JSFiddle - React, Tailwind, and code Playground

JavaScript

function countNum(array, num){
    var count = 0;
    for(var i = 0; i < array.length; ++i){
        if(array[i] == num)
            count++;
    }
    return count
}

var a=1, b=2, c=3, d=4;
//other stuff
array = ['a', 'b', 'a', 'd']
winner = [0, 0]
arr = ['a', 'b', 'c', 'd']

for (index in arr){
    c = countNum(array, arr[index])
    if (c > winner[0]) {
        winner = [c, arr[index]]
    }
}

window.alert(winner[1])