A commented and edited version of Jake Ryan `copyWithin` version of the native js method

jake-copyWithin-revisited-v1.js

// jake you forgot the end parameter, added!
function copyWithin(originalArray, target, start, end) {
  var originalLength = originalArray.length;
  var elementsToCopy = [];

  // the vars I need to break down the problem
  var copyFrom, copyTo, pasteFrom, pasteTo;

  // set copyFrom
  // if undefined let's set it to 0
  if (!start) {
    start = 0;
  }
  // set copyFrom to start, meaning 0 or a defined value
  copyFrom = start; 
  // if this value is negative let's start counting from the end
  if (copyFrom < 0) {
    copyFrom += originalLength;
  }

  // set copyTo
  var copyTo = end || originalLength;
  if (copyTo < 0) {
    copyTo += originalLength;
  } 

  // get elementsToCopy
  for (var i = copyFrom, k = 0; i < copyTo; i++) {
    elementsToCopy[k] = originalArray[i];
    k++;
  }

  // set pasteFrom
  // I commented out your calculation because it gave me problems if target < - originalLength
  // if (target < 0) {
  //   target = target + originalLength;
  //   if (target < 0) {
  //     return originalArray;
  //   }
  // }
  pasteFrom = target;
  if (pasteFrom < - originalLength) {
    pasteFrom = 0;
  }
  if (pasteFrom < 0) {
    pasteFrom += originalLength;
  }

  // set pasteTo
  pasteTo = Math.min(pasteFrom + elementsToCopy.length, originalLength);

  // paste elementsToCopy
  for (var i = pasteFrom, k = 0; i < pasteTo; i++) {
    originalArray[i] = elementsToCopy[k];
    k++;
  }

  return originalArray;
}