TypeScript 型号声明

Question

Can someone explain to me what exactly this declaration means in TypeScript? I know that with type and interface I can create new data types. But I really can not understand this statement.

 type ParameterizedContext<StateT = DefaultState, CustomT = DefaultContext> = ExtendableContext & {
        state: StateT;
    } & CustomT; 

I found this at https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/koa/index.d.ts

Thank you,

Answer 1

The & operator allow to create intersection-types . A & B is a type which is A and B, has all the properties of A and all the properties of B.

In your type, ParameterizedContext is an ExtendableContext plus a property called state of type StateT plus all the properties of CustomT

StateT and CustomT are generics . generics allow to reuse type definition by adding parameters.

StateT by default is an DefaultState which is defined as any . CustomT by default is a DefaultContext which is defined by a Record<string, any> .

Answer 2

ParameterizedContext<StateT = DefaultState, CustomT = DefaultContext>

This is the start of defining a generic type. StateT and CustomT are placeholders which you can fill in with any type you like, though if you don't fill them in it will use DefaultState and DefaultContext .

For example, if you create a ParameterizedContext<{ foo: string }, { bar: number }> , then StateT is becomes { foo: string } everywhere in the type and CustomT becomes { bar: number }

ExtendableContext & {
  state: StateT;
} & CustomT; 

This means that it has all of the properties of ExtendableContext, plus a state property who's type is StateT , plus all of the properties of CustomT .

So continuing the example from above, if StateT is { foo: string } and CustomT is { bar: number } , then this type is the following

{
  // ... imagine all the properties of ExtendableContext being here (i don't know what they are)
  state: { foo: string },
  bar: number,
}