JavaScript如何使用if语句

JavaScript的if语句可以让我们根据某些条件来执行不同的代码块。使用if语句的基本思路是将要执行的代码放在括号内，并使用if关键字进行匹配。下面是一些例子：

简单的if语句：

let age = 18;  
if (age >= 18) {  console.log("You are an adult");  
} else {  console.log("You are not an adult");  
}

上述代码会输出“You are an adult”，因为年龄大于或等于18岁。

多条件if语句：

let day = 25;  
let dayOfWeek = "Monday";  
if (day === 22) {  console.log("It's Tuesday");  
} else if (day < 22 || day === 22 && dayOfWeek !== "Saturday") {  console.log("It's still Monday");  
} else {  console.log("It's Tuesday");  
}

上述代码会输出“It’s Tuesday”，因为今天是星期二，而且星期二是不包括星期六的。如果星期六被认为是星期日，则输出“It’s still Monday”。

嵌套if语句：

let isStudent = true;  
let day = 25;  
if (isStudent && day >= 18) {  if (dayOfWeek === "Friday" || dayOfWeek === "Saturday" || dayOfWeek === "Sunday") {  isStudent = false;  } else {  console.log("You must be a student and weekday is valid");  }  
} else {  console.log("You are not a student and weekday is invalid");  
}

上述代码会输出“You must be a student and weekday is valid”，因为在日历上某一天周五或周六的晚上6点以后的某一天没有注册成为学生，输出该信息。在这种情况下，day变量为25，表示该晚上不在校。然后还可以通过条件执行如下的操作：弹出对话框询问“日历是否正常工作”以判断操作结果，输出到控制台“Please use weekdays instead”或其他结果，总之通过else-if或嵌套语句我们可以在基本if语句的基础上创造出复杂而有趣的条件语句。

大括号内的多条件判断：

let day = 25;  
let dayOfWeek = "Monday";  
let isStudent = true;  
if (day === 22) {  if (dayOfWeek === "Friday" || dayOfWeek === "Saturday" || dayOfWeek === "Sunday") {  isStudent = false;  } else {  console.log("You must be a student and weekday is valid");  }  
} else if (day < 22 || day === 22 && dayOfWeek !== "Saturday") {  console.log("It's still Monday");  
} else {  console.log("It's Tuesday");  
}

上述代码与第3个例子的代码效果相同，但是使用了大括号来明确代码块的结束位置。这种写法更加清晰易懂。

多个条件可以用花括号括起来的写法：

let day = 25;  
let dayOfWeek = "Monday";  
let isStudent = true;  
if ((day >= 18 && day <= 22) || dayOfWeek === "Friday" || dayOfWeek === "Saturday" || dayOfWeek === "Sunday") {  isStudent = false;  
} else {  console.log("You must be a student and weekday is valid");  
}

上述代码将多个条件放在一对花括号中，使用逗号分隔。这种写法也更加清晰易懂。

使用逻辑运算符进行多条件判断：

let day = 25;  
let dayOfWeek = "Monday";  
let isStudent = true;  
if (dayOfWeek === "Monday" || dayOfWeek === "Tuesday" || dayOfWeek === "Wednesday" || dayOfWeek === "Thursday" || dayOfWeek === "Friday" || dayOfWeek === "Saturday" || dayOfWeek === "Sunday") {  isStudent = false;  
} else if (day >= 18 && day <= 22) {  console.log("You are a student");  
} else if (day < 22 && day !== 22) {  console.log("You are not a student");  
} else {  console.log("It's a Monday");  
}

上述代码使用逻辑与运算符对多个条件进行判断。

在多条件if语句中使用switch语句：

let day = 25;  
let dayOfWeek = "Monday";  
let isStudent = true;  
switch (day) {  case 22:  if (dayOfWeek === "Friday" || dayOfWeek === "Saturday" || dayOfWeek === "Sunday") {  isStudent = false;  } else {  console.log("You must be a student and weekday is valid");  }  break;  case 18:  if (dayOfWeek === "Friday" || dayOfWeek === "Saturday" || dayOfWeek === "Sunday") {  isStudent = false;  } else {  console.log("You are a student and weekday is valid");  }  break;  default:  console.log("Other weekday is invalid");  
}

上述代码中使用了switch语句来判断day的值是否在18到22之间，并且根据dayOfWeek的值判断isStudent的值。

这些是JavaScript中if语句的一些基本用法，你可以根据自己的需求和喜好进行学习和使用。