Update solution for basic recursion

Author: basemath

Diff for: exercises/basic_recursion/solution/solution.js

@@ -1,8 +1,7 @@
 function reduce(arr, fn, initial) {
-  return (function reduceOne(index, value) {
-    if (index > arr.length - 1) return value // end condition
-    return reduceOne(index + 1, fn(value, arr[index], index, arr)) // calculate & pass values to next step
-  })(0, initial) // IIFE. kick off recursion with initial values
+  if (arr.length === 0) return initial // end condition
+  var subset = arr.slice(0, arr.length - 1) // setup recursive call with left subset
+  return fn(reduce(subset, fn, initial), arr[arr.length - 1]) // pass last element to traverse ltr
 }
 
 module.exports = reduce