javascript - 如何根据变量是否定义来更改 console.log() 输出消息

Question

您好，我是编码新手，我想尝试一下我学到了多少(不是很多)所以我做了这个:

var Name = prompt('What is your name?') 
var Name
var Age  = prompt('What is your age?') 
var Age 
const RealAge = 2018 - Age
if (Name && RealAge) {
    console.log("Hello " + Name + " you were born in " + RealAge + ' ...')
} else if (typeof Name == 'undefined') {
    console.log("Please insert your name")
} else if (typeof Age == 'undefined' ) {
    console.log("Please insert your age")
}

我想知道如果您将第一个提示留空，如何使 console.log 显示“请插入您的姓名”，如果第二个提示留空，则如何使 console.log 显示“请插入您的年龄”。

如果这是一个愚蠢的问题，我很抱歉，但我两天前才开始，我将感谢您的帮助:)

Answer 1

if (Name && RealAge) {
    console.log("Hello " + Name + " you were born in " + RealAge + ' ...')
} else if (!Name) {
    console.log("Please insert your name")
} else if (!Age) {
        console.log("Please insert your age")
}

如果值为假(未定义、null、空字符串)，

!Name 将求值为 true。

此外，正如 Frederico 提到的，最好在进行任何计算之前检查这些值:

var Name = prompt('What is your name?') 
var Name
var Age  = prompt('What is your age?') 
var Age 
if (!Name) {
    console.log("Please insert your name")
} else if (!Age) {
    console.log("Please insert your age")
} else {
    const RealAge = 2018 - +Age;
    console.log("Hello " + Name + " you were born in " + RealAge + ' ...')
}

如果您需要知道两者均未输入的时间:

var Name = prompt('What is your name?') 
var Name
var Age  = prompt('What is your age?') 
var Age 
if (!Name) {
    if (!Age) {
        console.log("Please insert your name and age")
    }
    else {
        console.log("Please insert your name")
    }
} else if (!Age) {
    console.log("Please insert your age")
} else {
    const RealAge = 2018 - +Age;
    console.log("Hello " + Name + " you were born in " + RealAge + ' ...')
}