As a new Typescript user looking to build a passport strategy, I came across a line of code that has me completely baffled. The snippet is as follows: here.
The type StrategyCreated<T, O = T & StrategyCreatedStatic> = {
[P in keyof O]: O[P];
};
I need someone to kindly break this down for me in everyday language. What exactly does it mean?
It's a specialized type known as a mapped type. If you want to learn more about them, check out this resource. The main concept behind a mapped type is that it associates one type (designated as O in your scenario) with another type.
This process involves iterating through each key of type O (keyof O) within the P type parameter ([P in keyof O]) and assigning a new type for each key. In this instance, the assigned type matches the original type of the P property in the O object (O[P]).
Specifically, this type maps the intersection of T and StrategyCreatedStatic onto a new type with identical properties as the initial intersection. The purpose likely aims to eliminate the intersection from the final type outcome.
In the part labeled keyof O, a complete list of keys belonging to an Object is specified.
The use of P in signifies that the value assigned to P should be found within a set of potential values, which in this scenario represents the keys of O. In reality, this serves as a representation for the type T & StrategyCreatedStatic. This statement can be interpreted as "[Properties contained within the keys of type O]: O[P];" where "O[P]" defines the value type associated with the Property.
To illustrate this concept:
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
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