TypeScript · Senior · Question 69 of 75

How do you type a function that preserves the exact keys of an object passed to it while transforming values?

function mapValues<T extends object, R>(obj: T, fn: (v: T[keyof T]) => R): { [K in keyof T]: R } { /* ... */ }

✓Correct answer: Use a mapped type over keyof T in the return type
2Return Record<string, R>
3Return object

Explanation

Callers keep autocompletion and typo checks on keys. Note Object.entries inside still loses key types and needs a cast.

Further reading

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