Enable type checking for .js files

By using the »@ts-check« comment/annotation in your plain JavaScript, you can turn on TypeScript’s type checking without actually moving the file to TypeScript.

Building on top of adding TypeScript typing via JSDoc comments for improved IntelliSense in JavaScript files, you can also enable proper type checking for plain .js files using the comment/annotation // @ts-check. Let’s take the following function as an example.

 * @param {string} title
 * @param {number} id
 * @returns string
const getPostSlug = (title, id) => {
  return `${title.toLowerCase()}-${id}`;

console.log(getPostSlug('Test', 123));
console.log(getPostSlug(123, 'Test'));

While IntelliSense already works and helps during development, the last line would cause a runtime exception due to a number being used where a string is expected (and number not having a toLowerCase() method).

By adding the line // @ts-check to the beginning of the .js file, TypeScript checking is enabled for that file, making you aware of that issue.

Of course, you can import any TypeScript typing for complex typing just as you would do in .ts files, e.g. of Webpack configuration files (i.e. import('webpack').Configuration in the following example).

// @ts-check
const path = require('path');
const { merge } = require('webpack-merge');

const { getCommonConfig } = require('../../webpack.common.config');

 * @param {string} env
 * @param {import('webpack').CliConfigOptions} argv
 * @return {import('webpack').Configuration}
const config = (env, argv) => {
  const commonConfig = getCommonConfig(env, argv);

  return merge(commonConfig, {
    entry: path.resolve(__dirname, './src/index.ts'),
    output: {
      filename: 'index.js',
      path: path.resolve(__dirname, 'dist'),
      libraryTarget: 'commonjs2',
    node: {
      __dirname: false,
    target: 'node',

module.exports = config;

This is especially useful for files where moving them to proper .ts files (such as webpack.config.js) may require bigger adjustments of the build process.