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6

In modern JavaScript development, using ECMAScript 6 (ES6) import and export statements has become a standard practice for organising code into reusable modules. However, developers often encounter the error:
Uncaught SyntaxError: Cannot use import statement outside a module.

This issue can be frustrating, especially for those new to ES6 modules or when transitioning legacy projects. Let’s explore why this error occurs, where it’s likely to appear, and how to resolve it efficiently.

The Uncaught SyntaxError: Cannot use import statement outside a module error arises because the JavaScript runtime cannot recognise the file as a module.

By default, browsers and Node.js interpret JavaScript files as plain scripts unless specifically instructed to treat them as modules. ES6 modules rely on the runtime identifying the file as a module before using import and export statements.

Common Scenarios Where This Error Appears:

1. Browser Environments: Importing a module file without a <script> tag specifying type="module" results in this error.
2. Node.js: Using ES6 module syntax in a CommonJS project or without proper module configuration.
3. Testing Tools or Bundlers: Tools like Jest, Webpack, or Parcel may require specific settings to handle ES6 modules correctly.

Here’s an example that triggers this issue:

Below are two approaches to solving the issue based on your environment:

Fix for Browser Environments

In browser-based projects, you must explicitly tell the browser to parse the script file as a module by adding type="module" in your <script> tag.

• Ensure the file being imported is hosted on a web server—modules often fail to load when opened directly via the file:// protocol in some browsers.

Fix for Node.js

In Node.js, ES6 modules are supported but must be specified within the package.json file or by using the .mjs file extension.

Steps:

1. Edit your package.json file and set "type": "module".
2. Use .mjs as the file extension if you want module behaviour in a project using CommonJS by default.

package.json

index.js

OR

Without altering the package.json:

Be cautious when converting legacy CommonJS projects to ES6 modules — migrating all require() calls to import statements may introduce breaking changes.

Further Considerations

1. Performance and Compatibility:

   • Some browsers do not support ES6 modules natively (e.g., older versions of Internet Explorer). Use tools like Babel to transpile ES6 code into compatible ES5 code when targeting older environments.

2. Security:

   • Ensure the modules used are from trusted sources, as malicious imports can expose vulnerabilities. Validate any third-party libraries you include.

3. Alternative Syntax:

   • Use CommonJS syntax (require() and module.exports) in Node.js projects still using the older standard. This ensures compatibility but limits the benefits of ES6 features.

4. Bundlers and Tooling:

   • Adopt tools like Webpack, Rollup, or Vite to automate handling of ES6 imports and resolving compatibility issues across environments.

Related Resources

1. MDN Web Docs: Modules
2. Node.js: ES Modules Documentation
3. Babel Official Website