What does <this> mean in TypeScript generics?

Question

We use express with TypeScript in our app. I came across their type definitions and was wondering what the following means:

export interface IRouter extends RequestHandler {
    all: IRouterMatcher<this>;
}

In particular, the IRouterMatcher<this> .

I read the docs a few times and couldn't find anything mentioning this use case. And it's pretty hard to search for <this> in either SO or the web as the angle brackets usually get stripped.

Answer 1

this refers to the current available type.

This might also be the subtype of the overridden class or interface.

See also Polymorphic this types in https://www.typescriptlang.org/docs/handbook/advanced-types.html

Example:

class Calculator {
    a: number;
    add(): this {
        a++;
        return this;
    } 
}

class AdvancedCalculator {
    substract(): this {
         a--;
         return this;
    } 
}

new AdvancedCalculator()
   .add() // returns AdvancedCalculator as "this" 
   .substact() // it compiles! 

Answer 2

I believe <this> , in your example, refers to the type that is currently implementing the interface. For example if a class named TestClass implemented IRouter, then the functions that return IRouterMatcher<this> would return an IRouterMatcher<TestClass> .

After doing some more research, using <this> might be functionally equivalent to saying <T extends IRouter> such as all: IRouterMatch<T extends IRouter> . Using this might just be a shorter way to express the same thing.