The only reason I can think of was for IE4 compatibility, it did not understand the undefined keyword (which is not actually a keyword, unfortunately), but of course values could be undefined, so you had to have this: var undefined; and the comparison above would work just fine. In yo...
Javascript无法访问document:null或undefined 在JavaScript中,当尝试访问一个未定义或未声明的变量时,会返回undefined。而document:null是一个无效的表达式,因为document是一个HTML文档的根元素,它不能被赋值为null。 如果你想要检查一个变量是否为null或undefined,可以使用typeof操作符: 代码语言:javascript 复制 if (type...
undefined has normal variable semantics that not even strict mode can fix and requires run-time look-up. It can be shadowed like any other variable, and the default global variable undefined is not read-only in ES3. void 0 is effectively a compile time bulletproof constant for undefined wit...
It’s important to note that although it may be treated as 0 in arithmetic operations, the value of null is not 0. Meanwhile, undefined returns NaN when used in arithmetic operations. console.log(4 + undefined); // NaN console.log(4 * undefined); // NaN console.log(4 - undefined);...
undefined是全局对象的属性,即它是全局范围内变量。undefined的初始值是原始值undefined。参见https://...
直观上来看,foo.a 是 null, foo.b 是undefined,所以null == undefined 为true,null === undefined 为false。null与undefined似乎都代表空,== 与 === 都代表相等判断,那么JavaScript为什么要引入这两对相似的概念,它们又有什么区别呢?我们往下看。
但是实际是console.log(x);//undefined console.log(y)//y is not defined 引用RedNax的发言: 虽然一开始我觉得就null和undefined写一篇文章太大惊小怪了,不过考虑了一下其实蛮多东西值得挖掘的: 1.null 和 undefined在现代JS语义里面是有明确区别的: ...
greet("Alice");// Output: Hello, Alice!greet();// Output: Hello, stranger!Code language:JavaScript(javascript) In this example, if the `name` parameter is not provided when calling the `greet()` function, its value will be `undefined`, and the function will use the default value “str...
Lets see below JavaScript statements to understand theundefined. varmyVar;//Variable declaration without assigning any value to it console.log( myVar );//undefined console.log(typeof(myVar) );//undefined console.log( undeclaredVar );//Uncaught ReferenceError: undeclaredVar is not defined ...
In JavaScript there isnulland there isundefined. They have different meanings. undefinedmeans that the variable value has not been defined; it is not known what the value is. nullmeans that the variable value is defined and set to null (has no value). ...