type-challenges-solutions/en/medium-merge about type-challenges-solutions HOT 5 OPEN

A version that is a few characters shorter - keyof F | keyof S is the same as keyof F & S. But I could be missing something. :)

type Merge<F, S> = {
  [P in keyof (F & S)]: P extends keyof S
    ? S[P]
    : P extends keyof F
    ? F[P]
    : never;
};

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@zavarka clever 😺 I think there could be some edge-cases with properties in a sense how they will be treated when intersecting them, but for common cases should work, yes.

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Using a util function admittedly ( also from the 'Append to Object' challenge ) but the Collapse ( or Flatten ) just satisfies the tests - without it the solution is equivalent.

type Merge<F, S> = Collapse<S & {
  [K in keyof F as K extends keyof S ? never : K]: F[K]
}>

type Collapse<T extends { [k: string]: any }> = {
  [K in keyof T]: T[K]
}

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Why should we use k extends key F again because k is in keyof F | keyof S？

type Merge<F, S> = {
  [P in keyof F | keyof S]: P extends keyof S
    ? S[P]
    : F[P]
};

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@BruceYuj yes, because your key can be either in first set or another one, and you need to filter out one of them.

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