test: add CJS/ESM dual-package compatibility tests

Mirrors the cjs.test.js suite from webpack/sass-loader: builds a
CommonJS bundle from src/ at test time via @babel/core and asserts the
shape promised by the build:cjs pipeline (commonjs package marker,
strict-mode prologue, exports.default unwrap) so require() returns the
loader function with .default pointing back at itself, matching the
ESM default export.

Author: claude

test/cjs.test.js

@@ -0,0 +1,116 @@
+import assert from "node:assert";
+import {
+  copyFileSync,
+  mkdtempSync,
+  readFileSync,
+  readdirSync,
+  rmSync,
+  writeFileSync,
+} from "node:fs";
+import { createRequire } from "node:module";
+import { tmpdir } from "node:os";
+import path from "node:path";
+import { after, before, describe, it } from "node:test";
+import { fileURLToPath } from "node:url";
+
+import { transformFileAsync } from "@babel/core";
+
+import src from "../src/index.js";
+
+const require = createRequire(import.meta.url);
+
+const srcDir = fileURLToPath(new URL("../src", import.meta.url));
+
+/**
+ * Transpile every `.js` file under `src/` to CommonJS in `outDir`, copy
+ * non-JS assets, then drop the `dist/cjs/package.json` type marker and
+ * the same `module.exports = exports.default` post-build append the
+ * `build:cjs` npm script writes. The test then `require()`s the result.
+ * @param {string} outDir target directory
+ */
+async function buildCjsBundle(outDir) {
+  for (const entry of readdirSync(srcDir, { withFileTypes: true })) {
+    const source = path.join(srcDir, entry.name);
+    const target = path.join(outDir, entry.name);
+
+    if (entry.name.endsWith(".js")) {
+      const result = await transformFileAsync(source, { envName: "cjs" });
+
+      writeFileSync(target, result.code);
+    } else {
+      copyFileSync(source, target);
+    }
+  }
+
+  writeFileSync(
+    path.join(outDir, "package.json"),
+    `${JSON.stringify({ type: "commonjs" })}\n`,
+  );
+
+  const indexPath = path.join(outDir, "index.js");
+
+  writeFileSync(
+    indexPath,
+    `${readFileSync(indexPath, "utf8")}module.exports = exports.default;\nmodule.exports.default = exports.default;\n`,
+  );
+}
+
+describe("cjs", () => {
+  let cjsDir;
+  let cjsIndexPath;
+  let cjsIndexSource;
+  let cjsPackage;
+  let cjsLoader;
+
+  before(async () => {
+    cjsDir = mkdtempSync(path.join(tmpdir(), "less-loader-cjs-"));
+
+    await buildCjsBundle(cjsDir);
+
+    cjsIndexPath = path.join(cjsDir, "index.js");
+    cjsIndexSource = readFileSync(cjsIndexPath, "utf8");
+    cjsPackage = JSON.parse(
+      readFileSync(path.join(cjsDir, "package.json"), "utf8"),
+    );
+    cjsLoader = require(cjsIndexPath);
+  });
+
+  after(() => {
+    if (cjsDir) rmSync(cjsDir, { recursive: true, force: true });
+  });
+
+  it("should expose the loader as the default export of the ESM entry", () => {
+    assert.strictEqual(typeof src, "function");
+  });
+
+  // Run Babel against `src/` directly (no dependency on the published
+  // `dist/`) and assert the resulting CommonJS bundle has the shape the
+  // `build:cjs` pipeline promises: it's marked CommonJS, opens with
+  // Babel's strict-mode prologue and `exports.default = lessLoader`, and
+  // ends with the `module.exports = exports.default;` unwrap so
+  // `require()` returns the loader function directly.
+  it("should produce a require()-able CommonJS bundle via @babel/core", () => {
+    assert.strictEqual(cjsPackage.type, "commonjs");
+
+    assert.match(cjsIndexSource, /^"use strict";/);
+    assert.match(cjsIndexSource, /exports\.default = lessLoader/);
+    assert.match(cjsIndexSource, /module\.exports = exports\.default;/);
+
+    assert.strictEqual(typeof cjsLoader, "function");
+    assert.strictEqual(cjsLoader.default, cjsLoader);
+  });
+
+  // Pre-refactor consumers loaded the loader two ways:
+  //   const loader = require("less-loader");      // function
+  //   import loader from "less-loader";           // function
+  // Both surfaces must continue to return a callable function, with a
+  // `.default` property pointing back at the same function so transitional
+  // `require("less-loader").default` calls keep working.
+  it("should expose the loader through `require` as a callable function (pre-refactor shape)", () => {
+    assert.strictEqual(typeof cjsLoader, "function");
+    assert.strictEqual(typeof src, "function");
+    assert.strictEqual(cjsLoader.default, cjsLoader);
+    assert.strictEqual(cjsLoader.name, src.name);
+    assert.strictEqual(cjsLoader.length, src.length);
+  });
+});