空值和逻辑运算符不起作用

Posted 2023-02-22

【中文标题】空值和逻辑运算符不起作用

【英文标题】：Null values & logical operators not working

【发布时间】：2017-08-02 00:25:37

【问题描述】：

我试图弄清楚为什么如果我输入空值，警报仍然会在不应该出现的时候出现。参数声明所有值都不能为空，否则函数应该结束。

有人有什么想法吗？谢谢！

@charset "UTF-8";
/* CSS Document */

body
	height:1000px;
	width:100%;
	background:#fff;
	margin:0;


.divider
	width:100%;
	height:auto;
	background:#CCC;
	display:block;
	margin:10px;


h2
	font-size:16px;
	display:block;

#confirm-paragraph
#global-variable-paragraph
#user-input
#expressions
#elephant-paragraph
#method-1
#method-2
#ml
#litres
#conditional-operator
#cast-1
#cast-2

<!-- Checklist: Obtain user input and store it in variables -->
<!-- Report variable text to the client window -->
<!-- Prompt -->
<section class="divider">
<h2>User Input Variables Example</h2>
<button onclick="nameFunction()">Click Me</button>
<p id="user-input">This text should change after clicking the button.</p>
<p style="color:red; font-weight:bold">NOT WORKING Version3!!!!!!!!</p>
<p>Should not alert if null values are present - null values are present by just clicking OK when entering nothing. All values should not be null in order for the alert to appear</p>
<script>
function nameFunction() 
	var yourforename = prompt("What is your first name?");
	var yoursurname = prompt("What is your last name?");
	var yourage = prompt("What is your age?");

	if (yourforename != null && yoursurname != null && yourage != null) 
	alert("Hello " + yourforename + " " + yoursurname + ". You are " + yourage + " years old.");
	

</script>
</section>

【问题讨论】：

很确定如果你没有在prompt 中输入任何内容，它会返回一个空字符串而不是null。此外，您可以删除 CSS/html 代码并删除两种语言的标签，因为它们与您的问题无关。

【参考方案1】：

我猜这是对返回值的混淆。如果您在提示中不输入任何内容，则返回空字符串 ("")。但是，如果单击取消按钮，则返回 null。您的代码只检查不是null。 "" 不等于 null，所以通过检查。一个快速的解决办法是改变

if (yourforename != null && yoursurname != null && yourage != null) 
    alert("Hello " + yourforename + " " + yoursurname + ". You are " + yourage + " years old.");
    

到

if (yourforename && yoursurname && yourage) 
    alert("Hello " + yourforename + " " + yoursurname + ". You are " + yourage + " years old.");
    

【讨论】：

您不希望在更正版本中使用前缀 ! 运算符。

好收获！我修好了。

【参考方案2】：

如果您提醒typeof yourforename 变量等...您将看到它返回一个字符串。因此，如果您这样重写 if 语句：

var yourforename = prompt("What is your first name?");
var yoursurname = prompt("What is your last name?");
var yourage = prompt("What is your age?");

if (yourforename > '' && yoursurname > '' && yourage > '') 
  alert("Hello " + yourforename + " " + yoursurname + ". You are " + yourage + " years old.");

它会起作用的。因为您现在正在检查字符串的长度是否大于空的''。