Declaration of TypeScript type

Question

Declaration of TypeScript type

Could anyone provide an explanation of what this TypeScript declaration means? I understand that type and interface can be used to define new data types, but this particular statement is confusing to me.

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

I came across this code snippet on https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/koa/index.d.ts

Your help would be greatly appreciated,

typescript typescript-typings

Answer 1

Answer №1

ParameterizedContext<StateT = DefaultState, CustomT = DefaultContext>

This code snippet marks the beginning of defining a generic type. The placeholders StateT and CustomT can be replaced with any type you prefer. If left blank, it defaults to DefaultState and DefaultContext.

For instance, if you instantiate a

ParameterizedContext<{ foo: string }, { bar: number }>

, then StateT will represent { foo: string } throughout the type, while CustomT will represent { bar: number }.

ExtendableContext & {
  state: StateT;
} & CustomT;

This signifies that the type inherits all properties from ExtendableContext, adds a state property of type StateT, and merges in all properties from CustomT.

To continue our example, if StateT is { foo: string } and CustomT is { bar: number }, this type would look like:

{
  // ... representing all ExtendableContext properties here (unknown)
  state: { foo: string },
  bar: number,
}

Answer 2

ParameterizedContext<StateT = DefaultState, CustomT = DefaultContext>

This code snippet marks the beginning of defining a generic type. The placeholders StateT and CustomT can be replaced with any type you prefer. If left blank, it defaults to DefaultState and DefaultContext.

For instance, if you instantiate a

ParameterizedContext<{ foo: string }, { bar: number }>

, then StateT will represent { foo: string } throughout the type, while CustomT will represent { bar: number }.

ExtendableContext & {
  state: StateT;
} & CustomT;

This signifies that the type inherits all properties from ExtendableContext, adds a state property of type StateT, and merges in all properties from CustomT.

To continue our example, if StateT is { foo: string } and CustomT is { bar: number }, this type would look like:

{
  // ... representing all ExtendableContext properties here (unknown)
  state: { foo: string },
  bar: number,
}

Answer 3

Answer №2

The & operator allows for the creation of intersection types. When you have a type A & B, it means that the type is both A and B, possessing all properties of A and all properties of B.

In your specific case, ParameterizedContext is essentially an extension of ExtendableContext with the addition of a property named state of type StateT, along with including all properties from CustomT.

StateT and CustomT are examples of generics. Generics allow for the reusability of type definitions by incorporating parameters.

By default, StateT is set as DefaultState, which is defined simply as any. On the other hand, CustomT defaults to being a DefaultContext defined by a Record<string, any>.

Answer 4

The & operator allows for the creation of intersection types. When you have a type A & B, it means that the type is both A and B, possessing all properties of A and all properties of B.

In your specific case, ParameterizedContext is essentially an extension of ExtendableContext with the addition of a property named state of type StateT, along with including all properties from CustomT.

StateT and CustomT are examples of generics. Generics allow for the reusability of type definitions by incorporating parameters.

By default, StateT is set as DefaultState, which is defined simply as any. On the other hand, CustomT defaults to being a DefaultContext defined by a Record<string, any>.

Declaration of TypeScript type

Answer №1

Answer №2

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