JavaScript的if語(yǔ)句可以讓我們根據(jù)某些條件來(lái)執(zhí)行不同的代碼塊。使用if語(yǔ)句的基本思路是將要執(zhí)行的代碼放在括號(hào)內(nèi)，并使用if關(guān)鍵字進(jìn)行匹配。下面是一些例子：

簡(jiǎn)單的if語(yǔ)句：

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

上述代碼會(huì)輸出“You are an adult”，因?yàn)槟挲g大于或等于18歲。

多條件if語(yǔ)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");  
}

上述代碼會(huì)輸出“It’s Tuesday”，因?yàn)榻裉焓切瞧诙?#xff0c;而且星期二是不包括星期六的。如果星期六被認(rèn)為是星期日，則輸出“It’s still Monday”。

嵌套if語(yǔ)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");  
}

上述代碼會(huì)輸出“You must be a student and weekday is valid”，因?yàn)樵谌諝v上某一天周五或周六的晚上6點(diǎn)以后的某一天沒(méi)有注冊(cè)成為學(xué)生，輸出該信息。在這種情況下，day變量為25，表示該晚上不在校。然后還可以通過(guò)條件執(zhí)行如下的操作：彈出對(duì)話框詢問(wèn)“日歷是否正常工作”以判斷操作結(jié)果，輸出到控制臺(tái)“Please use weekdays instead”或其他結(jié)果，總之通過(guò)else-if或嵌套語(yǔ)句我們可以在基本if語(yǔ)句的基礎(chǔ)上創(chuàng)造出復(fù)雜而有趣的條件語(yǔ)句。

大括號(hào)內(nèi)的多條件判斷：

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");  
}

上述代碼與第3個(gè)例子的代碼效果相同，但是使用了大括號(hào)來(lái)明確代碼塊的結(jié)束位置。這種寫法更加清晰易懂。

多個(gè)條件可以用花括號(hào)括起來(lái)的寫法：

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");  
}

上述代碼將多個(gè)條件放在一對(duì)花括號(hào)中，使用逗號(hào)分隔。這種寫法也更加清晰易懂。

使用邏輯運(yùn)算符進(jìn)行多條件判斷：

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");  
}

上述代碼使用邏輯與運(yùn)算符對(duì)多個(gè)條件進(jìn)行判斷。

在多條件if語(yǔ)句中使用switch語(yǔ)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");  
}

上述代碼中使用了switch語(yǔ)句來(lái)判斷day的值是否在18到22之間，并且根據(jù)dayOfWeek的值判斷isStudent的值。

這些是JavaScript中if語(yǔ)句的一些基本用法，你可以根據(jù)自己的需求和喜好進(jìn)行學(xué)習(xí)和使用。