Count lucky tickets regarding max length

tickets.js

var Tickets = function(maxLen){
    if (maxLen % 2) {
        throw Error('Max length must be even: 2, 4, 6, ...');
    }
    this.maxLen = parseInt((new Array(maxLen / 2 + 1)).join(9));
};
Tickets.prototype.count = function(num, undefined){
    var digits = {},
        i,
        tmp;
    for (i = 0; i <= this.maxLen; i += 1) {
        tmp = i % 10 + ((i % 100) - (i % 10)) / 10 + (i / 100) ^ 0;
        if (digits[tmp] === undefined) {
            digits[tmp] = 1;
        } else {
            digits[tmp] += 1;
        }
    }
    tmp = 0;
    for (i in digits) {
        tmp += digits[i] * digits[i];
    }
    return tmp;
};
console.log((new Tickets(2)).count()); // 10
console.log((new Tickets(4)).count()); // 670
console.log((new Tickets(6)).count()); // 55252
console.log((new Tickets(8)).count()); // 953668