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
问Javascript - if (变量)检查'undefined‘失败ENundefined 是 Undefined 类型的唯一值，它表示未定义的...
Javascript check if string is empty 1 2 const string = ' '; 3 4 function isEmpty(str) { 5 return !str || str.trim().length === 0; 6 } 7 8 function checkString() { 9 var result = isEmpty(string); 10 if (result) { 11 document.getElementById("output").innerHTM...
console.log("空string值是null值吗?用===判断,答案为:" + ("" ===null)); console.log("空string值是undefined值吗?用==判断,答案为:" + ("" ==undefined)); console.log("空string值是undefined值吗?用===判断,答案为:" + ("" ===undefined));console.log("空string值是0值吗?用==判断,...
if(str.match(reg)){ //包含; } 方法三: search()方法可在字符串内检索指定的值,或找到一个或多个正则表达式的匹配。 var str = "123" console.log(str.search("2") != -1); // true RegExp对象的方法search() 方法用于检索字符串中指定的子字符串,或检索与正则表达式相匹配的子字符串。如果没有...
规范 if("x"inwindow){// 只有 x 被全局性的定义 才会执行这些语句} Void 操作符和 undefined void操作符是第三种可以替代的方法。 js varx;if(x===void0){// 执行这些语句}// 没有声明 yif(y===void0){// 抛出一个 RenferenceError 错误(与 `typeof` 相比)}...
No compatible source was found for this media. htmlbodyvara;if(a===undefined){document.write('a is undefined');} Output Following is the output of the above code a is undefined Print Page Previous Next Advertisements
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...
We can use search method to check if a string contains a substring or not as shown below. var actualstring = "Javascript search method", var substringToCheck = "search"; var isContains=actualstring.search("search") !== -1; //isContains true ...
String 基本类型(null, undefined, bool, number, string)应该是值类型,没有属性和方法。 内置对象 Javascript 有一系列内置对象来创建语言的基本功能,具体有如下几种 Boolean Boolean 对象表示两个值:true或false。 当作为一个构造函数(带有运算符 new)调用时,Boolean() 将把它的参数转换成一个布尔值,并且返回一...