Eliminate context code based on process.browser (#5159)

* Allow deadCodeElemination when using process.browser

* Add process.browser test

build/webpack.js

@@ -274,6 +274,10 @@ export default async function getBaseWebpackConfig (dir: string, {dev = false, i
         // required not to cache removed files
         useHashIndex: false
       }),
+      // Removes server/client code by minifier
+      new webpack.DefinePlugin({
+        'process.browser': JSON.stringify(!isServer)
+      }),
       // This is used in client/dev-error-overlay/hot-dev-client.js to replace the dist directory
       !isServer && dev && new webpack.DefinePlugin({
         'process.env.__NEXT_DIST_DIR': JSON.stringify(distDir)

test/integration/production/pages/process-env.js

@@ -1,3 +1,12 @@
+
+if (process.browser) {
+  global.__THIS_SHOULD_ONLY_BE_DEFINED_IN_BROWSER_CONTEXT__ = true
+}
+
+if (!process.browser) {
+  global.__THIS_SHOULD_ONLY_BE_DEFINED_IN_SERVER_CONTEXT__ = true
+}
+
 export default () => (
   <div id='node-env'>{process.env.NODE_ENV}</div>
 )

test/integration/production/test/process-env.js

@@ -1,5 +1,11 @@
 /* global describe, it, expect */
 import webdriver from 'next-webdriver'
+import { readFile } from 'fs'
+import { promisify } from 'util'
+import { join } from 'path'
+
+const readFileAsync = promisify(readFile)
+const readNextBuildFile = (relativePath) => readFileAsync(join(__dirname, '../.next', relativePath), 'utf8')
 
 export default (context) => {
   describe('process.env', () => {
@@ -10,4 +16,20 @@ export default (context) => {
       browser.close()
     })
   })
+
+  describe('process.browser', () => {
+    it('should eliminate server only code on the client', async () => {
+      const buildId = await readNextBuildFile('./BUILD_ID')
+      const clientCode = await readNextBuildFile(`./static/${buildId}/pages/process-env.js`)
+      expect(clientCode).toMatch(/__THIS_SHOULD_ONLY_BE_DEFINED_IN_BROWSER_CONTEXT__/)
+      expect(clientCode).not.toMatch(/__THIS_SHOULD_ONLY_BE_DEFINED_IN_SERVER_CONTEXT__/)
+    })
+
+    it('should eliminate client only code on the server', async () => {
+      const buildId = await readNextBuildFile('./BUILD_ID')
+      const serverCode = await readNextBuildFile(`./server/static/${buildId}/pages/process-env.js`)
+      expect(serverCode).not.toMatch(/__THIS_SHOULD_ONLY_BE_DEFINED_IN_BROWSER_CONTEXT__/)
+      expect(serverCode).toMatch(/__THIS_SHOULD_ONLY_BE_DEFINED_IN_SERVER_CONTEXT__/)
+    })
+  })
 }