Optimize and flowify mapWithSeparator

Summary:
Update mapWithSeparator so that Flow can reason about the arguments and return
type. For simplicity, it is expected that the type of the separator will be the
same as that of the mapped item.

Reviewed By: vjeux

Differential Revision: D3323557

fbshipit-source-id: 75b59e928d4e8c309b5933499a14744370ee5660

Libraries/Utilities/__tests__/mapWithSeparator-test.js

@@ -0,0 +1,57 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+'use strict';
+
+jest.unmock('mapWithSeparator');
+
+describe('mapWithSeparator', () => {
+  const mapWithSeparator = require('mapWithSeparator');
+
+  it('mapWithSeparator returns expected results', () => {
+    const array = [1, 2, 3];
+    const result = mapWithSeparator(
+      array,
+      function(value) {
+        return value * 2;
+      },
+      function() {
+        return 0;
+      }
+    );
+    expect(result).toEqual([2, 0, 4, 0, 6]);
+  });
+
+  it('mapWithSeparator indices are correct', () => {
+    const array = [1, 2, 3];
+    const result = mapWithSeparator(
+      array,
+      function(value, index) {
+        return index;
+      },
+      function(index) {
+        return index;
+      }
+    );
+    expect(result).toEqual([0, 0, 1, 1, 2]);
+  });
+
+  it('mapWithSeparator passes correct array and indices', () => {
+    const array = [3, 2, 1];
+    const result = mapWithSeparator(
+      array,
+      function(value, index, arr) {
+        return arr[index];
+      },
+      function(index) {
+        return index;
+      }
+    );
+    expect(result).toEqual([3, 0, 2, 1, 1]);
+  });
+});

Libraries/Utilities/mapWithSeparator.js

@@ -1,19 +1,29 @@
 /**
- * Copyright 2004-present Facebook. All Rights Reserved.
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule mapWithSeparator
+ * @flow
  */
 'use strict';
 
-function mapWithSeparator(array, valueFunction, separatorFunction) {
-  var results = [];
-  for (var i = 0; i < array.length; i++) {
-    results.push(valueFunction(array[i], i, array));
-    if (i !== array.length - 1) {
-      results.push(separatorFunction(i));
+function mapWithSeparator<TFrom, TTo>(
+  items: Array<TFrom>,
+  itemRenderer: (item: TFrom, index: number, items: Array<TFrom>) => TTo,
+  spacerRenderer: (index: number) => TTo,
+): Array<TTo> {
+  const mapped = [];
+  if (items.length > 0) {
+    mapped.push(itemRenderer(items[0], 0, items));
+    for (let ii = 1; ii < items.length; ii++) {
+      mapped.push(spacerRenderer(ii - 1), itemRenderer(items[ii], ii, items));
     }
   }
-  return results;
+  return mapped;
 }
 
 module.exports = mapWithSeparator;