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This article mainly introduces whether there are built-in objects in javascript, which can be used for reference by interested friends. I hope you can learn a lot after reading this article.

There are built-in objects in javascript. Built-in objects are some objects that come with JS language, such as String object, Array object, Date object, Boolean object, Number object, Math object, RegExp object, Global object and so on.

The operating environment of this tutorial: windows7 system, javascript1.8.5 version, Dell G3 computer.

Objects in JavaScript are divided into three categories: custom objects, built-in objects, and browser objects.

The first two objects are the basic content of JS, and the third browser object of ECMAScript; belongs to our unique JS.

Built-in objects are objects that come with the JS language, which are available to developers and provide some common or most basic and necessary functions (properties and methods).

The greatest advantage of built-in objects is that they help us develop quickly.

JavaScript provides several built-in objects, such as Math, Date, Array, String, etc.

String object: a string object that provides properties and methods for manipulating strings.

Array object: an array object that provides properties and methods for array manipulation.

Date object: a date-time object that can get the date-time information of the system.

Boolean object: a Boolean object. A Boolean variable is a Boolean object. (no properties and methods are available)

Number object: numeric object. A numerical variable is a numeric object.

Math object: a mathematical object that provides properties and methods for mathematical operations.

Object object

RegExp object

Global object

Function object

.

Math object

/ / the Math mathematical object is not a constructor, so we do not need new to call it, but directly use the properties and methods in it to console.log (Math.PI); / / an attribute pi console.log (Math.max (1,2,99)); / / 99 console.log (Math.max (- 1,-12)) / /-1 console.log (Math.max (1,99, 'Mathematical object'); / / NaN console.log (Math.max ()); / /-Infinity

Case: encapsulating one's own mathematical object

Var myMath = {PI: 3.141592653, max: function () {var max = arguments [0]; for (var I = 1; I

< arguments.length; i++) { if (arguments[i] >

Max) {max = arguments [I];}} return max;}, min: function () {var min = arguments [0]; for (var I = 1; I

< arguments.length; i++) { if (arguments[i] < min) { min = arguments[i]; } } return min; } } console.log(myMath.PI); console.log(myMath.max(1, 5, 9)); console.log(myMath.min(1, 5, 9)); 1Math概述 Math对象不是构造函数，它具有数学常数和函数的属性和方法。跟数学相关的运行（求绝对值，取整，最大值等）可以使用Math中的成员。 Math.PI //圆周率Math.floor() //向下取整Math.ceil() //向上取整Math.round() //四舍五入版 就近取整 注意-3,5 结果是 -3Math.abs() //绝对值Math.max()/Math.min() //求最大和最小值// 1.绝对值方法 console.log(Math.abs(1)); //1 console.log(Math.abs(-1)); //1 console.log(Math.abs('-1')); //隐式转换 会把字符串型 -1 转换为数字型 console.log(Math.abs('wode')); //NaN// 2.三个取整方法// (1)Math.floor() 地板 向下取整 往最小了取整 console.log(Math.floor(1.1)); //1 console.log(Math.floor(1.9)); //1// (2)Math.ceil() ceil 天花板 向上取整 往最大了取整 console.log(Math.ceil(1.1)); //2 console.log(Math.ceil(1.9)); //2// (3)Math.round() 四舍五入 其他数字都是四舍五入，但是 .5特殊，它往大了取 console.log(Math.round(1.1)); //1 console.log(Math.round(1.5)); //2 console.log(Math.round(1.9)); //2 console.log(Math.round(-1.1)); //-1 console.log(Math.round(-1.5)); //这个结果是 -1 2随机数方法 random（） //1. Math对象随机数方法 random() 返回一个随机的小数 0 =< x < 1//2.这个方法里面不跟参数// 3.代码验证 console.log(Math.random());// 4.我们想要得到两个数之间的随机整数 并且包含这两个数 // return Math.floor(Math.random() * (max - min + 1)) + min; function getRandom(min, max) { return Math.floor(Math.random() * (max - min + 1)) + min; } console.log(getRandom(1, 10));// 5.随机点名 var arr = ['张三', '李四', '王五', '赵六', '张三疯'] // console.log(arr[0]); // console.log(arr[getRandom(0, 4)]); console.log(arr[getRandom(0, arr.length - 1)]); 案例：猜数字游戏 程序随机生成一个1~10之间的数字，并让用户输入一个数字， 1.如果大于该数字，就提示，数字大了，继续猜； 2.如果小于该数字，就提示，数字小了，继续猜； 3.如果等于该数字，就提示猜对了，结束程序。 // 1.随机生成一个1~10的整数，我们需要用到Math.random()方法// 2.需要一直猜到正确为止，所以一直循环// 3.用while循环合适更简单// 4.核心算法：使用if else if 多分支语句来判断大于，小于，等于 function getRandom(min, max) { return Math.floor(Math.random() * (max - min + 1)) + min; } var random = getRandom(1, 10); while (true) { //死循环 var num = prompt('你来猜，输入1~10之间的一个数字'); if (num >

