无法读取属性'length'的null(javascript)

Question

在尝试调试时,我得到此行的'length'null错误.它写的就像书中指示的那样,所以我不明白为什么它会给我错误？谢谢,=)

if (capital.length < 1) {

(这是所要求的完整代码.. SORRY)

<script type="text/javascript">
var capital = window.prompt("What is the capital of Missouri?","")

if (capital.length < 1) {
    document.getElementById("firstdiv").innerHTML="Sorry you don't feel like playing.<br /> The Capital of Missouri is Jefferson City.";
}
else {
    if (!window.confirm("Is that your final answer?")){ return true;

        document.getElementById("firstdiv").innerHTML = "The capital of Missouri is: <bold>" + capital + "</bold>, so says you.";
    }
    else{
        return false;
    }
}
</script> 

Answer 1

适当的测试是:

if (capital != null && capital.length < 1) {

这保证了capital是永远不空,当您执行长度检查.

此外,正如评论所示,capital是null因为您从未初始化它.