Allow environment overrides specifically for Storybook

Stories are rendered as full Ember Apps including the default
Environment which may have settings that aren't appropriate for only
rendering a single story in an iframe.

Now those can be overridden with a .storybook/environment.js file

index.js

@@ -5,7 +5,7 @@ const path = require('path');
 const YUIDocsGenerator = require('ember-cli-addon-docs-yuidoc/lib/broccoli/generator');
 const Funnel = require('broccoli-funnel');
 const mergeTrees = require('broccoli-merge-trees');
-const { parse, generatePreviewHead } = require('./lib/util');
+const { parse, generatePreviewHead, overrideEnvironment } = require('./lib/util');
 
 module.exports = {
   name: require('./package').name,
@@ -74,6 +74,7 @@ module.exports = {
     const previewHeadFilePath = path.resolve(process.cwd(), '.storybook/preview-head.html');
     const previewHeadDirectory = path.dirname(previewHeadFilePath);
     const envFilePath = path.resolve(process.cwd(), '.env');
+    const environmentOverridePath = path.resolve(process.cwd(), '.storybook/environment.js');
 
     let fileContents = '';
 
@@ -93,6 +94,22 @@ module.exports = {
 
     this.ui.writeDebugLine('Generating preview-head.html');
 
+    const environment = parsedConfig.meta.find(meta => meta.name.endsWith('config/environment'));
+
+    if (environment) {
+      // From the Ember App's environment.js file
+      const original = JSON.parse(decodeURIComponent(environment.content));
+
+      // When rootURL is anything other than "/" routing can't be started without erroring, so
+      // this is a sensible default.
+      const defaultOverride = { rootURL: '/' };
+
+      // Allow arbitrary overriding in the storybook environment
+      const environmentOverride = fs.existsSync(environmentOverridePath) && require(environmentOverridePath)(process.env);
+
+      environment.content = encodeURIComponent(JSON.stringify(overrideEnvironment(original, defaultOverride, environmentOverride)));
+    }
+
     if(config) {
       this.ui.writeDebugLine('Setting up overrides.');
 

lib/util.js

@@ -1,4 +1,5 @@
 const cheerio = require('cheerio');
+const merge = require('lodash.merge');
 
 const lookupTable = {
   meta: [{
@@ -146,9 +147,14 @@ function generatePreviewHead(parsedConfig) {
   return doc.join('\n')
 }
 
+function overrideEnvironment(env, ...overrides) {
+  return merge(env, ...overrides);
+}
+
 module.exports = {
   getDocumentValues,
   parse,
   objectToHTMLAttributes,
   generatePreviewHead,
+  overrideEnvironment,
 };

package.json

@@ -75,7 +75,8 @@
     "cheerio": "^1.0.0-rc.2",
     "ember-cli-addon-docs-yuidoc": "^1.0.0",
     "ember-cli-babel": "^7.23.0",
-    "ember-cli-htmlbars": "^5.3.1"
+    "ember-cli-htmlbars": "^5.3.1",
+    "lodash.merge": "^4.6.2"
   },
   "publishConfig": {
     "access": "public"