support js and json config file with same path

docs/guides/production.md

@@ -87,6 +87,15 @@ into your application.  One example of the file `_configs/production/config.json
 }
 ```
 
+If dynamic config is preferred, just replace `config.json` with file `config.js`. One example of `config.js` might look like this:
+
+```js
+module.exports = {
+  APP_ENV: process.env.NODE_ENV === "production" ? "prod" : "dev",
+  MY_GLOBAL_VARIABLE: process.env.NODE_ENV === "production" ? "foo" : "bar"
+}
+```
+
 To use these config variables inside your application, just use the `config()` function inside `react-server` and
 you're all set!  This works reliably on both server and client sides of the application--fully isomorphic! 
 Here's an example:

packages/react-server/core/config.js

@@ -1,4 +1,3 @@
-
 /**
  * Thin wrapper around the environment-specific configuration file
  */
@@ -12,17 +11,15 @@ if (SERVER_SIDE) {
 		if (null === config) {
 
 			//eslint-disable-next-line no-process-env
-			if (process.env.REACT_SERVER_CONFIG_SOURCE) {
+			if (process.env.REACT_SERVER_CONFIGS) {
 				var path = require('path');
 				//eslint-disable-next-line no-process-env
-				config = require(path.join(process.cwd(), process.env.REACT_SERVER_CONFIG_SOURCE));
-			}
-			//eslint-disable-next-line no-process-env
-			else if (process.env.REACT_SERVER_CONFIGS) {
-				var fs = require("fs");
-				//eslint-disable-next-line no-process-env
-				var configFile = fs.readFileSync(process.env.REACT_SERVER_CONFIGS + "/config.json");
-				config = Object.freeze(JSON.parse(configFile));
+				var configFilePath = process.env.REACT_SERVER_CONFIGS;
+
+				// Node.js tries to load `config.js` file first. If `config.js` doesn't exist, Node.js
+				// then try to load `config.json`.
+				var configFilePath = path.join(process.cwd(), configFilePath + "/config");
+				config = require(configFilePath);
 			} else {
 				config = Object.freeze({});
 			}