空检查正在产生问题

Question

let age = prompt('What is your name?');
if(age  != null){
document.write(`Your name is ${age}`) 
}else{
document.write("You didn't enter your age");
}

空检查不起作用，即使我把它留空，它也会显示：你的名字是

但当我写这篇文章时-

let age = prompt('What is your name?');
if(age == null){
document.write(`Your name is ${age}`) 
}else{
document.write("You didn't enter your age");
}

它总是显示"你没有输入你的年龄">

Answer 1

因为当您不输入内容时，age不包含null，而是一个空字符串(长度为0的字符串(。您可以通过以下方式确认：

let age = prompt('What is your name?');
console.log(age); // prints ''
console.log(age.length); // prints 0
if(age  != null){
document.write(`Your name is ${age}`) 
}else{
document.write("You didn't enter your age");
}

事实上，你只需要做：

let age = prompt('What is your name?');
if(age){
document.write(`Your name is ${age}`) 
}else{
document.write("You didn't enter your age");
}