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Extracting TypeScript types from functions, objects & arrays

Tips & tricks for reverse-engineering function, object & array TypeScript types from 3rd-party libraries that fail to export them

June 06, 2021 · 6 min read

A 3rd-party library written in TypeScript likely makes use of lots of internal types to support its API. Libraries typically export additional helper types that we may need in order to use the API. But sometimes the library forgets types or there are types that they did not expect we would need. So I want to share some TypeScript tips for extracting types trapped in functions, objects, and arrays.

Let’s say that we’re using the following function from a library:

// index.ts (in @third-party-lib/api)

interface Player {
  name: string
  dob: Date
  number: number
  position: string
  height: number
  weight: number
}

interface Location {
  city: string
  state: string
}

interface TeamInfo {
  name: string
  location: Location
  players: Player[]
}

interface Team extends TeamInfo {
  id: string
  dateCreated: Date
}

export const createTeam = (info: TeamInfo): Promise<Team> => {  // do stuff asynchronously...}

The createTeam function takes a single parameter that’s an object with three properties: name, location, and players. It returns a Team object wrapped in a Promise because the function is async.

Let’s say the common use case for calling createTeam() is to pass a big object literal:

import { createTeam } from '@third-party-lib/api'

const run = async () => {
  const team = await createTeam({
    name: 'Houston Rockets',
    location: { city: 'Houston', state: 'TX' },
    players: [
      {
        name: 'Christian Wood',
        dob: new Date('9/27/1995'),
        number: 35,
        position: 'C',
        height: 82,
        weight: 214,
      },
      {
        name: 'Kevin Porter Jr.',
        dob: new Date('5/4/2000'),
        number: 3,
        position: 'SG',
        height: 76,
        weight: 203,
      },
      // ...
    ],
  })
}

run()

Even though we never declared the Player or Location types directly, TypeScript is able to associate them with the object literals. This is because the types of the properties of the object literals match the properties defined in the TypeScript interfaces.

But what if we need to use the Player or Location type in our application outside of calling createTeam?

const validatePlayer = (player: Player): boolean => {  // code that validates a `Player` object
}

And what if this library failed to export the Player, Location, and other types? How would we go about getting them? Well, we could create our own mirror types from inspecting the declaration files. But then they could get out of sync. Instead, let’s extract the types directly from the createTeam() function.

Let’s first start with extracting the parameters of createTeam() using the Parameters<T> generic utility type.

type CreateTeamParams = Parameters<typeof createTeam>
// CreateTeamParams = [info: TeamInfo]

The CreateTeamParams is an array type of a single element, the TeamInfo type. Again, TeamInfo wasn’t exported, so we need a way to get the TypeScript type out of the array type. We can use indexed access types to extract the TeamInfo type.

type ExtractedTeamInfo = CreateTeamParams[0]
// ExtractedTeamInfo = TeamInfo

Nice! We can index into the array type much like we can index into a normal JavaScript array object. So we now have TeamInfo which we’ve named ExtractedTeamInfo. I would normally just name it TeamInfo, but to be clear that it’s a type alias of the actual TeamInfo interface, I’ve named it ExtractedTeamInfo.

So now that we have ExtractedTeamInfo we need to get both the Player and Location types. Let’s try getting Location first.

type ExtractedLocation = ExtractedTeamInfo['location']
// ExtractedLocation = Location

Simple enough right? Just like we can index into an array type, we can also get the types of properties of interfaces / object types using indexed access types. We combine both types of indexed access types to retrieve Player.

type ExtractedPlayers = ExtractedTeamInfo['players']
// ExtractedPlayers = Player[]
type ExtractedPlayer = ExtractedPlayers[number]
// ExtractedPlayer = Player

Noticed that we used [number] to get the ExtractedPlayer. ExtractedPlayers is an array of Player types, so we’re not looking for one at a specific index. Using [number] gets us the type of the array’s elements, i.e. Player.

Now we have ExtractedLocation & ExtractedPlayer as types to use as we need.

const validatePlayer = (player: ExtractedPlayer): boolean => {  // code that validates a `Player` object
}

Oh, but what about Team? It’s wrapped up in the Promise returned by createTeam(). So first let’s get that return type of createTeam().

type CreateTeamPromise = ReturnType<typeof createTeam>
// CreateTeamPromise = Promise<Team>

We use the ReturnType<T> generic utility type to get the type Promise<Team>.

And finally we need to unwrap the promise type so we can get just Team. Unfortunately, there’s no simple index type that we can use to remove the Promise<T>. But we can create our own conditional type to do the unwrapping.

// checks to see if the `Type` is a promise wrapping an underlying type.
// if so it returns the underlying type. if not, it returns back the type.
type Unwrapped<Type> = Type extends Promise<infer WrappedType>  ? WrappedType  : Type
type ExtractedTeam = Unwrapped<CreateTeamPromise>
// ExtractedTeam = Team

So this is definitely some advanced TypeScripting going on here. Let me try to break it down. Not only is Unwrapped<Type> a generic conditional type, but it’s using type inference within a generic conditional type. The infer keyword allows us to “peek into” the generic type to see what it might be. If it matches Promise<WrappedType> then the conditional returns WrappedType (the type the promise was wrapping). Otherwise, it’ll just return the entire type. This “unwrapping” pattern can be used for any custom generic types you define as well. Just replace Promise with your type.


I sometimes have to use this process to extract types of props from 3rd-party React components. Because React components are just functions, the process is the same.

import { SomeComponent } from '@third-party/components'

// get the parameters of the function component
// then grab the first parameter which is the props
type ComponentProps = Parameters<typeof SomeComponent>[0]

// grab the "players" prop type which is a Player[]
// grab the single type from the array
type PlayerProp = ComponentProps['players'][number]

All of this hoop jumping encourages me to be liberal in exporting types from my own shared libraries. However, I don’t really worry about missing types too much, because at least folks have a way to get out the types they need.

The funny thing is that all of this information is in the TypeScript docs. But they can be quite dense and it’s sometimes hard to apply the technical topics to our specific use cases. Hopefully this post helps you out the next time you need to extract missing types. Let me know if it does by hitting me up on Twitter at @benmvp.

Keep learning my friends. 🤓

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Hi, I'm Ben Ilegbodu. 👋🏾

I'm a Christian, husband, and father of 3, with 15+ years of professional experience developing user interfaces for the Web. I'm a Principal Frontend Engineer at Stitch Fix, frontend development teacher, Google Developer Expert, and Microsoft MVP. I love helping developers level up their frontend skills.

Discuss on Twitter // Edit on GitHub