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TypeScript: Low maintenance types

Stefan Baumgartner

Written by @ddprrt

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I write a lot about TypeScript and I enjoy the benefits it gives me in my daily work a lot. But I have a confession to make, I don’t really like writing types or type annotations. I’m really happy that TypeScript can infer so much out of my usage when writing regular JavaScript so I’m not bothered writing anything extra.

That’s how I write TypeScript in general: I write regular JavaScript, and where TypeScript needs extra information, I happily add some extra annotations. One condition: I don’t want to be bothered maintaining types. I rather create types that can update themselves if their dependencies or surroundings change. I call this approach creating low maintenance types.

Scenario 1: Information is already available #

Let’s take a look at this brief and possibly incomplete copy function. I want to copy files from one directory to another. To make my life easier, I created a set of default options so I don’t have to repeat myself too much:

const defaultOptions = {
from: "./src",
to: "./dest",
};

function copy(options) {
// Let's merge default options and options
const allOptions = { ...defaultOptions, ...options};

// todo: Implementation of the rest
}

That’s a pattern you might see a lot in JavaScript. What you see immediately is that TypeScript misses some type information. Especially the options argument of the copy function is any at the moment. So let’s better add a type for that!

One thing I could do is creating types explicitly:

type Options = {
from: string;
to: string;
};

const defaultOptions: Options = {
from: "./src",
to: "./dest",
};

type PartialOptions = {
from?: string;
to?: string;
};

function copy(options: PartialOptions) {
// Let's merge default options and options
const allOptions = { ...defaultOptions, ...options};

// todo: Implementation of the rest
}

That’s a very reasonable approach. You think about types, then you assign types, and then you get all the editor feedback and type checking you are used to. But what if something changes? Let’s assume we add another field to Options, we would have to adapt our code three times:

type Options = {
from: string;
to: string;
+ overwrite: boolean;
};

const defaultOptions: Options = {
from: "./src",
to: "./dest",
+ overwrite: true,
};

type PartialOptions = {
from?: string;
to?: string;
+ overwrite?: boolean;
};

But why? The information is already there! In defaultOptions, we tell TypeScript exactly what we’re looking for. Let’s optimize.

  1. Drop the PartialOptions type and use the utility type Partial<T> to get the same effect. You might have guessed this one already
  2. Make use of the typeof operator in TypeScript to create a new type on the fly.
const defaultOptions = {
from: "./src",
to: "./dest",
overwrite: true,
};

function copy(options: Partial<typeof defaultOptions>) {
// Let's merge default options and options
const allOptions = { ...defaultOptions, ...options};

// todo: Implementation of the rest
}

There you go. Just annotation where we need to tell TypeScript what we’re looking for.

  • If we add new fields, we don’t have to maintain anything at all
  • If we rename a field, we get just the information we care about: All usages of copy where we have to change the options we pass to the function
  • We have one single source of truth: The actual defaultOptions object. This is the object that counts because it’s the only information we have at run-time.

And our code becomes a little bit terser. TypeScript becomes less intrusive and more aligned to how we write JavaScript.

David pointed out another example that falls into this category. With the const context, typeof and index access operators, you are able to convert a tuple into a union:

const categories = [
"beginner",
"intermediate",
"advanced",
] as const;

// "beginner" | "intermediate" | "advanced"
type Category = (typeof categories)[number]

Again, we maintain just one piece, the actual data. We convert categories into a tuple type and index each element. Nice!

