[рони]

prototip,

... тут не переводимый на русский комментарий.

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

function countWords(str, number){
    str = str.split(" ");
    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));
    if(number == void 0) number = str.length;
    return str.slice(0, number).map(a => [a, obj[a]]);
}


alert(JSON.stringify(countWords(input)));

или так

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

function countWords(str){
    str = str.split(" ");
    const obj = str.reduce((a, b) => (a[b] = (a[b]||0) + 1, a), {});
    return Object.entries(obj).sort((a, b) => b[1] - a[1]);;
}


alert(JSON.stringify(countWords(input)));