Move internal modules to “internal” folder.

.internal/baseReduce.js

@@ -0,0 +1,23 @@
+/**
+ * The base implementation of `reduce` and `reduceRight` which iterates
+ * over `collection` using `eachFunc`.
+ *
+ * @private
+ * @param {Array|Object} collection The collection to iterate over.
+ * @param {Function} iteratee The function invoked per iteration.
+ * @param {*} accumulator The initial value.
+ * @param {boolean} initAccum Specify using the first or last element of
+ *  `collection` as the initial value.
+ * @param {Function} eachFunc The function to iterate over `collection`.
+ * @returns {*} Returns the accumulated value.
+ */
+function baseReduce(collection, iteratee, accumulator, initAccum, eachFunc) {
+  eachFunc(collection, (value, index, collection) => {
+    accumulator = initAccum
+      ? (initAccum = false, value)
+      : iteratee(accumulator, value, index, collection);
+  });
+  return accumulator;
+}
+
+export default baseReduce;