用toString生成随机字符

function getCode(len) {
    len = len || 4;
    if (len > 6) {
        // 如果大于6位，我们采用生成多组的方法来处理，每组6位
        var time = len/6|0;
        var lastNum = len%6;
        var code = [];
        for(var i=0; i<time; i++){
            code.push(generate(6));
        }
        if (lastNum) {
            code.push(generate(lastNum));
        }
        return code.join('');
    }else{
        return generate(len);
    }
    function generate(len) {
            var start = Math.pow(36,len-1);
            var end = (Math.pow(36,len)-1);
            var number = start + Math.random()*(end-start) | 0;
            var code = number.toString(36);
            return code;
    }
}