Fix: Load CommonJS .eslintrc.js files within a "type": "module" packa…

…ge scope (fixes #12319)

lib/cli-engine/config-array-factory.js

@@ -36,6 +36,7 @@
 const fs = require("fs");
 const path = require("path");
 const importFresh = require("import-fresh");
+const requireFromString = require("require-from-string");
 const stripComments = require("strip-json-comments");
 const { validateConfigSchema } = require("../shared/config-validator");
 const naming = require("../shared/naming");
@@ -188,6 +189,19 @@ function loadJSConfigFile(filePath) {
     try {
         return importFresh(filePath);
     } catch (e) {
+        if (e.code === "ERR_REQUIRE_ESM") {
+
+            /*
+             * The JS config file is a CommonJS .js file within a `"type": "module"``
+             * package scope. Node errors when trying to require such a file,
+             * so bypass Node’s check. There’s no need for `importFresh`, because
+             * this method never adds the loaded .js file into `require.cache`.
+             */
+            const content = readFile(filePath);
+
+            return requireFromString(content, filePath, { appendPaths: path.dirname(filePath) });
+        }
+
         debug(`Error reading JavaScript file: ${filePath}`);
         e.message = `Cannot read config file: ${filePath}\nError: ${e.message}`;
         throw e;

package.json

@@ -74,6 +74,7 @@
     "optionator": "^0.8.2",
     "progress": "^2.0.0",
     "regexpp": "^2.0.1",
+    "require-from-string": "^2.0.2",
     "semver": "^6.1.2",
     "strip-ansi": "^5.2.0",
     "strip-json-comments": "^3.0.1",