Use @ts-check and JSDoc to Experiment with TypeScript

Overview

You can add TypeScript type checking to JavaScript files without converting them to TypeScript. Use @ts-check at the top of a JS file and JSDoc annotations to add types. This lets you experiment with TypeScript gradually without committing to the full conversion.

When to Use This Skill

• Experimenting with TypeScript
• Gradually migrating JavaScript
• Adding types to JS files
• Teams learning TypeScript
• Validating JavaScript with types

The Iron Rule

Use @ts-check and JSDoc to add TypeScript checking to JavaScript files without full conversion.

Example

// @ts-check

/**
 * @param {string} name
 * @param {number} age
 * @returns {string}
 */
function greet(name, age) {
  return `Hello ${name}, you are ${age}`;
}

greet('Alice', 30); // OK
greet('Alice', '30'); // Type error!

/** @type {string[]} */
const names = ['Alice', 'Bob'];

/** @typedef {{ x: number, y: number }} Point */
/** @type {Point} */
const point = { x: 1, y: 2 };

Reference

• Effective TypeScript, 2nd Edition by Dan Vanderkam
• Item 80: Use @ts-check and JSDoc to Experiment with TypeScript