const mergeSort = function(a,b){ var arr = []; while(a.length && b.length){ a[0] < b [0] ? arr.push(a.shift()) : arr.push(b.shift()); } while(a.length) arr.push(a.shift()); while(b.length) arr.push(b.shift()); return arr; } const sort = function(arr){ if(arr.length < 2) return arr; var split = Math.floor(arr.length/2); return mergeSort( sort(arr.slice(0,split)), sort(arr.slice(split, arr.length)) ); } console.log(sort([4,9,7,5,3,1,2,10,8,6]));