Thanks to VSC we can use almost every Typescript feature directly inside Javascript files thanks to JSDoc annotations

Update 2022: Microsoft proposal at ECMAScript to implement typing on runtime using annotations

Types definitions help to structure the code giving better autocomplete in code editors like VSC.

There're 2 alternatives to have full typescript support at .js files

• Adding this comment at the top of the .js file:

// @ts-check

OR

• Creating jsconfig.json with the same info as tsconfig.json in the root of the project

{
  "compilerOptions": {
    "allowJs": true,
    "esModuleInterop": true,
    "checkJs": true,
    "module": "ES2022",
    "target": "ES6",
    "typeRoots": ["src/types"],
    "lib": ["DOM"],
    "moduleResolution": "node",
    "resolveJsonModule": true,
    "jsx": "preserve"
  },
  "exclude": ["node_modules", "./src/_ui"]
}

We can import type definitions from .ts files (only types and interfaces), even from 3rd party libraries inside our project.

/**
 * @typedef {import('./filePath').default} MyType
 */

const x = /** @type {MyType} */ (value);

IMPORTANT don't forget to souround value inside parenthesis around value.