Modify with-universal-configuration example (#4498) (#5948)

Explains in details how the "with-universal-configuration" example works and rename it to "with-universal-configuration-build-time". Changing the example name makes the purpose of the example clear.

The "env-config.js" file introduce one more sample of variable usage that instantiates an immediate value of the local environment variable. This makes it even clearer how build-time variable configuration works. The "index.js" page makes explicit the use of these configured environment variables.

The universal configuration confusion happens when the value of the environment variable is used directly in the application causing an effect in server-side but not on the client side.

examples/with-universal-configuration/README.md → examples/with-universal-configuration-build-time/README.md

-[![Deploy to now](https://deploy.now.sh/static/button.svg)](https://deploy.now.sh/?repo=https://github.com/zeit/next.js/tree/master/examples/with-universal-configuration)
+[![Deploy to now](https://deploy.now.sh/static/button.svg)](https://deploy.now.sh/?repo=https://github.com/zeit/next.js/tree/master/examples/with-universal-configuration-build-time)
 
 # With universal configuration
 
@@ -9,9 +9,9 @@
 Execute [`create-next-app`](https://github.com/segmentio/create-next-app) with [Yarn](https://yarnpkg.com/lang/en/docs/cli/create/) or [npx](https://github.com/zkat/npx#readme) to bootstrap the example:
 
 ```bash
-npx create-next-app --example with-universal-configuration with-universal-configuration-app
+npx create-next-app --example with-universal-configuration-build-time with-universal-configuration-build-time-app
 # or
-yarn create next-app --example with-universal-configuration with-universal-configuration-app
+yarn create next-app --example with-universal-configuration-build-time with-universal-configuration-build-time-app
 ```
 
 ### Download manually
@@ -19,18 +19,18 @@ yarn create next-app --example with-universal-configuration with-universal-confi
 Download the example:
 
 ```bash
-curl https://codeload.github.com/zeit/next.js/tar.gz/canary | tar -xz --strip=2 next.js-canary/examples/with-universal-configuration
-cd with-universal-configuration
+curl https://codeload.github.com/zeit/next.js/tar.gz/canary | tar -xz --strip=2 next.js-canary/examples/with-universal-configuration-build-time
+cd with-universal-configuration-build-time
 ```
 
 Install it and run:
 
 ```bash
 npm install
-npm run dev
+VARIABLE_EXAMPLE=next.js npm run dev
 # or
 yarn
-yarn dev
+VARIABLE_EXAMPLE=next.js yarn dev
 ```
 
 Deploy it to the cloud with [now](https://zeit.co/now) ([download](https://zeit.co/download))
@@ -41,9 +41,13 @@ now
 
 ## The idea behind the example
 
-This example show how to set custom environment variables for your application based on NODE_ENV using [transform-define](https://github.com/FormidableLabs/babel-plugin-transform-define).
+This example shows how to use environment variables and customize one based on NODE_ENV for your application using [transform-define](https://github.com/FormidableLabs/babel-plugin-transform-define)
+
+When you build your application the environment variable is transformed into a primitive (string or undefined) and can only be changed with a new build. This happens for both client-side and server-side. If the environment variable is used directly in your application it will only have an effect on the server side, not the client side.
+
+To set the environment variables in runtime you can follow the example [with-universal-configuration-runtime]((https://deploy.now.sh/?repo=https://github.com/zeit/next.js/tree/master/examples/with-universal-configuration-runtime))
 
 ## Caveats
 
 - Because a babel plugin is used the output is cached in `node_modules/.cache` by `babel-loader`. When modifying the configuration you will have to manually clear this cache to make changes visible. Alternately, you may skip caching for `babel-loader` as shown [here](https://github.com/zeit/next.js/issues/1103#issuecomment-279529809).
-- This example sets the environment configuration at build time, meaning the same build might not be used in e.g. both staging and production. For a solution which sets the environment at runtime, see [here](https://github.com/zeit/next.js/issues/1488#issuecomment-289108931). 
+- This example sets the environment configuration at build time, meaning the same build might not be used in e.g. both staging and production. For a solution which sets the environment at runtime, see [here](https://github.com/zeit/next.js/issues/1488#issuecomment-289108931).

examples/with-universal-configuration/env-config.js → examples/with-universal-configuration-build-time/env-config.js

@@ -3,5 +3,6 @@ const prod = process.env.NODE_ENV === 'production'
 module.exports = {
   'process.env.BACKEND_URL': prod
     ? 'https://api.example.com'
-    : 'https://localhost:8080'
+    : 'https://localhost:8080',
+  'process.env.VARIABLE_EXAMPLE': process.env.VARIABLE_EXAMPLE
 }

examples/with-universal-configuration-build-time/pages/index.js

+export default () => (
+  <div>
+    <p>Environment variable process.env.VARIABLE_EXAMPLE is "{process.env.VARIABLE_EXAMPLE}"</p>
+    <p>Custom environment variables process.env.BACKEND_URL is "{process.env.BACKEND_URL}"</p>
+  </div>
+)

examples/with-universal-configuration/pages/index.js

-export default () => <div>Loading data from {process.env.BACKEND_URL}</div>