Load environment file in development (#695)

* Load environment file via dotenv if .env file is present

* Document loading environment variables in .env file

* Minor doc tweaks

packages/react-scripts/scripts/build.js

@@ -12,6 +12,12 @@
 // Do this as the first thing so that any code reading it knows the right env.
 process.env.NODE_ENV = 'production';
 
+// Load environment variables from .env file. Surpress warnings using silent
+// if this file is missing. dotenv will never modify any environment variables
+// that have already been set.
+// https://github.com/motdotla/dotenv
+require('dotenv').config({silent: true});
+
 var chalk = require('chalk');
 var fs = require('fs-extra');
 var path = require('path');

packages/react-scripts/scripts/start.js

@@ -11,6 +11,12 @@
 
 process.env.NODE_ENV = 'development';
 
+// Load environment variables from .env file. Surpress warnings using silent
+// if this file is missing. dotenv will never modify any environment variables
+// that have already been set.
+// https://github.com/motdotla/dotenv
+require('dotenv').config({silent: true});
+
 var path = require('path');
 var chalk = require('chalk');
 var webpack = require('webpack');

packages/react-scripts/scripts/test.js

@@ -10,6 +10,12 @@
 process.env.NODE_ENV = 'test';
 process.env.PUBLIC_URL = '';
 
+// Load environment variables from .env file. Surpress warnings using silent
+// if this file is missing. dotenv will never modify any environment variables
+// that have already been set.
+// https://github.com/motdotla/dotenv
+require('dotenv').config({silent: true});
+
 const createJestConfig = require('./utils/createJestConfig');
 const jest = require('jest');
 const path = require('path');