Clever Geek Handbook

Typed Methods in TypeScript

How do I determine the type of a class method in TypeScript? For a regular function, I would do

interface Listener { (foo: string, bar: any): void; }

// foo and bar will be typed according to the Listener interface
let listener: Listener = function(foo, bar) { };

Is it possible to declare a method with the Listener interface?

class Foo {

  // will warn about implicit 'any' types
  listener1(foo, bar) { }

  // I want to avoid this
  listener2(foo: string, bar: any): void { }
}

This will make it easier for me to override the method in subclasses, knowing that the methods must conform to the Listener interface.

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2 answers


If you want to make sure that only objects are passed to, for example, a function that "has" a listener function, you can also use an interface.

 interface IListener{
     listener(type: string, payload: any):void
 }

 function somethingElse( l:IListener ){
     l.listener( "hello", "heavy payload" );
 }


Hope it helps.

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class Foo {
    //as method
    foo(): boolean {
        return true;
    }

    //as lambda method
    bar = (): boolean => {
        return true;
    }
}


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