空值和逻辑运算符不起作用

Question

我试图弄清楚为什么如果我输入空值，警告仍然出现在不应该出现的时候。参数声明所有的值都不能为空，否则函数应该结束。

有谁有什么想法吗？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>

​

Answer 1

我猜这是对返回值的混淆。如果在提示符中不键入任何内容，则返回空字符串("")。但是，如果单击cancel按钮，则返回null。您的代码只检查非null。""不等于null，因此pass会进行检查。一种快速的解决办法可能是改变

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

至

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

Answer 2

如果您警告typeof yourforename变量等...您将看到它返回一个字符串。因此，如果将if语句重写如下：

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.");
}

看起来不错。因为您现在正在检查字符串的长度是否大于空的''。