[рони]

prototip,

let input = "aaa bb bb rrrrr aaa r t";

function countWords(str, number){
    str = str.split(/\s+/);
    const obj = str.reduce((a, b) => (a[b] = (a[b]||0) + 1, a), {});
    str = Object.keys(obj).sort((a, b) => (obj[b] - obj[a]) || (b.length -  a.length));
    return str.slice(0, number);
}


alert(countWords(input, 2));