Missing render on observer throws (#3103)

.changeset/blue-hairs-itch.md

@@ -0,0 +1,5 @@
+---
+"mobx-react": patch
+---
+
+Missing `render` on component prototype throws.

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();
+})

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)
     }