Implement custom handler for unhandled promise rejections in Jest tests

Author: NShahri

jest.config.ts

@@ -1,3 +1,16 @@
+const _ = process;
+
+// @ts-expect-error we need to put this inside jest.config.js,
+// since it's being read before the process object is replaced by the jest runner.
+// More info:
+// https://johann.pardanaud.com/blog/how-to-assert-unhandled-rejection-and-uncaught-exception-with-jest/
+// https://github.com/jestjs/jest/issues/5620
+// https://github.com/jestjs/jest/issues/11165
+// https://codesandbox.io/p/devbox/z9qdp4?migrateFrom=zzjfzz
+global._onUnhandledRejection = handler => {
+  _.on('unhandledRejection', handler);
+};
+
 import { createDefaultEsmPreset } from 'ts-jest/dist/presets/create-jest-preset.js';
 
 const tsJestTransformCfg = createDefaultEsmPreset().transform;

src/__tests__/unhandled.test.ts

@@ -7,22 +7,22 @@
 
 import DataLoader from '../index.ts';
 import { describe, it, expect, jest } from '@jest/globals';
-import process from 'node:process';
 
 describe('Unhandled rejections', () => {
   it('Not catching a primed error is an unhandled rejection', async () => {
-    //process.removeAllListeners('unhandledRejection');
     const handler = jest.fn();
-    process.on('unhandledRejection', handler); //detectOpenHandles
+    // @ts-expect-error need to use injected _onUnhandledRejection as the original process.on may have been overridden by Jest
+    global._onUnhandledRejection(handler); //detectOpenHandles
 
     const identityLoader = new DataLoader<number, number>(async keys => keys);
     identityLoader.prime(1, new Error('Error: 1'));
 
-    // Ignore result.
     const promise = identityLoader.load(1);
 
     await new Promise(resolve => setTimeout(resolve, 1000));
     expect(handler).toHaveBeenCalled();
+
+    // Prevent Jest from complaining about the unhandled rejection
     promise.catch(() => {});
   });
 });