JSFiddle - React, Tailwind, and code Playground

HTML

<div id ='result'></div>

JavaScript

var carArray  = [1,8,4,6,5,7]; // ["1,8", "4,6,8", "8,9", "6,9"];

//1) create single array
var arr = carArray .join().split(','); 

//2) find most occurring
var counts = {}; //object to hold count for each occurence    
var max = 0, res;
arr.forEach(function(el){     
    var cnt = (counts[el] || 0); //previous count
    counts[el] = ++cnt;
    if(cnt > max && cnt > 1){ //only register if more than once (cnt>1)
        max=cnt;
        res = el;
    }
});


if(res){
    //there was an element more than once, maxOccuring contains that element
    setResult('Most occuring: ' + res + ' (' + max + ' times)');
}
else{
    //3 & 4)    
    
    var cmpArr = ['6','7','4','1','2','8','9','5'];
   	//sort based on the index of compare array: 
    
    res = function(){ for(var i= 0 ; i < cmpArr.length; i++){ if(arr.indexOf(cmpArr[i]) !== -1)return cmpArr[i];}}();    
    
    setResult('Element by unique sort order: ' +  res);
}

//variable res contains the found element in both cases

//below is only for test display purposes
function setResult(res){
    console.log(res);
    $('#result').text(res);
}