cjs-resolves-esm

Detect when CJS require() would resolve to an ESM file, causing ERR_REQUIRE_ESM at runtime.

Rule ID	types/cjs-resolves-esm
Category	types
Default Severity	🔴 error
Fixable	No
attw equivalent	CJSResolvesToESM

What it checks

This rule detects packages where a CommonJS require() call would resolve to an ESM file. Node.js throws ERR_REQUIRE_ESM when this happens, breaking CJS consumers.

Common patterns caught:

1. type: "module" with string exports — require() resolves to a .js file that is ESM
2. require condition pointing to .mjs — .mjs is always ESM
3. require condition pointing to .js with type: "module" — .js is ESM when type: "module"
4. default fallback to ESM — no require condition, CJS falls back to default which is ESM

Examples

Fail

{
  "type": "module",
  "exports": "./source/index.js"
}

{
  "exports": {
    ".": {
      "require": "./dist/index.mjs"
    }
  }
}

{
  "type": "module",
  "exports": {
    ".": {
      "import": "./dist/index.js",
      "default": "./dist/index.js"
    }
  }
}

Pass

{
  "type": "module",
  "exports": {
    ".": {
      "import": "./dist/index.js",
      "require": "./dist/index.cjs"
    }
  }
}

{
  "exports": {
    ".": {
      "import": "./dist/index.mjs",
      "require": "./dist/index.cjs"
    }
  }
}

How to fix

Provide a CJS-compatible file for the require condition:

• Use .cjs extension for CJS files when type: "module"
• Add a separate require condition pointing to a CJS build
• Use a bundler (like tspub build --format esm,cjs) to generate both formats