use global dependency when DataLoader initiated, refactor tests for easier mocking

Author: NShahri

src/__tests__/browser.test.ts

@@ -5,15 +5,21 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-// Set up mocks to simulate a recent browser environment.
-// Remove process.nextTick.
-// This mock must be imported before importing DataLoader.
-import './nextTick.mock.ts';
-
 import DataLoader from '../index.ts';
-import { describe, it, expect } from '@jest/globals';
+import { describe, it, expect, beforeEach, afterEach } from '@jest/globals';
 
 describe('Browser support', () => {
+  const originalProcess = global.process;
+
+  beforeEach(() => {
+    // @ts-expect-error testing a browser environment by removing process.nextTick
+    global.process = { nextTick: undefined };
+  });
+
+  afterEach(() => {
+    global.process = originalProcess;
+  });
+
   it('batches multiple requests without process.nextTick', async () => {
     const loadCalls: ReadonlyArray<number>[] = [];
     const identityLoader = new DataLoader<number, number>(async keys => {

src/__tests__/nextTick.mock.ts

@@ -5,16 +5,25 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-// Set up mocks to simulate an old browser environment.
-// Remove both setImmediate and process.nextTick.
-// These mocks must be imported before importing DataLoader.
-import './nextTick.mock.ts';
-import './setImmediate.mock.ts';
-
 import DataLoader from '../index.ts';
-import { describe, it, expect } from '@jest/globals';
+import { describe, it, expect, beforeEach, afterEach } from '@jest/globals';
 
 describe('Old browser support', () => {
+  const originalSetImmediate = global.setImmediate;
+  const originalProcess = global.process;
+
+  beforeEach(() => {
+    // @ts-expect-error testing an old browser environment by removing setImmediate
+    global.setImmediate = undefined;
+    // @ts-expect-error testing an old browser environment by removing process.nextTick
+    global.process = { nextTick: undefined };
+  });
+
+  afterEach(() => {
+    global.setImmediate = originalSetImmediate;
+    global.process = originalProcess;
+  });
+
   it('batches multiple requests without setImmediate', async () => {
     const loadCalls: ReadonlyArray<number>[] = [];
     const identityLoader = new DataLoader<number, number>(async keys => {

src/__tests__/oldbrowser.test.ts

@@ -348,8 +348,8 @@ class DataLoader<K, V, C = K> {
 // for enqueuing a job to be performed after promise microtasks and before the
 // next macrotask. For browser environments, a macrotask is used (via
 // setImmediate or setTimeout) at a potential performance penalty.
-const enqueuePostPromiseJob: (fn: () => void) => void =
-  typeof process === 'object' && typeof process.nextTick === 'function'
+function getEnqueuePostPromiseJob(): (fn: () => void) => void {
+  return typeof process === 'object' && typeof process.nextTick === 'function'
     ? function (fn) {
         if (!resolvedPromise) {
           resolvedPromise = Promise.resolve();
@@ -365,6 +365,7 @@ const enqueuePostPromiseJob: (fn: () => void) => void =
       : function (fn) {
           setTimeout(fn);
         };
+}
 
 // Private: cached resolved Promise instance
 let resolvedPromise: Promise<void> | undefined;
@@ -413,7 +414,7 @@ function getValidBatchScheduleFn<K, V, C>(
 ): (callback: () => void) => void {
   const batchScheduleFn = options && options.batchScheduleFn;
   if (batchScheduleFn === undefined) {
-    return enqueuePostPromiseJob;
+    return getEnqueuePostPromiseJob();
   }
   if (typeof batchScheduleFn !== 'function') {
     throw new TypeError(