more robust solutions and enables us to work more closely with engineers to avoid unpredictable or broken experiences.

Strict typing can help prevent errors in engineering, making software more predictable, harder to exploit, and easier to maintain. Applying the same concepts to product design, particularly in design systems, can provide similar benefits at a higher product level, speeding up the overall product development process, as well as exposing potential risk in a product or flow.