In JavaScript, undefined is the default value for variables that have been declared but not initialized. On the other hand, null is an intentional assignment that explicitly indicates the absence of a value. Methods to Check if a Variable is Undefined Using typeof Operator Using Strict Equality...
这是因为属性的基值受 CheckObjectCoercible (ECMA 5 9.10 到 11.2.1)的影响,在它尝试将 Undefined 类型转换为 Object 的时候会抛出 TypeError。(感谢 kangax 在 twitter 上提前发布的消息) 代码语言:javascript 代码运行次数:0 运行 变量引用永远会被解析,因为 var 关键字确保 VariableObject 总是被赋给基值。
If you want to check if a variable exist 1 2 3 if(typeofyourvar !='undefined')// Any scope if(window['varname'] != undefined)// Global scope if(window['varname'] != void 0)// Old browsers If you know the variable exists but don't know if there's any value stored in it:...
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 } ...
// 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...
person); //returns undefined console.log("Hello " + person); //returns Hello Friend })() console.log("Hello = " person)_; //returns reference error variable not defined. Listing 5-4Referencing a Variable from Inside and Outside the Function Scope 无需调用该函数,该函数将在加载到浏览器...
<!--varmyVar ="global";// Declare a global variablefunctioncheckscope( ){varmyVar ="local";// Declare a local variabledocument.write(myVar); }//--> 该例子产生如下结果: Local JavaScript 变量名称 JavaScript 中变量的命名规则如下: 不能使用...
Here's a Code Recipe to check whether a variable or value is either an array or not. You can use the Array.isArray() method. For older browser, you can use the polyfill 👍 constvariable=['🍝','🍜','🍲'];// ✅ NEWER BROWSERArray.isArray(variable);// 🕰 OLDER BROWSERObj...
URL = "http://new.com"; // 报错:Assignment to constant variable[^4] 1. 2. 对于对象或数组,属性或元素可以修改: const user = { name: "小明" }; user.name = "小红"; // 合法(对象属性可变)[^4] 1. 2. 块级作用域 const和let一样仅在块作用域内有效: ...
console.log(x + " " + y); // '1 undefined' function catName(name) { console.log("我的猫名叫 " + name); } y = 2; catName("Chloe") //'Choloe' 这样的一种声明方式就被叫做变量提升,从概念的字面意义上说,它意味着变量和函数的声明会在物理层面移动到代码的最前面。可这么说并不准确,毕...