JSFiddle - React, Tailwind, and code Playground

HTML

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

JavaScript

var res  = GetRes( [1,8,4,6,5,7] // ["1,8", "4,6,8", "8,9", "6,9"]
                  );

$('#result').text('Result: ' + res);

function GetRes(arr){
	//1) create single array
	arr = arr .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, return  that element
        console.log('Most occuring: ' + res + ' (' + max + ' times)');
        return res;    
    }

    //3 & 4)    
    var cmpArr = ['6','7','4','1','2','8','9','5'];
    
   	//return based on the index of compare array:     
    for(var i= 0 ; i < cmpArr.length; i++)
        if(arr.indexOf(cmpArr[i]) !== -1)return cmpArr[i];
}