window.undefined = "omg"; "omg" == undefined // true 正如@CMS 所指出的那样,这已在 ECMAScript 第 5 版中修补,而undefined是不可写的。 if (window.myVar)也会包含这些假值,所以它不是很健壮: false 0 "" NaN null undefined 感谢@CMS 指出你的第三种情况 - if (myVariable)也可以在两种情况...
JavaScript 如果要判断变量是否已定义,可以使用 typeof: 实例 if(typeof someVar=='undefined'){ document.write("变量 someVar 未定义"); }else{ document.write("变量 someVar 已定义"); } 尝试一下 » JavaScript 如果只想判断已定义变量是否为 true 可以直接使用以下方法: 实例 if(strValue){ // str...
We then use "if not" (!) to check if the player has entered a valid choice. If choice is null, undefined, or an empty string, the condition inside the first if statement is true, and the code inside the block is executed. Therefore, the output to the console is "You didn't ...
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...
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 === 'undefined'){ document.write('a is undefined'); } OutputFollowing is the...
如果基值是 undefined,则认为引用是无法被解析的。 因此,如果在.之前的变量值为 undefined,那么属性引用是不可被解析的。下面的示例本会抛出一个 ReferenceError,但实际上它不会,因为 TypeError 会先被抛出。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...
Undefined value primitive value is used when a variable has not been assigned a value. 当一个变量没有被赋值的时候,就会被赋值undefined。 这个标准清晰的定义了变量没有初始值,或者不存在的对象属性,或者不存在的数组元素,都会被赋值undefined。例如: ...
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...
One of the simplest ways to check if a variable is undefined is by using the typeof operator. This operator returns a string that indicates the type of the unevaluated operand. If the variable is not defined, typeof will return the string “undefined”. Here’s how you can use it: let...
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 ...