This is an example of the code. const person = { name: "John" }; console.log(person.age); // undefined 4. Array Indices If you try to access an element of an array that hasn’t been initialized, it returns undefined. Here’s an example of the code. let numbers = [1, 2]; ...
arr.includes(searchElement)方法用来判断一个数组是否包含一个指定的值,如果是返回 true,否则false。searchElement:必须。需要查找的元素值。 arr.includes(searchElement, fromIndex).fromIndex:可选。从该索引处开始查找 searchElement。如果为负值,则按升序从 array.length + fromIndex 的索引开始搜索。默认为 0。 va...
特别的,数字、布尔值、null、undefined、字符串是不可变。 // null、undefined null是JavaScript语言的关键字,它表示一个特殊值,常用来描述“空值”。 undefined是一个特殊值,表示变量未定义。 num = 123 123 b = typeof(num) //typeof用来判断属于那种类型,返回的是字符串 "number" 1、数字(Number) JavaScrip...
log(username); //returns undefined var username = "Hunter"; console.log(username); //returns Hunter; } checkVars() //executes function; function checkVars(){ var username; console.log(username); //returns undefined username = "Hunter"; console.log(username); //returns Hunter; } checkVars...
是否启用数据污点:" + navigator.taintEnabled() + ""; info += ""; document.getElementById("elInfo").innerHTML = info; return info; //director varappCodeName = navigatorappCodeName; //与浏览器相关的内部代码名 var appMinorVersion = navigatorappMinorVersion; //辅版本号(通常应用于浏览...
Once again, this method will fail on a null or undefined input. 3. JSON.stringify The JSON.stringify method is used to convert a JavaScript object to a JSON string. So we can use it to convert an object to a string, and we can compare the result with {} to check if the given ...
or anon-existingarray elementmovies[3](不存在的数组元素) ECMSScript手册定义了undefined值的类型: Undefined type is a type whose sole value is theundefinedvalue. Undefined是值是undefined的类型。 这样讲来,“typeof”操作一个未定义的值会返回字符串‘undefined’ ...
If the value exists, then the function will return the index value of the element, else it will return -1Syntax put-array-or-string-here.indexOf() Code //code to check if a value exists in an array using javascript indexOf var fruits_arr = ['Apple', 'Mango', 'Grapes', 'Orange'...
prototype.myFlat = function (num) { //判断第一层数组的类型 let type = checkType(this); //创建一个新数组,用于保存拆分后的数组 let result = []; //若当前对象非数组则返回undefined if (!Object.is(type, "Array")) { return; } //遍历所有子元素并判断类型,若为数组则继续递归,若不为数组...
function isElementInViewport (el) { //special bonus for those using jQuery if (typeof jQuery === "function" && el instanceof jQuery) { el = el[0]; } var rect = el.getBoundingClientRect(); return ( rect.top >= 0 && rect.left >= 0 && rect.bottom <= (wind...