feat: Support ESM test running via the test task

packages/hardhat-core/src/builtin-tasks/test.ts

@@ -5,11 +5,14 @@ import path from "path";
 
 import { HARDHAT_NETWORK_NAME } from "../internal/constants";
 import { subtask, task } from "../internal/core/config/config-env";
+import { HardhatError } from "../internal/core/errors";
+import { ERRORS } from "../internal/core/errors-list";
 import { isRunningWithTypescript } from "../internal/core/typescript-support";
 import { getForkCacheDirPath } from "../internal/hardhat-network/provider/utils/disk-cache";
 import { showForkRecommendationsBannerIfNecessary } from "../internal/hardhat-network/provider/utils/fork-recomendations-banner";
 import { pluralize } from "../internal/util/strings";
 import { getAllFilesMatching } from "../internal/util/fs-utils";
+import { getProjectPackageJson } from "../internal/util/packageInfo";
 
 import {
   TASK_COMPILE,
@@ -35,23 +38,26 @@ subtask(TASK_TEST_GET_TEST_FILES)
       return testFilesAbsolutePaths;
     }
 
-    const jsFiles = await getAllFilesMatching(config.paths.tests, (f) =>
-      f.endsWith(".js")
+    const jsFiles = await getAllFilesMatching(
+      config.paths.tests,
+      (f) => f.endsWith(".js") || f.endsWith(".cjs") || f.endsWith(".mjs")
     );
 
     if (!isRunningWithTypescript(config)) {
       return jsFiles;
     }
 
-    const tsFiles = await getAllFilesMatching(config.paths.tests, (f) =>
-      f.endsWith(".ts")
+    const tsFiles = await getAllFilesMatching(
+      config.paths.tests,
+      (f) => f.endsWith(".ts") || f.endsWith(".cts") || f.endsWith(".mts")
     );
 
     return [...jsFiles, ...tsFiles];
   });
 
 subtask(TASK_TEST_SETUP_TEST_ENVIRONMENT, async () => {});
 
+let testsAlreadyRun = false;
 subtask(TASK_TEST_RUN_MOCHA_TESTS)
   .addFlag("parallel", "Run tests in parallel")
   .addFlag("bail", "Stop running tests after the first test failure")
@@ -99,6 +105,29 @@ subtask(TASK_TEST_RUN_MOCHA_TESTS)
       const mocha = new Mocha(mochaConfig);
       taskArgs.testFiles.forEach((file) => mocha.addFile(file));
 
+      // if the project is of type "module" or if there's some ESM test file,
+      // we call loadFilesAsync to enable Mocha's ESM support
+      const projectPackageJson = await getProjectPackageJson();
+      const isTypeModule = projectPackageJson.type === "module";
+      const hasEsmTest = taskArgs.testFiles.some(
+        (file) => file.endsWith(".mjs") || file.endsWith(".mts")
+      );
+      if (isTypeModule || hasEsmTest) {
+        // Because of the way the ESM cache works, loadFilesAsync doesn't work
+        // correctly if used twice within the same process, so we throw an error
+        // in that case
+        if (testsAlreadyRun) {
+          throw new HardhatError(
+            ERRORS.BUILTIN_TASKS.TEST_TASK_ESM_TESTS_RUN_TWICE
+          );
+        }
+        testsAlreadyRun = true;
+
+        // This instructs Mocha to use the more verbose file loading infrastructure
+        // which supports both ESM and CJS
+        await mocha.loadFilesAsync();
+      }
+
       const testFailures = await new Promise<number>((resolve) => {
         mocha.run(resolve);
       });

packages/hardhat-core/src/internal/core/config/config-loading.ts

@@ -25,8 +25,17 @@ import { DEFAULT_SOLC_VERSION } from "./default-config";
 const log = debug("hardhat:core:config");
 
 function importCsjOrEsModule(filePath: string): any {
-  const imported = require(filePath);
-  return imported.default !== undefined ? imported.default : imported;
+  try {
+    const imported = require(filePath);
+    return imported.default !== undefined ? imported.default : imported;
+  } catch (e: any) {
+    if (e.code === "ERR_REQUIRE_ESM") {
+      throw new HardhatError(ERRORS.GENERAL.ESM_PROJECT_WITHOUT_CJS_CONFIG);
+    }
+
+    // eslint-disable-next-line @nomiclabs/hardhat-internal-rules/only-hardhat-error
+    throw e;
+  }
 }
 
 export function resolveConfigPath(configPath: string | undefined) {

packages/hardhat-core/src/internal/core/errors-list.ts

@@ -237,6 +237,16 @@ Either try using a new directory name, or remove the conflicting files.`,
         "Hardhat attempted to convert the input value to a BigInt, but no known conversion method was applicable to the given value.",
       shouldBeReported: false,
     },
+    ESM_PROJECT_WITHOUT_CJS_CONFIG: {
+      number: 18,
+      message: `Your project is an ESM project (you have "type": "module" set in your package.json) but your Hardhat config file uses a .js or .ts extension.
+
+Rename the file to use a .cjs or .cts extension to fix this problem.`,
+      title: "Hardht config with .js or .ts extension in an ESM project",
+      description:
+        "Your project is an ESM project but your Hardhat config uses an .js or .ts extension. Hardhat config files cannot be an ES module. To fix this, rename your Hardhat config to use the .cjs or .cts extension.",
+      shouldBeReported: false,
+    },
   },
   NETWORK: {
     CONFIG_NOT_FOUND: {
@@ -947,6 +957,14 @@ The first supported version is %firstSupportedVersion%`,
 Please use a newer, supported version.`,
       shouldBeReported: true,
     },
+    TEST_TASK_ESM_TESTS_RUN_TWICE: {
+      number: 609,
+      message: `Your project uses ESM and you've programmatically run your tests twice. This is not supported yet.`,
+      title: "Running tests twice in an ESM project",
+      description:
+        'You have run your tests twice programmatically and your project is an ESM project (you have `"type": "module"` in your `package.json`, or some of your files has a `.mjs` or `.mts` extension). This is not supported by Mocha yet.',
+      shouldBeReported: true,
+    },
   },
   ARTIFACTS: {
     NOT_FOUND: {

packages/hardhat-core/src/internal/util/packageInfo.ts

@@ -2,6 +2,8 @@ import findup from "find-up";
 import fsExtra from "fs-extra";
 import path from "path";
 
+import { assertHardhatInvariant } from "../core/errors";
+
 export function getPackageJsonPath(): string {
   return findClosestPackageJson(__filename)!;
 }
@@ -43,3 +45,17 @@ export function getHardhatVersion(): string | null {
     return null;
   }
 }
+
+/**
+ * Return the contents of the package.json in the user's project
+ */
+export function getProjectPackageJson(): Promise<any> {
+  const packageJsonPath = findup.sync("package.json");
+
+  assertHardhatInvariant(
+    packageJsonPath !== null,
+    "Expected a package.json file in the current directory or in an ancestor directory"
+  );
+
+  return fsExtra.readJson(packageJsonPath);
+}