SOURCE

const str = "Hello world!ahsdjaasjdzbxbaozdowqbsbad"
function stString(str) {
    var obj = {};
    var maxStr = { label: "", value: 0 };
    var strArr = str.split("");
    strArr.forEach((item, index) => {
        if (Object.keys(obj).includes(item)) {
            obj[item] = obj[item] + 1;
        } else {
            obj[item] = 1;
        }
        if (maxStr.value < obj[item]) {
            maxStr.label = item;
            maxStr.value = obj[item]
        }
    })
    return { maxStr, obj };
}

console.log(stString(str))