Scenario 2: Connected Models #

I’m not against laying out your models, though. On the contrary, I think in most cases it makes sense to be explicit and intentional about your models and your data. Let’s take a look at this toy shop:

type ToyBase = {
name: string;
price: number;
quantity: number;
minimumAge: number;
};

type BoardGame = ToyBase & {
kind: "boardgame";
players: number;
}

type Puzzle = ToyBase & {
kind: "puzzle";
pieces: number;
}

type Doll = ToyBase & {
kind: "doll";
material: "plastic" | "plush";
}

type Toy = BoardGame | Puzzle | Doll;

That’s some great data modelling here. We have a proper ToyBase which includes all properties that are available with all the distinct toy types like BoardGame, Puzzle, and Doll. With the kind attribute we can create a distinct union type Toy where we can differentiate properly:

function printToy(toy: Toy) {
switch(toy.kind) {
case "boardgame":
// todo
break;
case "puzzle":
// todo
break;
case "doll":
// todo
break;
default:
console.log(toy);
}
}

If we need the information of those models in different scenarios, we might end up with more types:

type ToyKind = "boardgame" | "puzzle" | "doll";

type GroupedToys = {
boardgame: Toy[];
puzzle: Toy[];
doll: Toy[];
};

And this is where maintenance starts again. The moment we add a type VideoGame:

type VideoGame = ToyBase & {
kind: "videogame";
system: "NES" | "SNES" | "Mega Drive" | "There are no more consoles";
};

We have to maintain at three different spots:

- type Toy = BoardGame | Puzzle | Doll;
+ type Toy = BoardGame | Puzzle | Doll | VideoGame;

- type ToyKind = "boardgame" | "puzzle" | "doll";
+ type ToyKind = "boardgame" | "puzzle" | "doll" | "videogame";

type GroupedToys = {
boardgame: Toy[];
puzzle: Toy[];
doll: Toy[];
+ videogame: Toy[];
};

This is not only a lot of maintenance but also very error-prone. Typos may happen, as I could misspell the videogame key in GroupedToys or the string "videogame" in the ToyKind union.

Let’s use some of TypeScript’s built-in functionality to change that. I think there is no reasonable way of changing the first type we need to maintain, Toy, but that’s ok. Here it’s good to be explicit because we only want to include actual toys and not something that accidentally has the same basic features.

If we want to have a union type ToyKind with all possible kind types, it’s better to not maintain them on the side, but rather access the types directly.

- type ToyKind = "boardgame" | "puzzle" | "doll";
+ type ToyKind = Toy["kind"]

That does the same trick, thanks to us creating the Toy union.

We can use the newly created and self-maintaining ToyKind type to create a new, better GroupedToys type using mapped types:

type GroupedToys = {
[Kind in ToyKind]: Toy[]
}

And that’s it! The moment we change the Toy type with new information, we have updated information in ToyKind and GroupedToys. Less to maintain for us.

We can even go further. The GroupedToys type is not exactly what we’re looking for. When we group toys, we want to make sure that we only add Doll type objects to doll, etc. So what we need to is split the union again.

The Extract type gives us a great utility to do exactly that.

// GetKind extracts all types that have the kind property set to Kind
type GetKind<Group, Kind> = Extract<Group, { kind: Kind }>

type DebugOne = GetKind<Toy, "doll"> // DebugOne = Doll
type DebugTwo = GetKind<Toy, "puzzle"> // DebugTwo = Puzzle

Let’s apply that to GroupedToys:

type GroupedToys = {
[Kind in ToyKind]: GetKind<Toy, Kind>[]
};

// this is equal to

type GroupedToys = {
boardgame: BoardGame[];
puzzle: Puzzle[];
doll: Doll[];
}

Great! Better, more correct types at no maintenance! But there’s one thing that still bugs me. The property keys. They’re singular. They should be plural:

type GroupedToys = {
[Kind in ToyKind as `${Kind}s`]: GetKind<Toy, Kind>[]
};

// this is equal to

type GroupedToys = {
boardgames: BoardGame[];
puzzles: Puzzle[];
dolls: Doll[];
}

Great! And again, no maintenance for us. The moment we change something in Toy, we get a proper update in all other types.

Defining low maintenance types #

Usually, this is my approach if I want to create low maintenance types:

  1. Model your data or infer from existing models
  2. Define derivates (mapped types, Partials, etc)
  3. Define behavior (conditionals)

I discuss the last point extensively in my book TypeScript in 50 Lessons. And, as always, enjoy the playground and fiddle around with the results.

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