Exploration of different config + expose webpack config (#222)

* Use next.config.js instead of package.json

* Remove irrelevant comment

* Integrate with custom webpack config

* Include hotReload option

* Remove async/await for getConfig

* Read package.json, show warning when webpack in config is defined

* Prepend warning message with WARNING

* Update log statements

* Documentation

* Restart server on change of config

* Fix process handling and cases where there is no config

* Also restart server when config file gets deleted

* Changed second parameter of webpack to config

* Support for returning Promise

* Update documentation, fix bug with webpack config

* Remove package.json, cdn and hotReload from config

README.md

@@ -285,6 +285,66 @@ Then run `now` and enjoy!
 
 Note: we recommend putting `.next` in `.npmignore` or `.gitignore`. Otherwise, use `files` or `now.files` to opt-into a whitelist of files you want to deploy (and obviously exclude `.next`)
 
+## Configuration
+
+While Next.js aims to work without any configuration, sometimes there is a need to add custom behaviour.
+You can define custom configuration in a file called `next.config.js` in the project root directory.
+An example of a configuration looks like this:
+
+```javascript
+// next.config.js
+module.exports = {
+  cdn: true
+}
+```
+
+### Customizing webpack config
+
+Sometimes the user needs to have custom configuration for webpack to add a specific behaviour in the build process.
+An example of this is using `eslint-loader` to lint the files before compiling. This can be done by defining 
+`webpack` in the config.
+
+```javascript
+module.exports = {
+  webpack: (webpackConfig, { dev }) => {
+    webpackConfig.module.preLoaders.push({ test: /\.js$/, loader: 'eslint-loader' })
+    return webpackConfig
+  }
+}
+```
+
+As you can see you need to provide a function which has two parameters `webpackConfig`, which is the config used by Next.js, and `options`, which contains
+`dev` (`true` if dev environment). The config you return is the config used by Next.js.
+You can also return a `Promise` which will be resolved first.
+
+_NOTE: Use this option with care, because you can potentially break the existing webpack build configuration by using this option._
+
+These are some more examples:
+
+```javascript
+const I18nPlugin = require('i18n-webpack-plugin');
+
+module.exports = {
+  webpack: (webpackConfig, { dev }) => {
+    // Read image files:
+    webpackConfig.module.loaders.push({
+      test: /\.png$/,
+      loader: 'file'
+    })
+
+    // Adding a plugin
+    webpackConfig.plugins.push(new I18nPlugin())
+
+    // Or adding an alias
+    // Create webpackConfig.resolve.alias if it doesn't exist yet:
+    webpackConfig.resolve.alias = webpackConfig.resolve.alias || {}
+    webpackConfig.resolve.alias.src = './src'
+
+    return webpackConfig
+  }
+}
+```
+
 ## FAQ
 
 <details>
@@ -423,7 +483,7 @@ For this reason we want to promote a situation where users can share the cache f
 
 We are committed to providing a great uptime and levels of security for our CDN. Even so, we also **automatically fall back** if the CDN script fails to load [with a simple trick](http://www.hanselman.com/blog/CDNsFailButYourScriptsDontHaveToFallbackFromCDNToLocalJQuery.aspx).
 
-To turn the CDN off, just set `{ “next”: { “cdn”: false } }` in `package.json`.
+To turn the CDN off, just set `module.exports = { cdn: false }` in `next.config.js`.
 </details>
 
 <details>

bin/next

@@ -2,6 +2,7 @@
 
 import { join } from 'path'
 import { spawn } from 'cross-spawn'
+import { watchFile } from 'fs'
 
 const defaultCommand = 'dev'
 const commands = new Set([
@@ -23,9 +24,26 @@ if (commands.has(cmd)) {
 
 const bin = join(__dirname, 'next-' + cmd)
 
-const proc = spawn(bin, args, { stdio: 'inherit', customFds: [0, 1, 2] })
-proc.on('close', (code) => process.exit(code))
-proc.on('error', (err) => {
-  console.error(err)
-  process.exit(1)
-})
+const startProcess = () => {
+  const proc = spawn(bin, args, { stdio: 'inherit', customFds: [0, 1, 2] })
+  proc.on('close', (code) => process.exit(code))
+  proc.on('error', (err) => {
+    console.error(err)
+    process.exit(1)
+  })
+  return proc
+}
+
+let proc = startProcess()
+
+if (cmd === 'dev') {
+  watchFile(join(process.cwd(), 'next.config.js'), (cur, prev) => {
+    if (cur.size > 0 || prev.size > 0) {
+      console.log('\n> Found a change in next.config.js, restarting the server...')
+      // Don't listen to 'close' now since otherwise parent gets killed by listener
+      proc.removeAllListeners('close')
+      proc.kill()
+      proc = startProcess()
+    }
+  })
+}

server/build/webpack.js

@@ -9,6 +9,7 @@ import WatchPagesPlugin from './plugins/watch-pages-plugin'
 import WatchRemoveEventPlugin from './plugins/watch-remove-event-plugin'
 import DynamicEntryPlugin from './plugins/dynamic-entry-plugin'
 import DetachPlugin from './plugins/detach-plugin'
+import getConfig from '../config'
 
 export default async function createCompiler (dir, { dev = false } = {}) {
   dir = resolve(dir)
@@ -166,7 +167,7 @@ export default async function createCompiler (dir, { dev = false } = {}) {
     [errorDebugPath, 'dist/pages/_error-debug.js']
   ])
 
-  return webpack({
+  let webpackConfig = {
     context: dir,
     entry,
     output: {
@@ -206,5 +207,11 @@ export default async function createCompiler (dir, { dev = false } = {}) {
     customInterpolateName: function (url, name, opts) {
       return interpolateNames.get(this.resourcePath) || url
     }
-  })
+  }
+  const config = getConfig(dir)
+  if (config.webpack) {
+    console.log('> Using Webpack config function defined in next.config.js.')
+    webpackConfig = await config.webpack(webpackConfig, { dev })
+  }
+  return webpack(webpackConfig)
 }

server/config.js

 import { join } from 'path'
-import { readFile } from 'mz/fs'
+import { existsSync } from 'fs'
 
 const cache = new Map()
 
-const defaultConfig = {}
+const defaultConfig = {
+  webpack: null
+}
 
 export default function getConfig (dir) {
   if (!cache.has(dir)) {
@@ -12,22 +14,16 @@ export default function getConfig (dir) {
   return cache.get(dir)
 }
 
-async function loadConfig (dir) {
-  const path = join(dir, 'package.json')
+function loadConfig (dir) {
+  const path = join(dir, 'next.config.js')
 
-  let data
-  try {
-    data = await readFile(path, 'utf8')
-  } catch (err) {
-    if (err.code === 'ENOENT') {
-      data = '{}'
-    } else {
-      throw err
-    }
-  }
+  let userConfig = {}
 
-  // no try-cache, it must be a valid json
-  const config = JSON.parse(data).next || {}
+  const userHasConfig = existsSync(path)
+  if (userHasConfig) {
+    const userConfigModule = require(path)
+    userConfig = userConfigModule.default || userConfigModule
+  }
 
-  return Object.assign({}, defaultConfig, config)
+  return Object.assign({}, defaultConfig, userConfig)
 }