var randomNumber = function (min, max, length, arr) {
    var num = parseInt(Math.random() * max);
    if (num < min) {
        randomNumber(min, max, length, arr);
        return;
    }
    if (arr.length >= length) {
        return arr;
    }
    arr.push(num);
    randomNumber(min, max, length, arr);
}

window.onload = function () {
    var arr = [];
    randomNumber(10, 21, 5, arr);
    arr.sort(function (a, b) {
        return b - a;
    });
    console.log(arr);
}

JavaScript 选取 min 到 max 之间的 length 个数字并排序