JavaScript 如何检查“undefined”值

在JavaScript中， ‘undefined’ 是一个原始值，表示已声明但尚未分配值的变量，没有返回语句的函数或不存在的对象属性。

有几种方法可以检查 ‘undefined’ 。

使用 ‘typeof’ 运算符
与 ‘undefined’ 值进行比较
方法1： 使用 ‘typeof’ 运算符：

语法：

Javascript

// Declare a variable 
let myVariable; 
  
// Condition to check variable is defined or not 
if (typeof myVariable === "undefined") { 
    console.log("myVariable is undefined"); 
} else { 
    console.log("myVariable is defined"); 
}

在这里，如果变量没有被赋值，’typeof’操作符会返回字符串’undefined’。

输出：

myVariable is undefined

方法2： 与 ‘undefined’ 值进行比较:

语法:

Javascript

// Declare a variable 
let myVariable; 
  
// Condition to check variable is defined or not 
if (myVariable === undefined) { 
    console.log("myVariable is undefined"); 
} else { 
    console.log("myVariable is defined"); 
}

在这里，===运算符检查变量的值是否完全等于’undefined’。

输出：

myVariable is undefined

例子1：

JavaScript

// Using the 'typeof' operator: 
  
// Declare a variable 
let fruit; 
  
// Condition for check variable is defined or not 
if (typeof fruit === "undefined") { 
  console.log("fruit is undefined"); 
} else { 
  console.log("fruit is defined"); 
}

输出：

fruit is undefined

例子2：

JavaScript

// Using the 'typeof' operator: 
  
// Declare a variable and assign a value 
// it will return fruit is defined 
let fruit = "apple"; 
  
// Condition for check variable is defined or not 
if (typeof fruit === "undefined") { 
  console.log("fruit is undefined"); 
} else { 
  console.log("fruit is defined"); 
}

输出:

fruit is defined

示例3：

JavaScript

// Comparing with the 'undefined' value: 
  
// Declare a variable 
let profile; 
  
// Condition for check variable is defined or not 
if (profile === undefined) { 
  console.log("profile is undefined"); 
} else { 
  console.log("profile is defined"); 
}

输出：

profile is undefined

示例 4：

JavaScript

// Comparing with the 'undefined' value: 
  
// Declare a variable and assign value 
let profile = "geeksforgeeks"; 
  
// Condition for check variable is defined or not 
if (profile === undefined) { 
  console.log("profile is undefined"); 
} else { 
  console.log("profile is defined as", profile); 
}