Hey there, TypeScript tinkerers! 👋

Ever thought about how doesReturnType<T> works under the hood? Today we’re going to build our own version of ReturnType<T> from the ground up, so you can see how it works under the hood. Let’s dive in!

Why do I need to extract a Function’s Return Type?

Imagine you are importing a function from an external library that did not export its return type.

You can easily grab its return type using ReturnType<T> util and use it anywhere you want -

import {fetchUser} from 'some-external-library';

export type User = ReturnType<typeof fetchUser>;

Before implementing this, let’s first understand the infer keyword.

Meet infer: TypeScript’s “Guess What This Is” Keyword

Think of infer like guessing someone’s coffee order based on the smell—TypeScript smells inside a type, grabs the bit you care about, and stores it in a new type variable.

The classic pattern looks like this:

type Foo = T extends /* something */ ? /* use infer here */ : never;

When T matches the pattern, the compiler “infers” the piece you asked for.

Building MyReturnType<T> Step-by-Step

Let’s write our own version, MyReturnType<T>:

type MyReturnType<T> = T extends (...args: any[]) => infer R ? R : never;

Breakdown

1. T extends (...args: any[]) => infer R
   We ask: “Hey TypeScript, is T a function type?”
   We promise: “If yes, you may infer its return type and call it R.”

2. ? R : never
   If the condition is true, yield that inferred R; otherwise, default to never.

That’s it — just four lines!

Putting MyReturnType<T> to Work

function makeSmoothie(flavor: "mango" | "berry") {  
  return flavor === "mango"  
    ? { flavor, calories: 250, fresh: true }  
    : { flavor, calories: 200, fresh: true };  
}

type Smoothie = MyReturnType<typeof makeSmoothie>;  
// { flavor: "mango" | "berry"; calories: number; fresh: boolean }

Conclusion

Knowing how to implement your own ReturnType<T> isn’t just a neat party trick—it deepens your understanding of conditional types and paves the way to craft bespoke type utilities for your codebase.