Add tests for behavior when process.nextTick does not exist

src/__tests__/browser.test.js

@@ -0,0 +1,31 @@
+/**
+ * Copyright (c) 2019-present, GraphQL Foundation
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+// Mock out process.nextTick as not existing for this test before requiring.
+process.nextTick = (null: any);
+const DataLoader = require('..');
+
+describe('Browser support', () => {
+  it('batches multiple requests without process.nextTick', async () => {
+    const loadCalls = [];
+    const identityLoader = new DataLoader<number, number>(async keys => {
+      loadCalls.push(keys);
+      return keys;
+    });
+
+    const promise1 = identityLoader.load(1);
+    const promise2 = identityLoader.load(2);
+
+    const [ value1, value2 ] = await Promise.all([ promise1, promise2 ]);
+    expect(value1).toBe(1);
+    expect(value2).toBe(2);
+
+    expect(loadCalls).toEqual([ [ 1, 2 ] ]);
+  });
+});

src/__tests__/oldbrowser.test.js

@@ -0,0 +1,33 @@
+/**
+ * Copyright (c) 2019-present, GraphQL Foundation
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+// Mock out process.nextTick and setImmediate as not existing for this test
+// before requiring.
+process.nextTick = (null: any);
+global.setImmediate = (null: any);
+const DataLoader = require('..');
+
+describe('Old browser support', () => {
+  it('batches multiple requests without setImmediate', async () => {
+    const loadCalls = [];
+    const identityLoader = new DataLoader<number, number>(async keys => {
+      loadCalls.push(keys);
+      return keys;
+    });
+
+    const promise1 = identityLoader.load(1);
+    const promise2 = identityLoader.load(2);
+
+    const [ value1, value2 ] = await Promise.all([ promise1, promise2 ]);
+    expect(value1).toBe(1);
+    expect(value2).toBe(2);
+
+    expect(loadCalls).toEqual([ [ 1, 2 ] ]);
+  });
+});