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.