main.js

    var sumId = 0;

    for (var i = 0; i < instructions.length; i++) {
        var instruction = instructions[i];
        var encrypted = instruction.match(/^[a-z]+/)[0];
        var id = parseInt(instruction.match(/\d+/)[0]);
        var checksum = instruction.match(/\[([a-z]{5})\]/)[1];

        Array.prototype.byCount = function () {
            var array_length = this.length, dict = {};
            for (var i = 0; i < array_length; i++) {
                var letter = this[i];
                if (!letter) {
                    continue
                }

                if (dict[letter] === undefined) {
                    dict[letter] = 1
                } else {
                    ++dict[letter]
                }
            }

            var keys = Object.keys(dict);

            return keys.sort(function (a, b) {
                if (dict[b] !== dict[a]) {
                    return dict[b] - dict[a];
                }
                return a.charCodeAt(0) - b.charCodeAt(0);
            });
        };
        var letters_table = encrypted.split("");
        var sorted_letters_table = letters_table.byCount();
        var counted_checksum = sorted_letters_table.slice(0, 5).join('');

        if (checksum === counted_checksum) {
            sumId += id
        }
    }

    return sumId;