Type checking in commonJS modeles

Question

Having such a simple JS app:

add.ts:

function add(r:number){
    return r + r;
}
exports.add = add;

app.ts :

var utils = require('./add.js');
console.log(utils.add(4));

I get no type checking error when passing in a string instead of a number:

...
console.log(utils.add("abc"));

There is no error info both in the IDE nor on the output of the npx tsc command. I know that using the ES6 syntax ( export...import ) type checking would work but my question is how to enable this checking in commonJS standard with exports...require ?

I've created the app using standard npm init -yes and have installed @types/node and typescript itself (using Visual Studio Code as IDE).

Answer 1

Typescript Doc shows you how to use that when module is CommonJS .

So in your snippets, do following changes.

// app.ts
import add = require('./add.js');

// add.ts
function add(r:number){
    return r + r;
}
export = add;