Merge pull request #237 from ember-fastboot/better-error-message

Better `ember fastboot` error message

Author: tomdale

lib/tasks/fastboot-server.js

@@ -97,11 +97,11 @@ module.exports = CoreObject.extend({
     this.restartPromise = this.stop()
       .then(() => this.clearRequireCache(options.outputPath))
       .then(() => this.start(options))
-      .catch(e => this.ui.writeLine(e))
+      .catch(e => this.printError(e))
       .finally(() => {
         this.restartPromise = null;
         if (this.restartAgain) {
-          debug('restart again')
+          debug('restart again');
           this.restartAgain = false;
           this.restart(options);
         }
@@ -120,5 +120,58 @@ module.exports = CoreObject.extend({
         delete require.cache[key];
       }
     });
+  },
+
+  /*
+   * Try to show a useful error message if we're not able to start the user's
+   * app in FastBoot.
+   */
+  printError: function(e) {
+    var preamble = ["There was an error trying to run your application in FastBoot.\n",
+      "This is usually caused by either your application code or an addon trying to access " +
+      "an API that isn't available in Node.js."];
+
+    // If there's a stack trace available, try to extract some information for
+    // it so we can generate a more helpful error message.
+    if (e.stack) {
+      // Look at the stack trace line by line
+      var stack = e.stack.split('\n');
+
+      // Verify there's a second line with path information.
+      // (First line is the error message itself.)
+      if (stack[1]) {
+        // Extract path and line number information. An example line looks like either:
+        //     at /Users/monegraph/Code/fastboot-test/dist/fastboot/vendor.js:65045:19
+        // or
+        //     at Module.callback (/Users/monegraph/Code/fastboot test/dist/fastboot/fastboot-test.js:23:31)
+        var match = stack[1].match(/\s*(?:at .* \(([^:]+):(\d+):(\d+)|at ([^:]+):(\d+):(\d+)$)/);
+        if (match) {
+          var fileName = match[1] || match[4];
+          var lineNumber = match[2] || match[5];
+
+          // Print file name and line number from the top of the stack. This is displayed
+          // anyway, of course, but not everyone knows how to read a stack trace. This makes
+          // it more obvious.
+          var badFilePath = path.relative(process.cwd(), fileName);
+          preamble.push("Based on the stack trace, it looks like the exception was generated in " + badFilePath + " on line " + lineNumber + ".");
+
+          // If the exception is coming from `vendor.js`, that usually means it's from an addon and thus may be
+          // out of the user's control. Give the user some instructions so they can try to figure out which
+          // addon is causing the problem.
+          if (fileName.substr(-9) === 'vendor.js') {
+            preamble.push("Because it's coming from vendor.js, an addon is most likely responsible for this error. You should look at this " +
+                          "file and line number to determine which addon is not yet FastBoot compatible.");
+
+          } else {
+            preamble.push("The exception is probably coming from your app. Look at this file and line number to determine what is triggering the exception.");
+          }
+        }
+      }
+
+      preamble.push("\nThe full stack trace is:");
+    }
+
+    this.ui.writeError(preamble.join('\n') + '\n');
+    this.ui.writeError(e);
   }
 });