How do you tell Node.js to treat your files as ES modules?

Node has to decide how to read each .js file, and it cannot guess from the content alone. It looks for two hints: the file extension and the nearest package.json.

• .mjs is always an ES module.
• .cjs is always CommonJS.
• .js follows the "type" field of the closest package.json: "module" means ESM, anything else (including no field) means CommonJS.

{
  "name": "fest-registration",
  "version": "1.0.0",
  "type": "module",
  "main": "src/index.js"
}

With this file, import express from "express" works in src/index.js. Without "type": "module" you would see SyntaxError: Cannot use import statement outside a module.

Rules that change once you switch to ESM:

• Relative imports need the full extension: ./utils.js, not ./utils.
• require, module.exports, __dirname are not available.
• You can use await at the top level.

• Forgetting the .js extension in relative imports.
• Leaving old require calls in a package that is now ESM.