} function second(){ first(); } first();
In this recursion pattern, two functions each call the other, such that an infinite loop is formed. This is the more troubling pattern and a far more difficult one to identify in large code bases. Most call stack errors are related to one of these two recursion patterns. A frequent cause of stack overflow is an incorrect terminal condition, so the first step after identifying the pattern is to validate the terminal condition. If the terminal condition is correct, then the algorithm contains too much recursion to safely be run in the browser and should be changed to use iteration, memoization, or both.
Iteration Any algorithm that can be implemented using recursion can also be implemented using iteration. Iterative algorithms typically consist of several different loops performing different aspects of the process, and thus introduce their own performance issues. However, using optimized loops in place of long-running recursive functions can result in performance improvements due to the lower overhead of loops versus that of executing a function. As an example, the merge sort algorithm is most frequently implemented using recursion. A simple JavaScript implementation of merge sort is as follows: function merge(left, right){ var result = []; while (left.length > 0 && right.length > 0){ if (left[0] < right[0]){ result.push(left.shift()); } else { result.push(right.shift()); } } }
return result.concat(left).concat(right);
function mergeSort(items){ if (items.length == 1) { return items; } var middle = Math.floor(items.length / 2), left = items.slice(0, middle),
76 | Chapter 4:â&#x20AC;&#x201A;Algorithms and Flow Control