( undefined ,与null不同,也可能在 ECMAScript 3 环境中重新定义,使其不可靠进行比较,尽管现在几乎所有常见环境都符合 ECMAScript 5 或更高版本)。 if (typeof someUndeclaredVariable == "undefined") { // Works } if (someUndeclaredVariable === undefined) { // Throws an error }首页 滇ICP备1400...
You can use the operator to check forundefinedvalues as shown below: console.log(typeof42);// output: "number"console.log(typeofundeclaredVariable);// output: "undefined"letmyObj={color:"red"};console.log(typeofmyObj.color);// output: "string"console.log(typeofmyObj.id);// output: ...
Below syntax is used to check undefined variable using the typeof operator.typeof variable_name === 'undefined' ExampleBelow is the example code given, that shows how to use the typeof operator to check the undefined value of the variable.Open Compiler var a; if(typeof a === 'unde...
log(username); //returns undefined username = "Hunter"; console.log(username); //returns Hunter; } checkVars() //executes function; Listing 3-7Variables Are Hoisted When They Are Declared, Not When They Are Assigned a Value. These Two Examples Produce the Same Result. 面试问题什么是吊装,...
if (typeof heart !=='object') { thrownewError('heart is not an object') } if (!('rate'in heart)) { thrownewError('rate in heart is undefined') } // Assume the caller wants to pass in a callback to receive the current frog's weight and height that he or she has set ...
JavaScriptundefined属性定义和用法undefined属性用于存放JavaScript的undefined值。语法undefined说明无法使用forin循环来枚举undefined属性也不能用delete运算符来删除它。undefined不是常量可以把它设置为其他值。当尝试读取不存在的对象属性时也会返回undefined。提示和注释提示只能用运算来测试某个值是否是未定义的因为运算符认...
When the string is not null or undefined, and you intend to check for an empty one, you can use the length property of the string prototype, as follows:Javascript empty string 1 2 3 4 let emptyStr = ""; if (!emptyStr && emptyStr.length == 0) { console.log("String is empty")...
(!) Missing global variable nameUse output.globals to specify browser global variable names corresponding to external modulesaxios (guessing 'axios...
如果作为一个函数(不带有运算符 new)调用时,Boolean() 只将把它的参数转换成一个原始的布尔值,并且返回这个值,如果省略 value 参数,或者设置为0、-0、null、""、false、undefined或NaN,则该对象设置为 false。否则设置为 true(即使 value 参数是字符串false)。
undefined 但是,如果省略var关键字,就不再声明变量,而是初始化它。它将返回一个ReferenceError并停止脚本的执行。 代码语言:javascript 代码运行次数:0 运行 AI代码解释 //在声明变量之前尝试使用它 console.log(x); //没有var的变量赋值 x = 100; 输出: 代码语言:javascript 代码运行次数:0 运行 AI代码解释 ...