Typescript Key-Value relation preserving Object.entries type

Here's a solution, but beware when using this as a return type for Object.entries; it is not always safe to do that (see below).

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When you want to pair each key with something dependent on that key's type, use a mapped type:

type Entries<T> = {
    [K in keyof T]: [K, T[K]];
}[keyof T][];

type Test = Entries<Obj>;
// (["a", number] | ["b", string] | ["c", number])[]

The second version, which has a tuple type containing the properties instead of a union, is much harder to construct; it is possible to convert a union to a tuple but you basically shouldn't do it.

The third version is manageable, but a bit more complicated than the first version: you need PickByValue from this answer.

type Entries3<T> = {
    [K in keyof T]: [keyof PickByValue<T, T[K]>, T[K]]
}[keyof T][];

type Test3 = Entries3<Obj>;
// (["a" | "c", number] | ["b", string])[]

Playground Link

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I guess I should also explain why Typescript doesn't give a stronger type to Object.entries. When you have a type like type Obj = {a: number, b: string, c: number}, it's only guaranteed that a value has those properties; it is not guaranteed that the value does not also have other properties. For example, the value {a: 1, b: 'foo', c: 2, d: false} is assignable to the type Obj (excess property checking for object literals aside).

In this case Object.entries would return an array containing the element ['d', false]. The type Entries<Obj> says this cannot happen, but in fact it can happen; so Entries<T> is not a sound return type for Object.entries in general. You should only use the above solution with Object.entries when you yourself know that the values will have no excess properties; Typescript won't check this for you.