test(accumulate): add test suite for accumulate function (#15159)

* refactor(typo): remove typo 'be'

* test(accumulate): add test suite for accumulate function

packages/events/__tests__/accumulate-test.internal.js

@@ -0,0 +1,49 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ */
+
+'use strict';
+
+let accumulate;
+
+describe('accumulate', () => {
+  beforeEach(() => {
+    accumulate = require('events/accumulate').default;
+  });
+
+  it('throws if the second item is null', () => {
+    expect(function() {
+      accumulate([], null);
+    }).toThrowError(
+      'accumulate(...): Accumulated items must not be null or undefined.',
+    );
+  });
+
+  it('return second item if first item is null', () => {
+    const a = [];
+    expect(accumulate(null, a)).toBe(a);
+  });
+
+  it('return concatenation of items if first item is an array', () => {
+    const a = ['hello'];
+    const b = 'world';
+    expect(accumulate(a, b)).toEqual(['hello', 'world']);
+  });
+
+  it('return concatenation of items if second item is an array', () => {
+    const a = 'hello';
+    const b = ['world'];
+    expect(accumulate(a, b)).toEqual(['hello', 'world']);
+  });
+
+  it('return an array containing both items if neither item is an array', () => {
+    const a = 'hello';
+    const b = 'world';
+    expect(accumulate(a, b)).toEqual(['hello', 'world']);
+  });
+});

packages/events/accumulate.js

@@ -22,7 +22,7 @@ function accumulate<T>(
 ): T | Array<T> {
   invariant(
     next != null,
-    'accumulate(...): Accumulated items must be not be null or undefined.',
+    'accumulate(...): Accumulated items must not be null or undefined.',
   );
 
   if (current == null) {