copyWithin

copyWithin.js

function copyWithin (array, startPasting, startCopying, endCopying) {
          /* TYPE ERRORS */
          // no arguments passed in
          if (arguments.length === 0) {
            throw new TypeError('undefined is not an array');
          }

          // The following is taken from underscore.js. Basically it checks if the length property of arguments[0] is there
          var shallowProperty = function (key) {
            return function (obj) {
              return obj == null ? void 0 : obj[key];
            };
          };
          // in Javascript arrays can't contain more elements than this
          var MAX_ARRAY_INDEX = Math.pow(2, 53) - 1; // 9007199254740991

          // getLength is a function, because shallowProperty returns a function
          var getLength = shallowProperty('length'); 

          var isArrayLike = function (collection) {
            var length = getLength(collection);
            return typeof length == 'number' && length >= 0 && length <= MAX_ARRAY_INDEX;
          };

          // first argument is not an array-like object
          if (isArrayLike(arguments[0]) === false) {
            throw new TypeError('value ' + JSON.stringify(arguments[0]) + ', of type ' + typeof arguments[0] + ', is not an array-like object');
          }

          /* CORE LOGIC */
          // step 1: get array part beginning and ending index
          var partFromIndex = getPartStart();
          var partToIndex = getPartEnd();

          // step 2: get array part
          var part = copyPart(array, partFromIndex, partToIndex);

          // step 3: get array beginning and ending paste index
          var pasteFromIndex = getPasteStart();
          var pasteToIndex = getPasteStop();

          // step 4: paste part into array
          pastePart(array, part);

          // by putting the code into functions I try to make the solution logic clear at first glance, just by reading var and functions name - which I choose carefully
          function getPartStart () {
            var from = startCopying || 0; 
            if (from < 0) {
                from += array.length;
            }
            return from;
          }

          function getPartEnd () {
            var to = endCopying || array.length;
            if (to < 0) {
                  to += array.length;
            } 
            return to;
          }

          function copyPart (array, fromIndex, toIndex) {
            var arrayPart = [];
            for (var i = fromIndex; i < toIndex; i++) {
              // copy also the array holes 
              if (i in array) {
                arrayPart[arrayPart.length] = array[i];
              } else {
                arrayPart.length++;
              }
            }
            return arrayPart;
          }

          function getPasteStart () {
            if (startPasting < - array.length) {
              startPasting = 0;
            }
            if (startPasting < 0) {
              startPasting += array.length;
            }
            return startPasting;
          }

          function getPasteStop () {
            var PasteStart = getPasteStart();
            var PasteStop = Math.min(PasteStart + part.length, array.length);
            return PasteStop;
          }

          function pastePart(array, arrayPart) {
            var j = 0;
            for (var i = pasteFromIndex; i < pasteToIndex; i++) {
                // also paste holes
                if (j in arrayPart) { 
                  array[i] = arrayPart[j];
                  j++;
                } else {
                  delete array[i];
                  j++;
                }
            }
          }

          return array;
        }