From 8a1ff856043d59396f623f8ca209822b1331d85f Mon Sep 17 00:00:00 2001
From: urugator <11457665+urugator@users.noreply.github.com>
Date: Mon, 6 Sep 2021 16:45:36 +0200
Subject: [PATCH] Missing render on observer throws (#3103)

---
 .changeset/blue-hairs-itch.md                   | 5 +++++
 packages/mobx-react/__tests__/observer.test.tsx | 9 +++++++++
 packages/mobx-react/src/observerClass.ts        | 8 ++++++++
 3 files changed, 22 insertions(+)
 create mode 100644 .changeset/blue-hairs-itch.md

diff --git a/.changeset/blue-hairs-itch.md b/.changeset/blue-hairs-itch.md
new file mode 100644
index 000000000..d2a9244a8
--- /dev/null
+++ b/.changeset/blue-hairs-itch.md
@@ -0,0 +1,5 @@
+---
+"mobx-react": patch
+---
+
+Missing `render` on component prototype throws.
diff --git a/packages/mobx-react/__tests__/observer.test.tsx b/packages/mobx-react/__tests__/observer.test.tsx
index 9a4d36310..147024156 100644
--- a/packages/mobx-react/__tests__/observer.test.tsx
+++ b/packages/mobx-react/__tests__/observer.test.tsx
@@ -880,3 +880,12 @@ test("Redeclaring an existing observer component as an observer should log a war
     observer(AlreadyObserver)
     expect(warn).toHaveBeenCalled()
 })
+
+test("Missing render should throw", () => {        
+    class Component extends React.Component<any, any> {
+        render = function() {
+            return <div />
+        }   
+    }          
+    expect(() => observer(Component)).toThrow();
+})
diff --git a/packages/mobx-react/src/observerClass.ts b/packages/mobx-react/src/observerClass.ts
index 7fddc23c4..66bd0513a 100644
--- a/packages/mobx-react/src/observerClass.ts
+++ b/packages/mobx-react/src/observerClass.ts
@@ -51,6 +51,14 @@ export function makeClassComponentObserver(
     makeObservableProp(target, "state")
 
     const baseRender = target.render
+    if (typeof baseRender !== 'function') {
+        const displayName = getDisplayName(target)
+        throw new Error(
+            `[mobx-react] class component (${displayName}) is missing \`render\` method.`
+            + `\n\`observer\` requires \`render\` being a function defined on prototype.`
+            + `\n\`render = () => {}\` or \`render = function() {}\` is not supported.`
+        )
+    }
     target.render = function () {
         return makeComponentReactive.call(this, baseRender)
     }