Random) {alert ('guess big');} else if (num

< random) { alert('猜小了'); } else { alert('猜对了'); break; } }// 要求用户猜1~50之间的一个数字 但是只有10次猜的机会 function getRandom(min, max) { return Math.floor(Math.random() * (max - min + 1)) + min; } var random = getRandom(1, 50); var i = 0; while (i < 10) { //死循环 var num = prompt('你来猜，输入1~50之间的一个数字'); if (num >

Random) {alert ('guess big');} else if (num

< random) { alert('猜小了'); } else { alert('猜对了'); break; //退出整个循环结束程序 } i++; } if (i = 10) { alert('全部猜错了'); } 日期对象 1Date概述 Date对象和Math对象不一样，他是一个构造函数，所以我们需要实例化后才能使用 Date实例用来处理日期和时间 2Date()方法的使用 1.获取当前时间必须实例化 var now = new Date();console.log(now); 2.Date()构造函数的参数 如果括号里面有时间，就返回参数里面的时间，例如日期格式字符串为'2019-5-1'，可以写成new Date('2019-5-1')或者new Date('2019/5/1') //Date() 日期对象 是一个构造函数 必须使用new 来调用创建我们的日期对象 var arr = new Date(); //创建一个数组对象 var obj = new Object(); //创建了一个对象实例// 1.使用Date 如果没有参数 返回当前系统的当前时间 var date = new Date(); console.log(date);// 2.参数常用的写法 数字型 2019,10,01 或者是 字符串型 '2019-10-1 8:8:8' var date1 = new Date(2019, 10, 1); console.log(date1); //返回的是 11月 不是 10月 var date2 = new Date('2019-10-1 8:8:8'); console.log(date2); 3日期格式化 我们想要2019-8-8 8:8:8格式的日期，要怎么办？ 需要获取日期指定的部分，所以我们要手动的得到这种格式 方法名说明代码getFullYears()获取当年dObj.getFullYears()getMonth()获取当月（0-11）dObj.getMonth()getDate()获取当天日期dObj.getDate()getDay()获取星期几（周日0 到周六6）dObj.getDay()getHours()获取当前小时dObj.getHours()getMinutes()获取当前分钟dObj.getMinutes()getSeconds()获取当前秒数dObj.getSeconds()// 格式化日期 年月日 var date = new Date(); console.log(date.getFullYear()); //返回当前日期的年 2020 console.log(date.getMonth() + 1); //月份 返回的月份小1个月 记得月份 +1 console.log(date.getUTCDate()); //返回的是几号 console.log(date.getDay()); //6 周一返回的是 1 周六返回的是 6 但是 周日返回的是 0// 写一个 2020年 5月 23日 星期六 var year = date.getFullYear(); var month = date.getMonth() + 1; var dates = date.getDate(); var arr = ['星期日', '星期一', '星期二', '星期三', '星期四', '星期五', '星期六']; var day = date.getDay(); console.log('今天是：' + year + '年' + month + '月' + dates + '日 ' + arr[day]); // 格式化日期 时分秒 var date = new Date(); console.log(date.getHours()); //时 console.log(date.getMinutes()); //分 console.log(date.getSeconds()); //秒// 要求封装一个函数返回当前的时分秒 格式是 08:08:08 function getTimer() { var time = new Date(); var h = date.getHours(); h = h < 10 ? '0' + h : h; var m = date.getMinutes(); m = m < 10 ? '0' + m : m; var s = date.getSeconds(); s = s < 10 ? '0' + s : s; return h + ':' + m + ':' + s; } console.log(getTimer()); 4获取日期的总的毫秒形式 Date对象是基于1970年1月1日（世界标准时间）起的毫秒数 我们经常利用总的毫秒数来计算时间，因为它更精确 // 获取Date总的毫秒数(时间戳) 不是当前时间的毫秒数 而是距离1970年1月1日过了多少毫秒数// 1.