From aaa694da9b8928cda53125598159817c776b5e9c Mon Sep 17 00:00:00 2001
From: Henrik Wenz <henrik.wenz@gmail.com>
Date: Wed, 8 Aug 2018 15:13:20 +0200
Subject: [PATCH 1/3] Throw error if getInitialProps is defined as as instance
 method

Omitting the static keyword happens pretty often. Therefore we should trigger a warning in devmode.

Closes: #4782
---
 lib/utils.js | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/lib/utils.js b/lib/utils.js
index 22faec6a62f2a..2474870175c41 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -54,6 +54,14 @@ export function isResSent (res) {
 }
 
 export async function loadGetInitialProps (Component, ctx) {
+  if (process.env.NODE_ENV !== 'production') {
+    if (Component.prototype && Component.prototype.getInitialProps) {
+      const compName = getDisplayName(Component)
+      const message = `"${compName}.getInitialProps()" is defined as an instance method.`
+      throw new Error(message)
+    }
+  }
+
   if (!Component.getInitialProps) return {}
 
   const props = await Component.getInitialProps(ctx)

From 212c44f37f12c2194158d652d47d3bd636f9d458 Mon Sep 17 00:00:00 2001
From: Henrik Wenz <henrik.wenz@gmail.com>
Date: Wed, 8 Aug 2018 18:47:22 +0200
Subject: [PATCH 2/3] Document getInitialProps error

---
 .../get-inital-props-as-an-instance-method.md | 39 +++++++++++++++++++
 lib/utils.js                                  |  2 +-
 2 files changed, 40 insertions(+), 1 deletion(-)
 create mode 100644 errors/get-inital-props-as-an-instance-method.md

diff --git a/errors/get-inital-props-as-an-instance-method.md b/errors/get-inital-props-as-an-instance-method.md
new file mode 100644
index 0000000000000..d9c58e617f182
--- /dev/null
+++ b/errors/get-inital-props-as-an-instance-method.md
@@ -0,0 +1,39 @@
+# getInitialProps was defined as an instance method
+
+#### Why This Error Occurred
+
+`getInitialProps` must be a static method in order to be called by next.js.
+
+#### Possible Ways to Fix It
+
+Use the static keyword.
+
+```js
+export default class YourEntryComponent extends React.Component {
+  static getInitialProps () {
+    return {}
+  }
+
+  render () {
+    return 'foo'
+  }
+}
+```
+
+or
+
+```js
+const YourEntryComponent = function () {
+  return 'foo'
+}
+
+YourEntryComponent.getInitialProps = () => {
+  return {}
+}
+
+export default YourEntryComponent
+```
+
+### Useful Links
+
+- [Fetching data and component lifecycle](https://github.com/zeit/next.js#fetching-data-and-component-lifecycle)
diff --git a/lib/utils.js b/lib/utils.js
index 2474870175c41..c1daa56f14994 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -57,7 +57,7 @@ export async function loadGetInitialProps (Component, ctx) {
   if (process.env.NODE_ENV !== 'production') {
     if (Component.prototype && Component.prototype.getInitialProps) {
       const compName = getDisplayName(Component)
-      const message = `"${compName}.getInitialProps()" is defined as an instance method.`
+      const message = `"${compName}.getInitialProps()" is defined as an instance method - visit https://err.sh/next.js/get-inital-props-as-an-instance-method for more information.`
       throw new Error(message)
     }
   }

From 7a0b3d7fa271d5efe1b07dfe66303e83c0ac2dc9 Mon Sep 17 00:00:00 2001
From: Henrik Wenz <henrik.wenz@gmail.com>
Date: Thu, 9 Aug 2018 10:10:50 +0200
Subject: [PATCH 3/3] Add unit tests for loadGetInitialProps

---
 test/unit/loadGetInitialProps.test.js | 52 +++++++++++++++++++++++++++
 1 file changed, 52 insertions(+)
 create mode 100644 test/unit/loadGetInitialProps.test.js

diff --git a/test/unit/loadGetInitialProps.test.js b/test/unit/loadGetInitialProps.test.js
new file mode 100644
index 0000000000000..860e90406b64c
--- /dev/null
+++ b/test/unit/loadGetInitialProps.test.js
@@ -0,0 +1,52 @@
+/* global describe, it, expect */
+import { loadGetInitialProps } from '../../dist/lib/utils'
+
+describe('loadGetInitialProps', () => {
+  it('should throw if getInitialProps is defined as an instance method', () => {
+    class TestComponent {
+      getInitialProps () {}
+    }
+    const rejectPromise = loadGetInitialProps(TestComponent, {})
+    const error = new Error('"TestComponent.getInitialProps()" is defined as an instance method - visit https://err.sh/next.js/get-inital-props-as-an-instance-method for more information.')
+    return expect(rejectPromise).rejects.toEqual(error)
+  })
+
+  it('should resolve to an object if getInitialProps is missing', async () => {
+    const result = await loadGetInitialProps(() => {}, {})
+    expect(result).toEqual({})
+  })
+
+  it('should resolve getInitialProps', async () => {
+    class TestComponent {
+      static async getInitialProps () {
+        return { foo: 1 }
+      }
+    }
+    const result = await loadGetInitialProps(TestComponent, {})
+    expect(result).toEqual({ foo: 1 })
+  })
+
+  it('should be able to return an invalid value if the request was already sent', async () => {
+    class TestComponent {
+      static async getInitialProps () {
+        return 'invalidValue'
+      }
+    }
+    const ctx = {
+      res: {
+        finished: true
+      }
+    }
+    const result = await loadGetInitialProps(TestComponent, ctx)
+    expect(result).toBe('invalidValue')
+  })
+
+  it('should throw if getInitialProps won\'t return an object ', () => {
+    class TestComponent {
+      static async getInitialProps () {}
+    }
+    const rejectPromise = loadGetInitialProps(TestComponent, {})
+    const error = new Error('"TestComponent.getInitialProps()" should resolve to an object. But found "undefined" instead.')
+    return expect(rejectPromise).rejects.toEqual(error)
+  })
+})