Detailed error and warnings upon next() call

errors/invalid-server-options.md

@@ -0,0 +1,25 @@
+# It looks like the next instance is being instantiated incorrectly.
+
+#### Why This Error Occurred
+
+You have passed a null or undefined parameter to the next() call.
+
+#### Possible Ways to Fix It
+
+Make sure you are passing the variables properly:
+
+```js
+const app = next()
+```
+
+And make sure you're passing dev as shown in the examples below:
+
+```js
+const app = next({ dev: boolean })
+```
+
+### Useful Links
+
+- [custom-server-express](https://github.com/vercel/next.js/blob/6ca00bfe312c8d3fc5c20d25a9cd8d2741a29332/examples/custom-server-express/server.js#L6)
+- [custom-server](https://github.com/vercel/next.js/blob/6ca00bfe312c8d3fc5c20d25a9cd8d2741a29332/examples/custom-server/server.js#L6)
+- [custom-server-typescript](https://github.com/vercel/next.js/blob/6ca00bfe312c8d3fc5c20d25a9cd8d2741a29332/examples/custom-server-typescript/server/index.ts#L7)

packages/next/server/next.ts

@@ -13,6 +13,12 @@ type NextServerConstructor = Omit<ServerConstructor, 'staticMarkup'> & {
 function createServer(options: NextServerConstructor): Server {
   const standardEnv = ['production', 'development', 'test']
 
+  if (typeof options === 'undefined') {
+    throw new Error(
+      'The server has not been instantiated properly. https://err.sh/next.js/invalid-server-options'
+    )
+  }
+
   if (
     !(options as any).isNextDevCommand &&
     process.env.NODE_ENV &&
@@ -22,6 +28,12 @@ function createServer(options: NextServerConstructor): Server {
   }
 
   if (options.dev) {
+    if (typeof options.dev !== 'boolean') {
+      console.warn(
+        "Warning: 'dev' is not a boolean which could introduce unexpected behavior. https://err.sh/next.js/invalid-server-options"
+      )
+    }
+
     const DevServer = require('./next-dev-server').default
     return new DevServer(options)
   }

test/integration/invalid-server-options/pages/index.js

@@ -0,0 +1 @@
+export default () => 'test'

test/integration/invalid-server-options/test/index.test.js

@@ -0,0 +1,59 @@
+import next from 'next'
+import { join } from 'path'
+const dir = join(__dirname, '../')
+const warningMessage =
+  "Warning: 'dev' is not a boolean which could introduce unexpected behavior. https://err.sh/next.js/invalid-server-options"
+
+describe('Invalid server options', () => {
+  test('next() called with no parameters should throw error', () => {
+    expect(() => next()).toThrowError(
+      'The server has not been instantiated properly. https://err.sh/next.js/invalid-server-options'
+    )
+  })
+
+  test('next() called with undefined parameters should throw error', () => {
+    expect(() => next(undefined)).toThrowError(
+      'The server has not been instantiated properly. https://err.sh/next.js/invalid-server-options'
+    )
+  })
+
+  test('next() called with dev as string should send warning', () => {
+    const consoleSpy = jest.spyOn(console, 'warn')
+    const dev = 'string'
+    next({ dev, dir })
+
+    expect(consoleSpy).toHaveBeenCalledWith(warningMessage)
+  })
+
+  test('next() called with dev as number should send warning', () => {
+    const consoleSpy = jest.spyOn(console, 'warn')
+    const dev = 123
+    next({ dev, dir })
+
+    expect(consoleSpy).toHaveBeenCalledWith(warningMessage)
+  })
+
+  test('next() called with dev as array should send warning', () => {
+    const consoleSpy = jest.spyOn(console, 'warn')
+    const dev = ['array']
+    next({ dev, dir })
+
+    expect(consoleSpy).toHaveBeenCalledWith(warningMessage)
+  })
+
+  test('next() called with dev as object should send warning', () => {
+    const consoleSpy = jest.spyOn(console, 'warn')
+    const dev = { test: 'goes here' }
+    next({ dev, dir })
+
+    expect(consoleSpy).toHaveBeenCalledWith(warningMessage)
+  })
+
+  test('next() called with dev as function should send warning', () => {
+    const consoleSpy = jest.spyOn(console, 'warn')
+    const dev = () => console.log('test')
+    next({ dev, dir })
+
+    expect(consoleSpy).toHaveBeenCalledWith(warningMessage)
+  })
+})