未捕获的TypeError无法读取未定义的属性“ toLowerCase”

Question

当前从API获取数据以创建像游戏一样的危险游戏，当我尝试从API中获取变量并将其传递给checkUserAnswer ，然后继续在其上使用toLowerCase()时，出现了我的问题，我收到了该错误，从其他问题中收集的信息，我不知道如何解决。

码：

var gamePts;

function game(){

    this.start = function(click){

        if(click == true){
            checkUserAnswer();
        }else{
            getQuestion();
        }
    }
    function getQuestion(){
        $.ajax({
            url:"http://jservice.io/api/random",
            dataType: "json"
        })
        //after the ajax call the data is sent to the populateQuestion method.
        .done(function(data){populateQuestion(data);})
        .fail(function(jqXHR, textStatus, errorThrown){
            showError(errorThrown);
        });
    }
    function populateQuestion(data){

        $("#Category").html(data[0].category.title);
        $("#question").html(data[0].question);
        var $correct = (data[0].answer);
        checkUserAnswer($correct);
    }
    function checkUserAnswer(correct){
        //allows the user answer to appear.
        var Correct = this.correct;
        $("#useAns").show();
        var $userGuess = ($('#AnsBx').val()).toLowerCase();
        //var $Correct = correct.toUpperCase();
        //$("#userA").val(userGuess);

        //$("#userA").html(userGuess);
        //$("#userA").html($userGuess);
        $("#Result").show();
        $("#Result").html(Correct);


        if($userGuess === Correct.toLowerCase()){
            $("#userA").html($userGuess)
        }
        else{
            $("#userA").html("wrong");
        }
        // can't figure out why the toUpperCase won't work, getting cannot read property 'toUpperCase' of undefined
    }
    function guessCount(){
        var guesses;

        //if user answer = game anwser 1 try and win = true, print number of tries

        //if user answer != game anwser ct+1 anwser again

        //if user try = 10 end game show answer to question.
    }
}
//was last working on the comparasion to allow the game to be functional.

Answer 1

this.start = function(click){

    if(click == true){
       //no passing argument.
        checkUserAnswer();
    }else{
        getQuestion();
    }
}

现在，您正在调用该方法，但是此处正确的变量具有空值。

function checkUserAnswer(correct){
    //allows the user answer to appear.
    var Correct = this.correct;
    //here Correct is null
    $("#useAns").show();

    var $userGuess = ($('#AnsBx').val()).toLowerCase();
      //so null.toLowerCase();uncaught error .
    if($userGuess === Correct.toLowerCase()){
        $("#userA").html($userGuess)
    }
    .......
}

编辑1：我现在可以解决的问题是硬编码正确的字符串

$("#Result").html("correct");


        if($userGuess === "correct".toLowerCase()){
            $("#userA").html($userGuess)
        }