feat(config): adjust to expect a function rather than an object

More closely mirrors the config of `ember-cli-fastboot` itself and helps to avoid confusion.

Author: gilest

blueprints/fastboot-test-config/files/__config__/fastboot-testing.js

@@ -1,7 +1,14 @@
-module.exports = {
-  resilient: false,
-  sandboxGlobals: {},
-  setupFastboot: fastbootInstance => {
-    // here you can access the fastboot instance which runs the tests
-  }
+module.exports = () => {
+  return {
+    resilient: false,
+    buildSandboxGlobals(defaultGlobals) {
+      return {
+        ...defaultGlobals
+        // here you can add globals to the Fastboot renderer
+      };
+    },
+    setupFastboot(fastbootInstance) {
+      // here you can access the fastboot instance which runs the tests
+    },
+  };
 };

lib/helpers.js

@@ -100,9 +100,18 @@ function makeFastbootTestingConfig(config, pkg) {
     'fastboot-testing.js'
   );
 
-  const customized = fs.existsSync(fastbootTestConfigPath)
-    ? require(fastbootTestConfigPath)
-    : {};
+  let customized = {};
+
+  if (fs.existsSync(fastbootTestConfigPath)) {
+    const fastbootTestConfig = require(fastbootTestConfigPath);
+    if (typeof fastbootTestConfig !== 'function') {
+      throw new Error(
+        `Expected export of ${fastbootTestConfigPath} to be a function. Instead found (${typeof fastbootTestConfig}).`
+      );
+    }
+
+    customized = fastbootTestConfig();
+  }
 
   return Object.assign({}, config, defaults, customized);
 }

tests/dummy/app/templates/docs/fastboot-configuration.md

@@ -5,14 +5,17 @@ When using this addon a new FastBoot instance will be created specifically for t
 ```js
 // my-app/config/fastboot-tesitng.js
 
-module.exports = {
-  resilient: true,
-  buildSandboxGlobals(defaultGlobals) {
-    return Object.assign({}, defaultGlobals, {
-      SampleGlobal: `TestSampleGlobal`,
-      najax
-    });
-  }
+module.exports = () => {
+  return {
+    resilient: true,
+    buildSandboxGlobals(defaultGlobals) {
+      return {
+        ...defaultGlobals,
+        SampleGlobal: `TestSampleGlobal`,
+        najax,
+      };
+    },
+  };
 };
 ```
 

tests/dummy/config/fastboot-testing.js

@@ -11,13 +11,15 @@ if (semver.lt(version, '3.0.0')) {
     },
   };
 } else {
-  module.exports = {
-    resilient: false,
-    buildSandboxGlobals(defaultGlobals) {
-      return Object.assign({}, defaultGlobals, {
-        SampleGlobal: 'TestSampleGlobal',
-        najax,
-      });
-    },
+  module.exports = () => {
+    return {
+      resilient: false,
+      buildSandboxGlobals(defaultGlobals) {
+        return Object.assign({}, defaultGlobals, {
+          SampleGlobal: 'TestSampleGlobal',
+          najax,
+        });
+      },
+    };
   };
 }