通过 valueOf() getTime() var date = new Date(); console.log(date.valueOf()); //就是 我们现在时间 距离1970.1.1 总的毫秒数 console.log(date.getTime());// 2.简单的写法(最常用的写法) var date1 = +new Date(); //+new Date() 返回的就是总的毫秒数 console.log(date1);// 3.H5 新增的 获得总的毫秒数 console.log(Date.now()); 案例：倒计时效果 // 倒计时效果// 1.核心算法：输入的时间减去现在的时间就是剩余的时间，即倒计时，但是不能拿着时分秒相减，比如05分减去25分，结果会是负数的// 2.用时间戳来做，用户输入时间总的毫秒数减去现在时间的总的毫秒数，得到的就是剩余时间的毫秒数// 3.把剩余时间总的毫秒数转换为天、时、分、秒(时间戳转换为时分秒)// 转换公式如下：// d = parseInt(总秒数 / 60 / 60 / 24); //计算天数// h = parseInt(总秒数 / 60 / 60 % 24); //计算小时// m = parseInt(总秒数 / 60 % 60); //计算分数// s = parseInt(总秒数 % 60); //计算当前秒数 function countDown(time) { var nowTime = +new Date(); //返回的是当前时间总的毫秒数 var inputTime = +new Date(time); //返回的是用户输入时间总的毫秒数 var times = (inputTime - nowTime) / 1000; //times是剩余时间总的秒数 var d = parseInt(times / 60 / 60 / 24); //天 d = d < 10 ? '0' + d : d; var h = parseInt(times / 60 / 60 % 24); //小 h = h < 10 ? '0' + h : h; var m = parseInt(times / 60 % 60); //分 m = m < 10 ? '0' + m : m; var s = parseInt(times % 60); //当前秒数 s = s < 10 ? '0' + s : s; return d + '天' + h + '时' + m + '分' + s + '秒'; } console.log(countDown('2020-5-24 00:00:00')); var date = new Date(); console.log(date); 数组对象 1数组对象的创建 创建数组对象的两种方式 字面量方式 new Array() // 创建数组的两种方式// 1.利用数组字面量 var arr = [1, 2, 3]; console.log(arr);// 2.利用new Array() // var arr1 = new Array(); //创建了一个空的数组 // var arr1 = new Array(2); //这个2 表示 数组的长度为 2 里面有两个空的数组元素 var arr1 = new Array(2, 3); //等价于[2,3] 这样写表示 里面有2个数组元素 是2和3 console.log(arr1); 2检测是否为数组 // 翻转数组 function reverse(arr) { // if (arr instanceof Array) { if (Array.isArray(arr)) { var newArr = []; for (var i = arr.length - 1; i >

= 0; iMel -) {newArr [newArr.length] = arr [I];} return newArr;} else {return 'error this parameter must be in array format [1je 2m 3]'}} console.log (reverse ([1m 2m 3])) Console.log (reverse (1,2,3)); / / [] / / detect whether it is an array / / (1) instanceof operator it can be used to detect whether it is an array var arr = []; var obj = {}; console.log (arr instanceof Array); console.log (obj instanceof Array); / / (2) Array.isArray (parameter) The new method IE9 in H5 supports console.log (Array.isArray (arr)) and console.log (Array.isArray (obj))

3 methods of adding and deleting array elements

The method name indicates the return value push (parameter 1.) Add one or more elements at the end, pay attention to modify the original array and return the new length pop ()

Delete the last element of the array, subtract the length of the array by 1 without parameters, and modify the original array.

