Variable Scope Solution 2: We can use typeofoperatorto check the variable is defined or not. if (typeof variable !== 'undefined') { // the variable is defined } Solution 3: You can use this code: if (typeof variable === 'undefined') { // variable is undefined } ...
const person = { name: "John" }; if (person.age === undefined) { console.log("Age is not defined"); } 4. Use typeof for Safe Checks Prefer using the typeof operator to check if a variable is undefined, especially when the variable may be undeclared. This method is safe because...
If you want to check whether a variable has been initialized or defined (i.e. test whether a variable has been declared and assigned a value) you can use the typeof operator.The most important reason of using the typeof operator is that it does not throw the ReferenceError if the ...
// Check if variable is equal to value if (username === "sammy_shark") { console.log(true); } 输出: 代码语言:javascript 代码运行次数:0 运行 AI代码解释 true 如前所述,变量可以用来表示任何JavaScript数据类型。在本例中,我们将使用字符串、数字、对象、布尔值和null值声明变量。 代码语言:javascr...
const x = 1; x // 1 x = 2 // TypeError: Assignment to constant variable.; const 所不能改变的并不是值,而是变量指向的内存地址所保存的值不能变动,下面看图 对于简单类型(数值、字符串、布尔值),值就保存在变量所指向的内存地址中。而对于复合类型(数组、对象),变量指向的内存地址,保存的只是一个...
error: Uncaught ReferenceError: somevariable is not defined Note:This isn't to say thattypeofis inherently a bad choice - but it does entail this implication as well. Using Lodash to Check if Variable isnull,undefinedornil Finally - you may opt to choose external libraries besides the built...
//例如var(变量声明 ),if(条件判断),while(循环判断) //2.用于定义常量和变量的关键字:const、let和var //要点1.const定义的变量不能修改,其实就是常量的定义,所以const是定义常量的关键字 const a = 1; //a = 2;//会报错,提示:Uncaught TypeError: Assignment to constant variable. ...
log(i) //returns error Listing 3-6When Creating a Variable Using the var Keyword Inside a Function, the Execution Context is Local to the Function 当处理变量时,在var上使用let将确保变量只存在于你创建的代码块中。变量表现不同的原因是因为变量提升。下一节将更详细地解释吊装。
lalala;//error, variable is not defined!}catch(err) { console.log(err.name);//ReferenceErrorconsole.log(err.message);//lalala is not definedconsole.log(err.stack);//ReferenceError: lalala is not defined at (...call stack)//也可以将一个 error 作为整体显示出来//error 信息被转换为像 "nam...
a = 2; // 运行报错: Uncaught TypeError: Assignment to constant variable. 1. 2. 3. 4. 5. 4. 使用 var 定义变量时有意思的案例 (1) var 可重复定义变量和变量提升的特性导致的问题 var a = 100; function func() { if (!a) { var ...