This comprehensive guide will help you understand the ParseError that occurs when 'import' and 'export' are not used with 'sourceType: module'. We'll walk through the reasons behind this error, and provide a step-by-step solution to resolve it. Additionally, we will cover an FAQ section with common questions and answers.
Table of Contents
Introduction to ES6 Modules
JavaScript ES6 (ECMAScript 2015) introduced a new feature called modules, which allows developers to organize and share their code more efficiently. Modules are a way to encapsulate functionality into separate files and import or export them as needed.
In ES6, there are two key module-related keywords:
import: Used to bring in functionality from another module.export: Used to expose functionality from the current module to others.
Example:
Let's say we have a file named utils.js with some utility functions:
// utils.js
export function add(a, b) {
return a + b;
}
export function subtract(a, b) {
return a - b;
}
And we want to use these functions in another file called app.js:
// app.js
import { add, subtract } from './utils.js';
console.log(add(3, 2)); // Output: 5
console.log(subtract(3, 2)); // Output: 1
Understanding the Error
The error 'import' and 'export' may only appear with 'sourceType: module' occurs when a JavaScript file is parsed as a script instead of a module. This can happen if the parser is not configured correctly to handle ES6 modules.
This error is commonly seen when using tools like Babel or ESLint, which have their own parsers to understand and transform JavaScript code.
Step-by-Step Solution
To resolve this error, follow these steps:
- Make sure you are using the correct parser configuration for ES6 modules.
- Verify the file extensions of your JavaScript files.
- Ensure your HTML file includes the
type="module"attribute when linking to your JavaScript files.
Step 1: Configure the Parser
Babel
For Babel users, make sure you have the babel-eslint package installed and configured in your .babelrc file:
{
"parser": "babel-eslint",
"parserOptions": {
"sourceType": "module"
}
}
ESLint
For ESLint users, configure the parserOptions in your .eslintrc file:
{
"parserOptions": {
"ecmaVersion": 6,
"sourceType": "module"
}
}
Step 2: Verify File Extensions
Make sure your JavaScript files have the correct file extensions. Some tools and bundlers expect a specific file extension for modules, such as .mjs. Check your tool's documentation for the recommended file extension.
Step 3: Include type="module" in HTML
When using modules in a browser environment, make sure to include the type="module" attribute in your <script> tags:
<script type="module" src="app.js"></script>
This tells the browser to treat the script as a module instead of a regular script.
FAQ
1. Can I use ES6 modules in all browsers?
Most modern browsers support ES6 modules natively. However, Internet Explorer and some older browsers do not support them. You can use a bundler like Webpack or Rollup to bundle your modules into a single JavaScript file that works across all browsers.
2. Can I mix ES6 modules with CommonJS modules?
Yes, you can use tools like Babel or Webpack to transpile and bundle your code, allowing you to use both ES6 and CommonJS modules together.
3. Can I use import and export in Node.js?
Starting from Node.js v12, you can use ES6 modules natively by setting the "type": "module" field in your package.json file or using the .mjs file extension. For earlier versions of Node.js, you can use the esm package to enable ES6 module support.
4. How do I use named and default exports together?
You can use both named and default exports in the same module like this:
// utils.js
export function add(a, b) {
return a + b;
}
export default function subtract(a, b) {
return a - b;
}
And import them like this:
// app.js
import subtract, { add } from './utils.js';
console.log(add(3, 2)); // Output: 5
console.log(subtract(3, 2)); // Output: 1
5. How do I use dynamic imports?
Dynamic imports allow you to load modules on demand, which can be useful for code splitting and lazy loading. Here's an example:
// app.js
import('./utils.js').then((utils) => {
console.log(utils.add(3, 2)); // Output: 5
console.log(utils.subtract(3, 2)); // Output: 1
});