You can stick types to JavaScript code, that's named "TypeScript Type Declaration", and the extension is .d.ts. When TypeScript (or also an editor with intellisense) sees a javascript file and a .d.ts file with the same name, it will import the type definitions from that file.

There's a project that aims to provide type definitions for every JavaScript library, maybe you've heard talking about DefinitelyTyped.

For example:

// example.js
function hello(name) {
  console.log(`Hello ${name}!`);
}


// example.d.ts
function hello(name: string);


// main.ts
import { hello } from 'example';
hello(42); // Error! "name" must be a string

The same system has been used to provide types in React