Infer Function Return Type In Typescript

Scenario

You are using a function called doFunkyStuff from a library called funky-lib. Do doFunkyStuff returns an interface FunkyStuff, but funky-lib does not export FunkyStuff

The lib code

// node_modules/funcky-lib/index.ts

interface FunkyStuff {
    message: string,
    funkinessLevel: number
}

export function doFunkyStuff(): FunkyStuff {
   console.log('doing funky stuff')

   return { 
      message: 'did some funky stuff',
      funkinessLevel: Math.random(),
   }
}

Our Code

// index.ts
import { doFunkyStuff } from 'funky-stuff'

// eslint yells no-explicit-any
function repeatFunkyStuff(fStuff: any) {
  console.log('repeating funky stuff', fStuff)

  return {...fStuff}
}

repeatFunkyStuff(doFunkyStuff)

Depending on your eslint-config, eslint would either yell "no-implicit-any" or "no-explicit-any"

Bad Solution

Redefine FunkyStuff

function repeatFunkyStuff(fStuff: { message: string; funkinessLevel: number})

The solution above is bad because it becomes unreadable if FunkyStuff has more than a handful of properties.

Also, we must update our code each time something changes in FunkyStuff.

Good solution

Use the ReturnType utility type.

// our code in index.ts
...
type AppFunkyStuff = ReturnType<typeof doFunkyStuff>

function repeatFunkyStuff(fStuff: AppFunkyStuff) {
   ...
}
...

Bonus

You can combine ReturnType<T> with Awaited<T> util type for functions that return promises.

Assuming doFunkyStuff returns Promise, then AppFunkyStuff would be defined like this.

type AppFunkyStuff = Awaited<ReturnType<typeof doFunkyStuff>>

⚠️ Awaited util became only available in typescript 4.5. Achieving the same effect can be a little tricky. Previous versions

Summary

This tip is just the tip of the iceberg regarding what typescript offers.