Add file location when SyntaxError happens in ESM (fixes #4551)

lib/esm-utils.js

@@ -3,7 +3,29 @@ const url = require('url');
 
 const formattedImport = async file => {
   if (path.isAbsolute(file)) {
-    return import(url.pathToFileURL(file));
+    try {
+      return await import(url.pathToFileURL(file));
+    } catch (err) {
+      // This is a hack created because ESM in Node.js (at least in Node v15.5.1) does not emit
+      // the location of the syntax error in the error thrown.
+      // This is problematic because the user can't see what file has the problem,
+      // so we add the file location to the error.
+      // This `if` should be removed once Node.js fixes the problem.
+      if (
+        err instanceof SyntaxError &&
+        err.message &&
+        err.stack &&
+        !err.stack.includes(file)
+      ) {
+        const newErrorWithFilename = new SyntaxError(err.message);
+        newErrorWithFilename.stack = err.stack.replace(
+          /^SyntaxError/,
+          `SyntaxError[ @${file} ]`
+        );
+        throw newErrorWithFilename;
+      }
+      throw err;
+    }
   }
   return import(file);
 };

test/integration/esm.spec.js

@@ -1,5 +1,7 @@
 'use strict';
-var run = require('./helpers').runMochaJSON;
+var helpers = require('./helpers');
+var run = helpers.runMochaJSON;
+var runMochaAsync = helpers.runMochaAsync;
 var utils = require('../../lib/utils');
 var args =
   +process.versions.node.split('.')[0] >= 13 ? [] : ['--experimental-modules'];
@@ -38,6 +40,17 @@ describe('esm', function() {
     });
   });
 
+  it('should show file location when there is a syntax error in the test', async function() {
+    var fixture = 'esm/syntax-error/esm-syntax-error.fixture.mjs';
+    const err = await runMochaAsync(fixture, args, {stdio: 'pipe'}).catch(
+      err => err
+    );
+    expect(err.output, 'to contain', 'SyntaxError').and(
+      'to contain',
+      'esm-syntax-error.fixture.mjs'
+    );
+  });
+
   it('should recognize esm files ending with .js due to package.json type flag', function(done) {
     if (!utils.supportsEsModules(false)) return this.skip();
 

test/integration/fixtures/esm/syntax-error/esm-syntax-error.fixture.mjs

@@ -0,0 +1,3 @@
+// This is intentionally a syntax error
+it('should never run because of a syntax error here', => {
+});