Functions

ReturnType

The type a function gives back.

What it is

A conditional type with infer in the return position — the canonical example of pattern-matching a type. infer R declares a variable that the compiler fills in by matching, and the true branch is where you get to use it.

Examples

```
ReturnType<typeof makeUser>
```
→{ id: number; name: string; }
```
ReturnType<() => void>
```
→void
```
ReturnType<() => string | number>
```
→string | number
A union return type comes back whole; nothing distributes here.

Each resolved type above was printed by TypeScript 6.0.3, not written by hand.

What it does not do

It does not take a function *value*. It takes a function *type*, which is why ReturnType<typeof fn> is the shape you always end up writing.
It does not resolve overloads the way a call would. Given an overloaded function it reports the last signature, not the one your arguments would have picked.

Takeaway

typeof crosses from the value world to the type world; ReturnType then works inside it. Almost every use of one involves the other.