function round2(val, len) {
    len = (len || len === 0)? len : 2;
    var flag = Math.pow(10, len);
    if (!val || isNaN(val)) {
        return def;
    }
    val = parseFloat(val);
    if (val.toString().indexOf('.') == -1) {
        return val;
    }
    var decimal = val.toString().split('.')[1];
    var n = val * flag
    if (decimal.charAt(len) >= 5) {
        return Math.floor((n + 1)) / flag;
    } else {
        return Math.floor(n) / flag;
    }
}

round2(3.145,2)
3.15
round2(3.135,2)
3.14
round2(3.134,2)
3.13
round2(3.1345,3)
3.135
round2(3.1344,3)
3.134