Add test for generateBuildId (vercel#5816)

* Add docs for returning `null` from generateBuildId

* Add test for setting custom buildid

* Fix linting

package.json

@@ -20,9 +20,9 @@
   },
   "pre-commit": "lint-staged",
   "lint-staged": {
-    "*.js": "standard --fix",
-    "*.ts": "standard --parser typescript-eslint-parser --plugin typescript --fix",
-    "packages/**/bin/*": "standard"
+    "*.js": ["standard --fix", "git add"],
+    "*.ts": ["standard --parser typescript-eslint-parser --plugin typescript --fix", "git add"],
+    "packages/**/bin/*": ["standard --fix", "git add"]
   },
   "standard": {
     "parser": "babel-eslint",

packages/next/README.md

@@ -1206,8 +1206,6 @@ Or use a function:
 
 ```js
 module.exports = (phase, {defaultConfig}) => {
-  //
-  // https://github.com/zeit/
   return {
     /* config options here */
   }
@@ -1296,6 +1294,21 @@ module.exports = {
 }
 ```
 
+To fall back to the default of generating a unique id return `null` from the function:
+
+```js
+module.exports = {
+  generateBuildId: async () => {
+    // When process.env.YOUR_BUILD_ID is undefined we fall back to the default
+    if(process.env.YOUR_BUILD_ID) {
+      return process.env.YOUR_BUILD_ID
+    }
+
+    return null
+  }
+}
+```
+
 ### Customizing webpack config
 
 <details>

test/integration/production-config/next.config.js

@@ -19,5 +19,8 @@ module.exports = withCSS(withSass({
     }
 
     return config
+  },
+  async generateBuildId () {
+    return 'custom-buildid'
   }
 }))

test/integration/production-config/test/index.test.js

@@ -11,6 +11,7 @@ import webdriver from 'next-webdriver'
 
 jasmine.DEFAULT_TIMEOUT_INTERVAL = 1000 * 60 * 5
 
+let appPort
 let server
 
 describe('Production Config Usage', () => {
@@ -23,6 +24,7 @@ describe('Production Config Usage', () => {
       quiet: true
     })
     server = await startApp(app)
+    appPort = server.address().port
   })
   afterAll(() => stopApp(server))
 
@@ -34,10 +36,22 @@ describe('Production Config Usage', () => {
       await testBrowser()
     })
   })
+
+  describe('with generateBuildId', () => {
+    it('should add the custom buildid', async () => {
+      const browser = await webdriver(appPort, '/')
+      const text = await browser.elementByCss('#mounted').text()
+      expect(text).toMatch(/ComponentDidMount executed on client\./)
+
+      const html = await browser.elementByCss('html').getAttribute('innerHTML')
+      expect(html).toMatch('custom-buildid')
+      return browser.close()
+    })
+  })
 })
 
 async function testBrowser () {
-  const browser = await webdriver(server.address().port, '/')
+  const browser = await webdriver(appPort, '/')
   const element = await browser.elementByCss('#mounted')
   const text = await element.text()
   expect(text).toMatch(/ComponentDidMount executed on client\./)