Is it possible to implement a function interface in TypeScript?

Question

I would like to be able to do this

class MyFunc extends ((s: string) => boolean) { ... }

so that an instance of MyFunc can be used as a function which takes a string as input and returns a boolean as follows:

const f = new MyFunc();
const b: boolean = f('someString');

Is this possible in TypeScript?

In languages such as Scala, one can extend the type String => Boolean, and provide an apply method to achieve this.

class MyFunc extends (String => Boolean)
val f = new MyFunc()
val b: Boolean = f("someString")

Answer 1

Perhaps you are thinking of something like this?

interface FunctionInterface {
    (s: string): boolean;
}

const f: FunctionInterface = s => true;
const b: boolean = f('someString');

Answer 2

There is no default apply concept in TypeScript, but there are ways of creating typed objects that are also functions.

interface MyCallable {
    (param1: string, param2: number): string;

    prop1: string;
    prop2: number;
    extraMethod: (param1: boolean) => boolean;
}

function makeMyCallable(prop1: string, prop2: number): MyCallable {
    let that = ((param1: string, param2: number) => param1 + param2) as MyCallable;

    that.prop1 = prop1;
    that.prop2 = prop2;
    that.extraMethod = (param1: boolean) => !param1;

    return that;
}

let mc = makeMyCallable("3", 4);

mc("3", 4);
mc.prop1 = "string";
mc.prop2 = 5;
mc.extraMethod(false);