Move internal modules to “internal” folder.

.internal/arrayMap.js

@@ -0,0 +1,20 @@
+/**
+ * A specialized version of `map` for arrays.
+ *
+ * @private
+ * @param {Array} [array] The array to iterate over.
+ * @param {Function} iteratee The function invoked per iteration.
+ * @returns {Array} Returns the new mapped array.
+ */
+function arrayMap(array, iteratee) {
+  let index = -1;
+  const length = array == null ? 0 : array.length;
+  const result = Array(length);
+
+  while (++index < length) {
+    result[index] = iteratee(array[index], index, array);
+  }
+  return result;
+}
+
+export default arrayMap;