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Coding problem · Easy · Language Idioms

Typed pick() and omit()

Easy Language practicals Seen at Microsoft, Atlassian

Implement two helpers that are common in API layers (e.g. stripping password from a user before returning it):

function pick<T extends object, K extends keyof T>(obj: T, keys: readonly K[]): Pick<T, K>
function omit<T extends object, K extends keyof T>(obj: T, keys: readonly K[]): Omit<T, K>

Neither may mutate obj. pick only includes keys that exist on the object.

Hints

Hint 1

K extends keyof T means only real keys of T can be passed — typos become compile errors.

Hint 2

Copy with { ...obj } and delete for omit, or build a new object from entries.

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Solution and explanation

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Time O(keys) for pick, O(fields) for omit · Space O(fields)

keyof T constrains the key list at compile time, and the built-in mapped types Pick<T, K> / Omit<T, K> describe the result, so omit(user, ["password"]).password is a type error. At runtime, pick copies own properties only and omit deletes from a shallow copy.

Note that omit is a shallow copy — nested objects are still shared with the original.

Reference solution

function pick<T extends object, K extends keyof T>(obj: T, keys: readonly K[]): Pick<T, K> {
  const out = {} as Pick<T, K>;
  for (const key of keys) {
    if (Object.prototype.hasOwnProperty.call(obj, key)) out[key] = obj[key];
  }
  return out;
}

function omit<T extends object, K extends keyof T>(obj: T, keys: readonly K[]): Omit<T, K> {
  const copy = { ...obj };
  for (const key of keys) delete copy[key];
  return copy as Omit<T, K>;
}

Common mistakes

  • Mutating the input with delete obj[key].
  • Typing keys as string[], which loses the connection to T.

Follow-up questions

  • Write a DeepReadonly<T> type.
  • Use satisfies to validate config objects without widening.