SOURCE

var arr=[1,2,3,4,5,6,7,8,9,1,2,3,1,2];
var obj={};
arr.forEach((item)=>{obj[item]?obj[item]++:obj[item]=1});

console.log(Object.keys(obj).sort((a,b)=>obj[b]-obj[a]).filter((item,index,arr)=>obj[item]===obj[arr[0]]))