无法读取属性'length'的null（javascript） [英] Cannot read property 'length' of null (javascript)

问题描述

尝试调试时，我遇到此行的'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，=）

While trying to debug I am get the 'length' null error with this line. It is written just like the book instructed, so I don't understand why it is giving me the error? Thanks, =)

if (capital.length < 1) {

（这是所要求的完整代码.. SORRY）

( here is the full code as requested.. 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> 

推荐答案

正确的测试是：

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

当您执行长度检查时，这确保 capital 始终非空。

This ensures that capital is always non null, when you perform the length check.

此外，正如评论所示，资本是 null 因为你从未初始化它。

Also, as the comments suggest, capital is null because you never initialize it.

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