In-Depth Analysis and Practical Guide to Resolving ESLint Error: Must Use Import to Load ES Module

Problem Background and Error Analysis

In modern frontend development, ESLint is widely used for code quality checking, but complex project configurations often lead to module-related errors. Based on the provided Q&A data, the user encountered the error "Error: Must use import to load ES Module" when running ESLint, with details as follows:

/Users/ben/Desktop/development projects/react-boilerplate-styled-context/src/api/api.ts
  0:0  error  Parsing error: Must use import to load ES Module: /Users/ben/Desktop/development projects/react-boilerplate-styled-context/node_modules/eslint/node_modules/eslint-scope/lib/definition.js
require() of ES modules is not supported.
require() of /Users/ben/Desktop/development projects/react-boilerplate-styled-context/node_modules/eslint/node_modules/eslint-scope/lib/definition.js from /Users/ben/Desktop/development projects/react-boilerplate-styled-context/node_modules/babel-eslint/lib/require-from-eslint.js is an ES module file as it is a .js file whose nearest parent package.json contains "type": "module" which defines all .js files in that package scope as ES modules.
Instead rename definition.js to end in .cjs, change the requiring code to use import(), or remove "type": "module" from /Users/ben/Desktop/development projects/react-boilerplate-styled-context/node_modules/eslint/node_modules/eslint-scope/package.json

The error message clearly indicates that the issue stems from the require() function attempting to load an ES module file, where the package.json in its directory sets "type": "module", defaulting all .js files to ES modules. The user is confused because the error persists even though their code uses only import statements or no imports, suggesting the problem lies within the toolchain's dependencies.

Root Cause: Parser Compatibility Issues

By analyzing the best answer from the Q&A data, the core issue is the use of the outdated babel-eslint parser. Last updated a year ago, this parser does not fully support ES6 module systems, causing it to incorrectly try loading ES modules with require() when parsing dependencies with "type": "module". Key points include:

• Module System Evolution: Node.js has supported ES modules since version 13.2.0, defined via the "type": "module" field in package.json. This is incompatible with traditional CommonJS modules (using require()), requiring toolchain updates.
• Parser Role: ESLint relies on parsers (e.g., babel-eslint) to parse JavaScript code and generate an Abstract Syntax Tree (AST). If the parser lacks ES module support, it triggers loading errors in internal dependencies.
• Error Propagation: The error appears in user code but actually originates from module loading issues in ESLint's internal dependencies (e.g., eslint-scope), explaining why it occurs even without require() in user files.

Solution: Upgrade and Configuration Adjustments

Based on the best answer, resolving this error requires upgrading the parser and adjusting configurations. Here are the detailed steps, illustrated with code examples:

1. Update package.json Dependencies: Replace the outdated babel-eslint with the officially maintained @babel/eslint-parser. Modify the devDependencies section in package.json:

   "devDependencies": {
     // Remove old version
     // "babel-eslint": "^10.0.2",
     // Add new parser
     "@babel/eslint-parser": "^7.19.1",
     // Keep other dependencies unchanged
   }

   Run npm install to install the new dependency.
2. Modify .eslintrc Configuration: Update the parser settings and add necessary options to avoid configuration file lookup errors. In the .eslintrc file:

   {
     "extends": ["airbnb", "prettier"],
     // Change parser
     "parser": "@babel/eslint-parser",
     "plugins": ["prettier", "@typescript-eslint"],
     "parserOptions": {
       "ecmaVersion": 8,
       // Add this option to prevent Babel config file lookup
       "requireConfigFile": false,
       "ecmaFeatures": {
         "experimentalObjectRestSpread": true,
         "impliedStrict": true,
         "classes": true
       }
     },
     // Keep other configurations unchanged
   }

3. Verify the Solution: Run the ESLint command (e.g., npm run lint:eslint:quiet). The error should disappear, replaced by normal code check outputs. If issues persist, check for conflicts with other configurations (e.g., TypeScript support).

In-Depth Analysis: Module Loading Mechanisms and Toolchain Optimization

To fully understand the problem, this section explores the underlying mechanisms of module loading and best practices for toolchains:

• ES Module and CommonJS Interoperability: In Node.js, ES modules and CommonJS modules can interoperate under specific rules, but toolchains must handle this correctly. For example, ESLint's parser needs to recognize import/export syntax while managing module type declarations in dependencies.
• Parser Workflow Example: Here is a simplified code example showing how a parser transforms code:

  // Original ES module code
  import React from 'react';
  export const Component = () => <div>Hello</div>;
  
  // Parser-generated AST (simplified representation)
  {
    type: 'Program',
    body: [
      {
        type: 'ImportDeclaration',
        specifiers: [{ type: 'ImportDefaultSpecifier', local: { name: 'React' } }],
        source: { type: 'Literal', value: 'react' }
      },
      {
        type: 'ExportNamedDeclaration',
        declaration: {
          type: 'VariableDeclaration',
          declarations: [/* ... */]
        }
      }
    ]
  }

  If the parser lacks ES module support, this process fails, leading to loading errors.
• Toolchain Compatibility Recommendations: Ensure all related tools (e.g., Webpack, Babel, TypeScript) have versions compatible with ES modules. For instance, in Webpack configuration, use experiments: { outputModule: true } to support module output.

Conclusion and Preventive Measures

By upgrading the parser and adjusting configurations, the "Must use import to load ES Module" error can be resolved. Key takeaways include: regularly updating dependencies to avoid outdated toolchain issues, understanding module system differences, and correctly setting configuration files. To prevent similar problems, it is recommended to:

• Use npm outdated to periodically check dependency versions.
• Choose officially maintained parsers (e.g., @babel/eslint-parser) during project initialization.
• Read tool documentation to ensure configurations support ES modules.

Through these measures, developers can build more stable frontend projects compatible with modern JavaScript features.