Typescript: how to define an invokable object?

Question

This comes from @types/d3:

export interface ScaleContinuousNumeric<Range, Output> {
    (value: number | { valueOf(): number }): Output;

but that's an interface. How do I define a class, the instances of which can be invoked like functions? Something like this:

const inverter = new Inverter();
const question = inverter(42);

Maybe there is some kind of special ES6 Symbol like

class Inverter {
    [Symbol.invokable]() {

    }
}

or TypeScript syntax?

Answer 1

You just define a regular function, that will satisfy the interface with the call signature:

export interface ScaleContinuousNumeric<Range, Output> {
    (value: number | { valueOf(): number }): Output;
}

let a : ScaleContinuousNumeric<number, string> = (value) => value.toString()

If the interface has other methods you can use Object.assign to create the function and add the properties:

export interface ScaleContinuousNumeric<Range, Output> {
    (value: number | { valueOf(): number }): Output;
    otherValue: Range
}

let a: ScaleContinuousNumeric<number, string> = Object.assign(
    (value: number | { valueOf(): number }) => value.toString(), {
        otherValue: 1
    });

Or in typescript 3.0 you can create the function, assign the extra properties and then the function will be compatible with the interface:

export interface ScaleContinuousNumeric<Range, Output> {
    (value: number | { valueOf(): number }): Output;
    otherValue: Range
}

function myFunc(value: number | { valueOf(): number }) {
    return value.toString()
}
myFunc.otherValue = 1
let a: ScaleContinuousNumeric<number, string> = myFunc