No. 213 · July 21, 2026 · Moderate

Unwrap

Implement `Unwrap<T>` so it strips every layer of `Promise`, however deeply nested. A non-promise passes through unchanged.

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unwrap-promise.ts

type Unwrap<T> = ???

type Unwrap<T> = ???

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Published July 22, 2026

type Unwrap<T> = T extends Promise<infer Inner> ? Unwrap<Inner> : T

type Unwrap<T> = T extends Promise<infer Inner> ? Inner : T

This peels exactly one layer. `Promise<Promise<number>>` becomes `Promise<number>` and stops. The recursive call is what makes it total.

T extends Promise<infer Inner>
Pattern-matches `T` against the shape `Promise<something>` and binds that something to `Inner`.
? Unwrap<Inner>
Recursion. `Inner` may itself be a promise, so the same question is asked again until the answer is no.
: T
The base case. Not a promise, so `T` is already the value type.

A conditional type that calls itself in its true branch is the type-level equivalent of a `while` loop. The false branch is the exit.