Apply let/const transform.

_baseFlatten.js

@@ -13,14 +13,14 @@ import isFlattenable from './_isFlattenable.js';
  * @returns {Array} Returns the new flattened array.
  */
 function baseFlatten(array, depth, predicate, isStrict, result) {
-  var index = -1,
-      length = array.length;
+  let index = -1;
+  const length = array.length;
 
   predicate || (predicate = isFlattenable);
   result || (result = []);
 
   while (++index < length) {
-    var value = array[index];
+    const value = array[index];
     if (depth > 0 && predicate(value)) {
       if (depth > 1) {
         // Recursively flatten arrays (susceptible to call stack limits).