JSFiddle - React, Tailwind, and code Playground

JavaScript

var arr = [3, 6, 6, 1, 5, 8, 9, 6, 6];
arr.sort();

function mine(a) {
    var count = 0;
    var integer = 0;
    var tempCount = 1;
    var tempInteger = 0;
    var prevInt = null
    for (var i = 0; i < arr.length; i++) {
        tempInteger = arr[i]
        if (i > 0) {
            prevInt = arr[i - 1]
        }
        if (prevInt == arr[i]) {
            tempCount += 1
            if (tempCount > count) {
                count = tempCount
                integer = tempInteger
            }
        } else {
            tempCount = 1
        }
    }
    console.log("most repeated is: " + integer)
}

function theirs(a) {
    var count = 1,
        tempCount;
    var popular = a[0];
    var temp = 0;
    for (var i = 0; i < (a.length - 1); i++) {
        temp = a[i];
        tempCount = 0;
        for (var j = 1; j < a.length; j++) {
            if (temp == a[j]) tempCount++;
        }
        if (tempCount > count) {
            popular = temp;
            count = tempCount;
        }
    }
    console.log("most repeated is: " + popular)
}

function better(a) {
    var top = a[0],
        count = 0,
        i = len = a.length - 1;
    while (i--) {
        var j = len,
            temp = 0;
        while (j--) {
            if (a[j] == a[i]) ++temp;
        }
        if (temp > count) {
            count = temp;
            top = a[i];
        }
    }
    console.log("most repeated is " + top);
}

console.clear();

console.time("mine");
for (var i = 0; ++i <= 500;) mine(arr);
console.timeEnd("mine")

console.time("theirs")
for (var i = 0; ++i <= 500;) theirs(arr)
console.timeEnd("theirs")

console.time("better")
for (var i = 0; ++i <= 500;) better(arr)
console.timeEnd("better")