fix: throw useful error on invalid custom matchers (#12488)

CHANGELOG.md

@@ -33,6 +33,7 @@
 ### Fixes
 
 - `[expect]` Move typings of `.not`, `.rejects` and `.resolves` modifiers outside of `Matchers` interface ([#12346](https://github.com/facebook/jest/pull/12346))
+- `[expect]` Throw useful error if `expect.extend` is called with invalid matchers ([#12488](https://github.com/facebook/jest/pull/12488))
 - `[jest-config]` Correctly detect CI environment and update snapshots accordingly ([#12378](https://github.com/facebook/jest/pull/12378))
 - `[jest-config]` Pass `moduleTypes` to `ts-node` to enforce CJS when transpiling ([#12397](https://github.com/facebook/jest/pull/12397))
 - `[jest-config, jest-haste-map]` Allow searching for tests in `node_modules` by exposing `retainAllFiles` ([#11084](https://github.com/facebook/jest/pull/11084))

packages/expect/src/__tests__/extend.test.ts

@@ -184,3 +184,27 @@ it('allows overriding existing extension', () => {
 
   jestExpect('foo').toAllowOverridingExistingMatcher();
 });
+
+it('throws descriptive errors for invalid matchers', () => {
+  expect(() =>
+    jestExpect.extend({
+      default: undefined,
+    }),
+  ).toThrow(
+    'expect.extend: `default` is not a valid matcher. Must be a function, is "undefined"',
+  );
+  expect(() =>
+    jestExpect.extend({
+      default: 42,
+    }),
+  ).toThrow(
+    'expect.extend: `default` is not a valid matcher. Must be a function, is "number"',
+  );
+  expect(() =>
+    jestExpect.extend({
+      default: 'foobar',
+    }),
+  ).toThrow(
+    'expect.extend: `default` is not a valid matcher. Must be a function, is "string"',
+  );
+});

packages/expect/src/jestMatchersObject.ts

@@ -6,6 +6,7 @@
  *
  */
 
+import {getType} from 'jest-get-type';
 import {AsymmetricMatcher} from './asymmetricMatchers';
 import type {
   Expect,
@@ -56,6 +57,15 @@ export const setMatchers = (
 ): void => {
   Object.keys(matchers).forEach(key => {
     const matcher = matchers[key];
+
+    if (typeof matcher !== 'function') {
+      throw new TypeError(
+        `expect.extend: \`${key}\` is not a valid matcher. Must be a function, is "${getType(
+          matcher,
+        )}"`,
+      );
+    }
+
     Object.defineProperty(matcher, INTERNAL_MATCHER_FLAG, {
       value: isInternal,
     });