Solution

src/arrayMethodSort.js

@@ -4,8 +4,48 @@
  * Implement method Sort
  */
 function applyCustomSort() {
-  [].__proto__.sort2 = function(compareFunction) {
-    // write code here
+  [].__proto__.sort2 = function (compareFunction) {
+    const compare = compareFunction || ((a, b) => String(a) > String(b));
+
+    const mergeSort = (arr) => {
+      if (arr.length <= 1) {
+        return arr;
+      }
+
+      const middle = Math.floor(arr.length / 2);
+      const left = arr.slice(0, middle);
+      const right = arr.slice(middle);
+
+      return merge(mergeSort(left), mergeSort(right));
+    };
+
+    const merge = (left, right) => {
+      const resultArray = [];
+      let leftIndex = 0;
+      let rightIndex = 0;
+
+      while (leftIndex < left.length && rightIndex < right.length) {
+        if (compare(left[leftIndex], right[rightIndex]) <= 0) {
+          resultArray.push(left[leftIndex]);
+          leftIndex++;
+        } else {
+          resultArray.push(right[rightIndex]);
+          rightIndex++;
+        }
+      }
+
+      return resultArray
+        .concat(left.slice(leftIndex))
+        .concat(right.slice(rightIndex));
+    };
+
+    const sortedArray = mergeSort(this);
+
+    for (let i = 0; i < this.length; i++) {
+      this[i] = sortedArray[i];
+    }
+
+    return this;
   };
 }