Include the filename in the error when an ES module has syntax errors

* Fixes #168

lib/loader.js

@@ -8,7 +8,18 @@ function Loader(options) {
 
 Loader.prototype.load = function(path) {
   if (path.endsWith('.mjs')) {
-    return this.import_(path);
+    return this.import_(path).catch(function(e) {
+      if (e.message.indexOf(path) !== -1 || e.stack.indexOf(path) !== -1) {
+        return Promise.reject(e);
+      } else {
+        // When an ES module has a syntax error, the resulting exception does not
+        // include the filename. Add it. We lose the stack trace in the process,
+        // but the stack trace is usually not useful since it contains only frames
+        // from the Node module loader.
+        const updatedError = new Error(`While loading ${path}: ${e.constructor.name}: ${e.message}`);
+        return Promise.reject(updatedError);
+      }
+    });
   } else {
     return new Promise(resolve => {
       this.require_(path);

spec/fixtures/esm-load-exception/jasmine.json

@@ -0,0 +1,7 @@
+{
+  "spec_dir": ".",
+  "spec_files": [
+    "throws_on_load.mjs"
+  ],
+  "helpers": []
+}

spec/fixtures/esm-load-exception/throws_on_load.mjs

@@ -0,0 +1 @@
+throw new Error("nope");

spec/fixtures/esm-syntax-error/jasmine.json

@@ -0,0 +1,7 @@
+{
+  "spec_dir": ".",
+  "spec_files": [
+    "syntax_error.mjs"
+  ],
+  "helpers": []
+}

spec/fixtures/esm-syntax-error/syntax_error.mjs

@@ -0,0 +1 @@
+function(

spec/integration_spec.js

@@ -27,12 +27,26 @@ describe('Integration', function () {
     expect(output).toMatch(/at .*throws_on_load.js/);
   });
 
+  it('handles load-time exceptions from ESM specs properly', async function () {
+    const {exitCode, output} = await runJasmine('spec/fixtures/esm-load-exception');
+    expect(exitCode).toEqual(1);
+    expect(output).toContain('Error: nope');
+    expect(output).toMatch(/at .*throws_on_load.mjs/);
+  });
+
   it('handles syntax errors in CommonJS specs properly', async function () {
     const {exitCode, output} = await runJasmine('spec/fixtures/cjs-syntax-error');
     expect(exitCode).toEqual(1);
     expect(output).toContain('SyntaxError');
     expect(output).toContain('syntax_error.js');
   });
+
+  it('handles syntax errors in ESM specs properly', async function () {
+    const {exitCode, output} = await runJasmine('spec/fixtures/esm-syntax-error');
+    expect(exitCode).toEqual(1);
+    expect(output).toContain('SyntaxError');
+    expect(output).toContain('syntax_error.mjs');
+  });
 });
 
 async function runJasmine(cwd) {

spec/loader_spec.js

@@ -28,9 +28,20 @@ describe('loader', function() {
         await expectAsync(loaderPromise).toBeResolved();
       });
 
-      it('propagates the error when import fails', async function () {
+      it("adds the filename to errors that don't include it", async function() {
+        const underlyingError = new SyntaxError('some details but no filename, not even in the stack trace');
+        const importShim = () => Promise.reject(underlyingError);
+        const loader = new Loader({importShim});
+
+        await expectAsync(loader.load('foo.mjs')).toBeRejectedWithError(
+          "While loading foo.mjs: SyntaxError: some details but no filename, not even in the stack trace"
+        );
+      });
+
+      it('propagates errors that already contain the filename without modifying them', async function () {
         const requireShim = jasmine.createSpy('requireShim');
         const underlyingError = new Error('nope');
+        underlyingError.stack = underlyingError.stack.replace('loader_spec.js', 'foo.mjs');
         const importShim = jasmine.createSpy('importShim')
           .and.callFake(() => Promise.reject(underlyingError));
         const loader = new Loader({requireShim, importShim});