The typeof operator can be used to check if a variable is undefined by comparing its type with the string ‘undefined’. Here is an example of the code. let a; console.log(typeof a === 'undefined'); // true This method is safe because it won’t throw an error if the variable ...
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...
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...
//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. ...
If you put a number in quotes, it will be treated as a text string. Example constpi =3.14; letperson ="John Doe"; letanswer ='Yes I am!'; Try it Yourself » Declaring a JavaScript Variable Creating a variable in JavaScript is called "declaring" a variable. ...
// No instance is created: console.log(c); // undefined // A global variable is created: console.log(name); // green 在严格模式下,你会得到一个异常: function StrictColor(name) { 'use strict'; this.name = name; } var c = StrictColor('green'); // TypeError: Cannot set property ...
x = 3 + y; // assigning a value to the variable `x` foo(x, y); // calling function `foo` with parameters `x` and `y` obj.bar(3); // calling method `bar` of object `obj` // A conditional statement if (x === 0) { // Is `x` equal to zero?
if(window.someVariable) { ... } 这是执行此检查的安全方式。如果someVariable未声明,则以下语句会引发异常: // Don’t do thisif(someVariable) { ... } 您可以通过window进行两种额外的检查方式;它们大致等效,但更加明确: if(window.someVariable!==undefined) { ... }if('someVariable'inwindow) { ...
错误提示:开发工具(idea)提示:mutable variable is accessible from closure 浏览器报错提示: 可以正确执行的代码: var yy = document.getElementsByClassName("div_inTop_001"); for (var i = 0; i < yy.length; i++) { yy[i].addEventListener("mouseover", myfunction0000); yy[i].addEventListener("mo...