TypeScript Interfaces Generic Interfaces
Example
Like classes, interfaces can receive polymorphic parameters (aka Generics) too.
Declaring Generic Parameters on Interfaces
interface IStatus<U> {
code: U;
}
interface IEvents<T> {
list: T[];
emit(event: T): void;
getAll(): T[];
}
Here, you can see that our two interfaces take some generic parameters, T and U.
Implementing Generic Interfaces
We will create a simple class in order to implements the interface IEvents.
class State<T> implements IEvents<T> {
list: T[];
constructor() {
this.list = [];
}
emit(event: T): void {
this.list.push(event);
}
getAll(): T[] {
return this.list;
}
}
Let's create some instances of our State class.
In our example, the State class will handle a generic status by using IStatus<T>. In this way, the interface IEvent<T> will also handle a IStatus<T>.
const s = new State<IStatus<number>>();
// The 'code' property is expected to be a number, so:
s.emit({ code: 200 }); // works
s.emit({ code: '500' }); // type error
s.getAll().forEach(event => console.log(event.code));
Here our State class is typed as ISatus<number>.
const s2 = new State<IStatus<Code>>();
//We are able to emit code as the type Code
s2.emit({ code: { message: 'OK', status: 200 } });
s2.getAll().map(event => event.code).forEach(event => {
console.log(event.message);
console.log(event.status);
});
Our State class is typed as IStatus<Code>. In this way, we are able to pass more complex type to our emit method.
As you can see, generic interfaces can be a very useful tool for statically typed code.
