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,...
Example 3 : vue js check if string is empty In this third example of this tutorial, we use Vue.js programming language to check if a string is empty or null, undefined using `str && str.trim() !== ""`. This is a common task when we want to validate user input or filter out ...
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 ...
它的参数是一个回调函数,所有数组元素依次遍历该回调函数,直到找出第一个返回值为true的元素,然后返回该元素,否则返回undefined。 find() 方法返回通过测试(函数内判断)的数组的第一个元素的值。find() 方法为数组中的每个元素都调用一次函数执行: 当数组中的元素在测试条件时返回true时, find() 返回符合条件的元...
Note: string.charAt(index) is preferred over string[index] (bracket notation). This is because "".charAt(0) returns an empty string whereas ""[0] returns undefined.This removes the need to check if a variable is undefined, and avoids exceptional states in your code....
Thetypeofoperator is used to find the type of the value you add as its operand. The operator returns astringrepresenting the type of the value. You can use the operator to check forundefinedvalues as shown below: console.log(typeof42);// output: "number"console.log(typeofundeclaredVariable...
In this tutorial, we are going to show you the ways of checking whether the JavaScript string is empty, undefined, or null. Just follow the guidelines.
company;// => undefinedletperson = {name:'John Smith'}; person.age;// => undefined 而null是代表了缺失对象引用。js是不能给null设置变量或者对象属性的。一些原生方法像String.prototype.match()会返回null来表示缺失对象,看一下下面这个例子:
f = undefined; typeof f; //"string" f; //"washing machine" 毋庸置疑,给 undefined 变量重新赋值是非常不好的做法。事实上,ECMA 5 不允许这样做(不过,在当前的浏览器中,只有 Safari 强制执行了)。 然后是 null? 是的,一般都很好理解,但是还需要重申的是:undefined与null不同,null表示有意的缺少值的原...
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)} 规范 Specification ECMAScript Language Specification #sec-undefined 浏览器兼容性 ...