Typescript 2.5 adds JSDoc type assertion support for javascript file via ts-check service.

First of all, you should make sure you have typescript@2.5 install:

sudo npm i -g typescript@2.5

Then add @ts-check to the top of js file:

// @ts-check

This tell typescript to check the file.

Check type assertion we can do:

// index.js

function shouldBeNumber (/** @type {Number} */ num) {

    'use strict';

    console.log(num)

}

In VScode, if will use the function by passing the wrong param, we will get error in IDE.

shouldBeNumber("100");

But this doesn't stop the compiler, the code will still be compiled without error.