Declaring JavaScript extended functions in TypeScript

Question

I have this type of Node.js module written in JavaScript:

function main(options) {
    return "some string";
}

main.methodName = function () {
    // implementation;
};

main.objectName = {
    // a namespace;
};

main.propertyName = 123;

module.exports = main;

What is the proper way of declaring such an interface in TypeScript?

CLARIFICATION

I'm asking about how to properly declare such an interface in TypeScript for an existing Node.js module, so it can be used correctly from TypeScript files, not how to re-implement such interfaces in TypeScript.

UPDATE

Following suggestion from @toskv, I have added the following interface:

declare module "my-module" {

    // Default library interface
    interface main {
        (options?:{}):string,
        methodName():any,
        propertyName:any,
        objectName:Object
    }

    export default main;
}

But if I use it like this:

import * as myModule from "my-module";
var s = myModule({});

Then I'm getting error Cannot invoke an expression whose type lacks a call signature..

Any idea why?

TypeScript is a superset of JavaScript, which means this code, which is valid JavaScript, is also valid TypeScript. — Michał Perłakowski
– Michał Perłakowski, Commented Apr 12, 2016 at 7:32
@Gothdo while that is true, TypeScript makes it a pain to extend objects after their types are defined because of the static typing. Like adding properties to a function. It's much easier and cleaner to transform the code to a class. :) — toskv
– toskv, Commented Apr 12, 2016 at 7:34

toskv · Accepted Answer · 2016-04-12 07:56:24Z

3

A TypeScript interface describing that code would be:

interface MainIf {
    (options) : string ; // the main function
    methodName() : any;
    propertyName: number;
    objectName: Object;
}

edited Apr 12, 2016 at 7:56

answered Apr 12, 2016 at 7:32

toskv

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9 Comments

toskv Over a year ago

indeed he doesn't, but it is cleaner this way. :) the alternative is to add type assertions to <any> when adding the extra properties on the main function. I'll make an example of that too. :)

vitaly-t Over a year ago

I think there maybe some misunderstanding. I was asking for a declaration interface for an existing Node.js module, not for a way to implement a new class that works in the same way.

toskv Over a year ago

@vitaly-t I added another example that just patches the current code to have it work in the typescript compiler. The generated js code for the 2 should be equivalent.

toskv Over a year ago

@vitaly-t I did, does that fit?

vitaly-t Over a year ago

@toskv in my module main(options) is a function that returns a string, it is not in any way a class. How to amend it then?

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