The typeof operator can be used to check if a variable is undefined by comparing its type with the string ‘undefined’. Here is an example of the code. let a; console.log(typeof a === 'undefined'); // true This method is safe because it won’t throw an error if the variable ...
var value = document.getElementById("文本框ID").value; if(value==""){ alert("值为空"); } 三、是否为undefined var exp = undefined; if (typeof(exp) == "undefined" && !exp) { alert("undefined"); } 四、是否为null var exp = null; if (typeof(exp) == "object" && !exp) { ...
varcheckId = $("#txtCheckId").val();if(checkId!="" &&typeof(checkId) != "undefined"){ //OnBtnMaterialChoose(checkId); }
33,16,40];functioncheckAdult(age){returnage<=14;}functionmyFunction(){ages.filter(checkAdult);console.log(ages.filter(checkAdult))}myFunction()//[]
可能是变量被显式地赋值为undefined。 或者变量名拼写错误,实际上指向了另一个已定义的变量。 解决方法: 确认变量确实未被赋值。 检查变量名的拼写是否正确。 示例代码 代码语言:txt 复制 function checkUndefined(value) { if (typeof value === 'undefined') { console.log('The value is undefined'); } ...
var array = ['1','2',undefined, '3.png', ‘’, undefined, ‘a’, ' '];let newArray=array.filter(i=>i && i.trim()); // 注意：IE9以下的版本没有这个trim()方法console.log(newArray); //返回结果:['1','2', '3.png', ‘a’,]5.把对象数组a中的某个属性值取出来存...
if (check([0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A])) { return { ext: 'png', mime: 'image/png' }; } return undefined } 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
Check window.location to verify if the app is in OAuth callback state or not. This function is synchronous and returns true or false.if (authClient.isLoginRedirect()) { // callback flow try { await authClient.handleRedirect(); } catch (e) { // log or display error details } } ...
为false时:空字符串 '' 、undefined、0、NAN、null 为true时:除上述false的情况都为true 此判断会经常应用于判断一个变量是否有返回的if 语句中 varo ={'name':'lee'};vara = ['reg','blue'];functioncheckBoolean(a){if(a){returntrue; }else{returnfalse; ...
此代码由Java架构师必看网-架构君整理if(variable1!==null||variable1!==undefined||variable1!==''){varvariable2=variable1;} 上面的意思是说如果variable1不是一个空对象,或者未定义,或者不等于空字符串,那么声明一个variable2变量,将variable1赋给variable2。也就是说如果variable1存在那么就将variable1的值...