javascript - 空值和逻辑运算符不起作用

Question

我想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。该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>

Answer 1

我猜这是对返回值的混淆。如果您在提示中未键入任何内容，则会返回空字符串 ("")。但是，如果您单击取消按钮，则会返回 null。您的代码仅检查非 null。 "" 不等于 null，因此通过检查。快速修复可能是更改

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.");
    }