`void` Return

void as a return type is subtle. It means “the caller won’t use the return value” — not “the function MUST return nothing”.

The Surprise

type Callback = () => void;

const fn: Callback = () => 42;   // ✓ — returning 42 is allowed

A function returning 42 is assignable to () => void. The “return type” of void is a contract about callers, not about the function’s body.

Why It Helps — forEach

Array.forEach’s callback is typed (item: T, ...) => void. If void meant “must return undefined”, THIS would error:

const arr2: string[] = [];
["a", "b"].forEach(x => arr2.push(x));
//                       ~~~~~~ push returns a number — would error

But void is loose — push returning a number is fine because forEach doesn’t read the return value. The pattern just works.

When You DO Want “Returns Nothing”

If you genuinely require the function to return undefined, type it that way:

type StrictCallback = () => undefined;

Rare. Most of the time, void is what you want.

In an Annotated Function

function log(msg: string): void {
  console.log(msg);
  // return 42;   // ✗ Type 'number' is not assignable to type 'void'.
}

In a function with an EXPLICIT void return, you can’t return a value. The asymmetry is:

• Annotating void on the function: you can’t return a value
• Assigning a function to a () => void type: the implementation CAN return anything; callers can’t read it

void vs undefined — Quick Reference

Up Next

The last function lesson — predicates that narrow.