Question

我正在尝试在字符串中的＃后测试空值。我已经尝试过各种各样的方法，但在提交测试数据时，我总是得到一个无法读取属性'1'的null。我已经找出了我能想到的错误，但这个我似乎无法解决。请记住，我是一个初学者，我没有编程，因为cobol天和我最后一次在javascript工作是在21世纪初。

//启动测试数据，可能通过的5个可能的字符串

elt.message = '#1a' //goes through the script good
elt.message = '#12b' // goes through
elt.message = '#123c' //goes through
elt.message = '' //is ignored
elt.message = '# ' //crashes server

//结束测试数据

//First lets test to see if # is in the message. If true then we will parse it and add it to the database.
var str = elt.message;
var substr = '#';
var vtest = str.indexOf(substr) > -1;
if (vtest == 1){
var Vname = elt.author;
console.log('We tested for # and the value is true');

//extracts the number and the letter after the # from incoming chat messages
var test = elt.message; // replace with message text variable. 
var pstr = test.match(/#(\d{1,3})([a-zA-Z])/);
if (pstr) {
var numbers = pstr[1];
var character = pstr[2];
var chupp = character.toUpperCase(); //Converts the lowercase to uppercase
}

//Tests to see if neither the question number or the possible answer is left out
//if (pstr[1] !== '' && pstr[2] !== ''){ //doesn't work =(
if (pstr[1] !== null && pstr[2] !== null){ //doesn't work either =(

console.log('we processed the numbers after the #sign and assigned the numbers and letter into variables.')
console.log('The question number is: ' + pstr[1]);
console.log('The letter processed is: ' + pstr[2]);

// Grabs the date and converts it into the YYYYMMDD string.
var dobj = new Date();
var dstr = dobj.toString();
var dsplit = dstr.split(' ');
let currentdate = `${dobj.getMonth() < '9' ? `0${dobj.getMonth() + 1}` :
  dobj.getMonth() + 1}`;
  currentdate = `${dsplit[3]}${currentdate}${dsplit[2]}`;
console.log(currentdate)//remove when done

//checks to see what the highest question number is in the database
var sel = con.query("SELECT * FROM questions WHERE ClassID = "+ currentdate + " ORDER BY QuesID DESC LIMIT 1", function (err, result){
    if (err) throw err;
    console.log('Total number of question records: '+result[0].QuesID);
    console.log('the script is querying with' + pstr[1]);
    console.log('the scripts answer letter is ' + pstr[2]);

    if (pstr[2] != '' && pstr[1] <= result[0].QuesID ){
        var query =  con.query("SELECT * FROM questions WHERE ClassID = " + currentdate + " AND QuesID = " + pstr[1], function (err, result) { // Selects the record based on the Date and the question number variables provided above
        if (err) throw err;
        console.log('it got past the test')

    if (result[0].AnsweredFirst === '' && result[0].AnswerLetter === chupp) { //Test to see if the AnsweredFirst is empty and that the Answer letter matchs with whats on file
    console.log('MATCH!');//remove when done

    var sql = "UPDATE questions SET AnsweredFirst = '"+ Vname + "' WHERE ClassID = " + currentdate + " AND QuesID = " + pstr[1]; //Updates the record with the first person who answered the question in the AnsweredFirst field
     con.query(sql, function (err, result) {
      if (err) throw err;
      console.log(Vname + " answered question " + pstr[1] + " First!");
     });
    }
  });
 }
});
} else {
    console.log('Either the question number or the letter was left blank so we are skipping'); //the viewer did not put in a proper number and letter after the # sign
    }
} else {
    console.log('No signs of # so skipping queries') //if there is no # sign the message is not processed 
    };

我添加了脚本的其余部分以获得更好的主意。消息从聊天客户端传递到服务器。

我会尝试将代码块移动到第一个if语句中。我知道它的凌乱，但说实话，我很惊讶我到目前为止。

Answer 1

看。如果您的测试字符串与您的正则表达式不匹配，则pstr指定为null。除了在下一个if条件中，您还尝试检查pstr的第一个元素，而不检查空值：

if (pstr[1] !== null && pstr[2] !== null){ //doesnt work either =(

因此，我认为您需要在第二个pstr!==null中添加if，或者从此if内移动所有条件分支，然后再移动前一个if语句的一部分。

Answer 2

var pstr = test.match(/#(\d{1,3})([a-zA-Z])/);

表示如果找不到正则表达式的匹配项，则pstr为null

在这种情况下，pstr的任何索引（如pstr [1]，pstr [2]）都会抛出你所描述的错误：

无法阅读财产＆＃39; n＆＃39;为null

<强>解决方案：

在使用索引之前，请检查变量是否具有值

if(pstr !== null) {
    // do something with pstr[1]
}

修改

正如nnnnnn正确指出的那样，你不能在字符串中明确地存储一个空值。

检查null只会在nodejs中获取“无法读取null的属性'1”

2 个答案: