JSFiddle - React, Tailwind, and code Playground

JavaScript

var array1 = [1,5,50,4,12,3,60];
      var array2 = [12,1,5,4,43,3,60];
      
      
      alert(mySpecialMergeLeft(array1,array2));
      // 43,1,5,4,12,3,60
      // same as array1 with added 43 at left and removed 50

      alert(mySpecialMergeRight(array1,array2));
      // 1,5,4,12,3,60,43
      // same as array1 with added 43 at right and removed 50
      
      function mySpecialMergeLeft(array1,array2) {
          var pos;
          var array3=[];
          var array4=array2.slice();
          for(var i=0;i<array1.length;i++) {
              pos=array4.indexOf(array1[i]);
            if(pos!=-1) {
              array3.push(array1[i]);
              array4.splice(pos,1);
            }
          }
          return array4.concat(array3);
      }

      function mySpecialMergeRight(array1,array2) {
          var pos;
          var array3=[];
          var array4=array2.slice();
          for(var i=0;i<array1.length;i++) {
              pos=array4.indexOf(array1[i]);
            if(pos!=-1) {
              array3.push(array1[i]);
              array4.splice(pos,1);
            }
          }
          return array3.concat(array4);
      }