No. 257 · September 9, 2026 · Moderate

Tuple to object

Implement TupleToObject<T> so a tuple of literal keys becomes an object mapping each of them to itself.

Try the puzzle yourself

Par 3

Puzzle

tuple-to-object.ts

type TupleToObject<T extends readonly PropertyKey[]> = ???

type TupleToObject<T extends readonly PropertyKey[]> = ???

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

Checks

TupleToObject<['a', 'b']>—
→ { a: 'a'; b: 'b' }
TupleToObject<['x']>—
→ { x: 'x' }
TupleToObject<[]>—
→ {}

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

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Fewer than 5 people have solved this one so far. The distribution appears once there is enough of a sample to mean anything.

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Annotated solution

Published September 10, 2026

The solution

type TupleToObject<T extends readonly PropertyKey[]> = { [K in T[number]]: K }

The common wrong answer

type TupleToObject<T extends readonly PropertyKey[]> = { [K in keyof T]: T[K] }

Mapping over keyof T when T is a tuple gives you a *tuple* back, not an object — the mapping is homomorphic, so it preserves the arrayness along with everything else. ["a", "b"] maps to ["a", "b"], which is a perfectly good answer to a question nobody asked.

Line by line

T[number]
Indexing a tuple by number asks for "the type at any index", which is the union of everything in it: "a" | "b". That union is the key set you actually want to iterate.
{ [K in T[number]]: K }
Because the key set is computed rather than keyof T, the mapping is no longer homomorphic — and that is exactly why the result comes back as a plain object.

Takeaway

keyof T on a tuple means its indices; T[number] means its contents. Reaching for the wrong one turns an object into a tuple, or the other way round, and the error message rarely says so.

Uses

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