print out big number in javascript (length > 22)

BigNumber_lite.js

/**
 * var big1 = new BigNum('234234234234234');
 * var big2 = new BigNum('234234234234234');
 * big1.multiplyBy(big2).toString();  // will output "54865676487297999188377566756" instead of 5.4865676487298e+28
 */

var BigNum = function(num) {
    if (typeof num === 'string') {
        this.parts = [];
        while (num.length) {
            this.parts.push(parseInt(num.slice(-5),10));
            num = num.substring(0,num.length-5)
        }    
    } else {
        this.parts = num;
    }
    
}

BigNum.prototype.multiplyBy = function(bigNum) {
    var newParts = [];
    for (var i = 0, l1 = this.parts.length; i < l1; ++i) {
        for (var j = 0, l2 = bigNum.parts.length; j < l2; ++j) {
            var newPartIndex = i + j;
            newParts[newPartIndex] = this.parts[i] * bigNum.parts[j] + (newParts[newPartIndex] || 0);
        }
    }
    this._fixOverflows(newParts);
    return new BigNum(newParts);
};

BigNum.prototype.add = function(bigNum) {
    var newParts = [];
    var length = Math.max(this.parts.length, bigNum.parts.length);
    for (var i = 0; i < length; ++i) {
        newParts[i] = (this.parts[i] || 0) + (bigNum.parts[i] || 0);
    }
    this._fixOverflows(newParts);
    return new BigNum(newParts);
}

BigNum.prototype._fixOverflows = function(parts) {
    for (var k = 0; k < parts.length; ++k) {
        var currentPart = parts[k].toString();
        if (currentPart.length > 5) {
            var overflowLength = currentPart.length - 5;
            var overflow = parseInt(currentPart.substr(0, overflowLength), 10);
            parts[k] = parseInt(currentPart.substr(overflowLength), 10)
            parts[k + 1] = overflow + (parts[k + 1] || 0);
        }
    }
}

BigNum.prototype.toString = function() {
    var fullNumber = this.parts.map(function(num) {
        num = num.toString();
        paddingAmount = 5 - num.length;
        for (var i = 0; i < paddingAmount; ++i) {
            num = '0' + num;
        }
        return num
    }).reverse().join('');
    while( fullNumber.charAt(0) === '0' && fullNumber.length !== 1) {
        fullNumber = fullNumber.substring(1);
    }
    return fullNumber;
};