No. 256 · September 8, 2026 · Moderate

Invert

Implement Invert<T> so the values of T become the keys and the keys become the values.

Try the puzzle yourself

Par 3

Puzzle

invert-object.ts

type Invert<T extends Record<string, string>> = ???

type Invert<T extends Record<string, string>> = ???

Stroke 1 of 3Not run yet

Replace ??? — your solution is checked against the cases below. Tab indents; press Escape then Tab to move focus out.

Checks

Invert<{ a: 'x'; b: 'y' }>—
→ { x: 'a'; y: 'b' }
Invert<{ a: 'x' }>—
→ { x: 'a' }
Invert<{}>—
→ {}

How a check is judged Exact type equality, not assignability — an intersection is not the same as the flattened object.

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Annotated solution

Published September 9, 2026

The solution

type Invert<T extends Record<string, string>> = { [K in keyof T as T[K]]: K }

The common wrong answer

type Invert<T extends Record<string, string>> = { [K in keyof T as T[K]]: T[K] }

Half the swap. The key is renamed to the old value, which is right, and then the value is set to the old value as well — producing { x: "x" } where the check wants { x: "a" }. Renaming a key does not move the old key anywhere; if you want it, you have to put it in the value clause yourself.

Line by line

as T[K]
The new key is the old value. as accepts any expression that produces a key type, and an indexed access is one.
: K
And the new value is the old key, still in scope, still a literal. The two clauses are independent — which is what makes a clean swap possible in one line.

Takeaway

A mapped type makes two separate decisions per key: what it is called, and what it holds. Most mapped-type bugs are one of those two answers accidentally reused for the other.

Uses

Mapped types Mapped types Partial Pick Omit Record Make Omit catch typos Flatten an intersection Require at least one property Make two prop shapes mutually exclusive