Can someone explain what '[P in keyof O]: O[P];' means?

user_4634044 0
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I am a typescript starter and need to create a passport strategy. I stumbled across a line which I completely don't understand (complete code see here).

type StrategyCreated<T, O = T & StrategyCreatedStatic> = {
    [P in keyof O]: O[P];
};

Can someone explain in plain English what this means?

typescript
2 Answers

It a mapped type. You can read mode about them here. The basic idea is that it maps a given type (O in your case) to another type.

It does this by iterating each key from O (keyof O) in the P type parameter ([P in keyof O]) and assigning a new type for that key. In this case the type is the same as the original type of the P property in O (O[P]).

This particular type just maps O (which is an intersection of T and StrategyCreatedStatic) to a type with the same properties as the original intersection. The aim is probably to remove the intersection from the resulting type.

user_1257340

The keyof O part is a list of all keys of an Object. P in declares that the value of P should be contained in a list of possible values which in your example would be the keys of O, which is actually just a proxy for the type T & StrategyCreatedStatic. So the line reads "[Properties contained in the keys of type O]: O[P];" "O[P]" determines the value type of the Property.

As an example:

interface Foo {
    hello: string;
    world: number;
}

type StrategyCreated<T, O = T & StrategyCreatedStatic> = {
    [P in keyof O]: O[P];
};

const a: StrategyCreated<Foo> = { hello: "one", world: 2 }; // valid
const b: StrategyCreated<Foo> = { foo: true, bar: false }; // invalid
user_53805840
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