function lookup() { firstName = /\w+/i(); if (!firstName) window.alert (RegExp.input + "非法输入"); else { count=0; for (i=0;i 输入你的姓然后按回车键。 <FORM><INPUT TYPE:"TEXT" NAME="FirstName" onChange="lookup(this);"></FORM> </HTML> 1. 2. 3. 4. 5. 6. 7. g...
如果变量是有意设置为空,可以使用 null。 示例代码 代码语言:txt 复制 function checkValue(value) { if (value === undefined) { console.log('Value is undefined'); } else if (value === null) { console.log('Value is null'); } else { console.log('Value is defined and not null');...
我们刚修改了全局变量myscope="local";// 这里显式地声明了一个新的全局变量return[scope,myscope];// 返回两个值}console.log(checkscope2());// ["local", "local"],产生了副作用console.log(scope);// "local",全局变量修改了console.log(myscope);// "local",全局命名空间搞乱...
error: Uncaught ReferenceError: somevariable is not defined Note: This isn't to say that typeof is inherently a bad choice - but it does entail this implication as well. Using Lodash to Check if Variable is null, undefined or nil Finally - you may opt to choose external libraries besides...
If false, carousel will not automatically cycle. pause string | null "hover" If set to "hover", pauses the cycling of the carousel on mouseenter and resumes the cycling of the carousel on mouseleave. If set to null, hovering over the carousel won't pause it. wrap boolean true Whether ...
If false, carousel will not automatically cycle. pause string | null "hover" If set to "hover", pauses the cycling of the carousel on mouseenter and resumes the cycling of the carousel on mouseleave. If set to null, hovering over the carousel won't pause it. wrap boolean true Whether ...
Null:null Undefined:undefined. 和一种单独的对象类型:{name: "Dmitri"},["apple", "orange"]. 就这6种基本类型来说,undefined是一个 Undefined类型的特殊值。参照ECMAScript specification: Undefined value primitive value is used when a variable has not been assigned a value. ...
function CheckVal(){ console.log(nickname); //wuyanzu console.log(username); //Uncaught ReferenceError: username is not defined } echoName(); CheckVal(); 案例1-Fake Protect 像我这种开发功底不好、安全功底也不强的程序员,就容易会写出如下这样的代码 ...
functioncheckCookie() { letusername = getCookie("username"); if(username !="") { alert("Welcome again "+ username); }else{ username = prompt("Please enter your name:",""); if(username !=""&& username !=null) { setCookie("username", username,365); ...
Is null false in JavaScript? Null is not considered false in JavaScript, but it is considered falsy. This means that null is treated as if it’s false when viewed through boolean logic. However, this is not the same thing as saying null is false or untrue. ...