未捕获的TypeError：无法读取null的属性“ toUpperCase”

Question

我的代码似乎正确，但我不知道为什么会收到此错误：

未捕获的TypeError：无法读取null的属性“ toUpperCase”

这是我的代码：

    //The function is executed after someone clicks the "Take the Quiz" 
    function startquiz() {
        //The variable for the first question
        var FirstAnwser = prompt("Who posted the first youtube video?");
        //The if statement for the first question
        if (FirstAnwser.toUpperCase()  === 'JAWED KARIM') {
            //If the person is correct a dialog box that says correct pops up
           alert("Correct");
            //The Variable for the second question
            var SecondAnwser = prompt("When was the domain name youtube.com     activated?");
            if (SecondAnwser.toUpperCase() === 'FEBUARY 14, 2005') {
                alert("Correct");
                var ThirdAnwser = prompt("What was the first video on youtube called?");
                if (ThirdAnwser.toUpperCase() === 'ME AT THE ZOO') {
                    alert("Correct");
                } else {
                    alert("Sorry, That is Wrong");
                }

            } else {
                alert("Sorry, That is Wrong");
            }
        } else {
            //If the person is wrong a dialog box pops up which says "Sorry,  That is wrong"
            alert("Sorry, That is Wrong");
        }
    }

错误是在说 if (SecondAnwser.toUpperCase() === 'FEBUARY 14, 2005') {

Answer 1

如果用户单击“确定”，则hint（）方法将返回输入值。如果用户单击“取消”，则该方法返回null，并且您的脚本报告错误，因为null对象上没有任何功能。

解决方案：致电前检查答案是否为空 toUpperCase()

if (SecondAnswer != null && SecondAnwser.toUpperCase() === 'FEBUARY 14, 2005')