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
直接比较undefined是很麻烦的,因为undefined可以被覆盖。 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 ...
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...
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 ...
JavaScript Undefined Check - Learn how to effectively check for undefined values in JavaScript. Explore methods and best practices to handle undefined variables in your code.
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...
// 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...
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: ...
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 ...
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...