Returns the value unshift (parameter 1...) of the element it deletes. Add one or more elements to the beginning of the array, notice to modify the original array and return the new length shift () delete the first element of the array, the length of the array minus 1 has no parameters, modify the original array and return the first element value / / 1.push () add one or more array elements to the end of our array push push var arr = [1, 2, 3] / / arr.push (4, 'white'); console.log (arr.push (4,' white')); / / 5 array length console.log (arr) / / (1) push can append new elements to the array / / (2) push () parameter directly write array elements / / (3) after push is finished The result returned is the length of the new array / / (4) the original array will also change / / 2.unshift add one or more array elements console.log (arr.unshift ('red',' green')) at the beginning of our array / / 7 console.log (arr) / / (1) unshift can append a new element / / (2) unshift () parameter to the front of the array and write the array element directly. / / (3) after unshift is finished The result returned is the length of the new array / / (4) the original array will also change / / 3.pop () it can delete the last element of the array console.log (arr.pop ()) / / white console.log (arr) / / (1) pop is the last element that can be deleted from the array. Only one element can be deleted at a time. / / (2) pop () has no parameters / / (3) after pop is over The result returned is that the deleted element / / (4) the original array will also change / / 4.shift () it can delete the first element of the array console.log (arr.shift ()) / / red console.log (arr); / / (1) shift is the first element that can be deleted only one element at a time / / (2) shift () has no parameters / / (3) after shift is finished, the result is that the deleted element / / (4) the original array will also change

Case: filter array

/ there is an array containing wages [1500, 1200, 2000, 2100, 1800]. Delete the salary over 2000 in the bar array, and put the rest in the new array var arr = [1500, 1200, 2000, 2100, 1800]; var newArr = []; for (var I = 0; I < arr.length) If +) {if (arr [I] < 2000) {/ / newArr [newArr.length] = arr [I]; newArr.push (arr [I]);} console.log (newArr)

4 array sorting

The method name indicates whether to modify the original array reverse () to reverse the order of the elements in the array. Without parameters, the method will change the original array and return the new array sort () to sort the elements of the array. This method will change the original array and return the new array / array sort / / 1. Flip array var arr = ['red',' white', 'blue']; arr.reverse (); console.log (arr); / / 2. Array sort (bubble sort) var arr1 = [2, 5, 77, 4, 7, 11, 1]; arr1.sort (function (a, b) {/ / return a-b; return b-a; / / descending order}); console.log (arr1)

5 array indexing method

Method name description returns the value indexOf () Array looking for the first index of a given element returns the index number if it does not exist, returns the index number of the last index of-1lastIndexOf () in the array if it exists If it does not exist, return-1 var arr / return array element index number method indexOf (array element) returns the index number of the array element from the previous search / / it only returns the first index number that meets the criteria / / if it cannot find the element in the array, it returns-1 var arr = ['red',' green', 'blue',' index 'bule'] Console.log (arr.indexOf ('blue')); / / 2 console.log (arr.indexOf (' black')); / /-1 green', / return array element index number method lastIndexOf (array element) function is to return the index number of the array element to look for var arr = ['red',' green', 'blue',' white', 'blue'] from behind. Console.log (arr.lastIndexOf ('blue')); / / 4

Case: array deduplication

/ / the array deduplicates ['centering,' axioms, 'zeales,' axioms, 'xtrees,' axioms, 'xweights,' caches,'b'] requires the removal of duplicate elements / / 1 in the array. Goal: pick out the non-repeating elements in the old array and put them in the new array, keep only one repeated element, and put it in the new array to remove / / 2. Core algorithm: we traverse the old array, and then take the old array element to query the new array, if the element does not appear in the new array, we will add, otherwise do not add / / 3. How do we know that the element does not exist? Using the new array .indexOf (array element) if the return is-1, it means that there is no such element in the new array / / encapsulating a deduplicated function unique unique function unique (arr) {var newArr = []; for (var I = 0; I < arr.length) ) {if (newArr.indexOf (arr [I]) = =-1) {newArr.push (arr [I]);}} return newArr } var demo = unique (['red',' blue', 'blue']) console.log (demo); console.log (demo1)

6 an array is converted to a string

Method name description return value toString () converts the array to a string, comma separates each item returns a string join (delimiter) method is used to convert all elements in the array to a string return a string / array to string / / 1.toString () convert our array to the string var arr = [1, 2, 3]; console.log (arr.toString ()) / / 1green-blue-red console.log 2 2.join / 2.join (delimiter) var arr1 = ['green',' blue', 'red']; console.log (arr1.join ()); / / green,blue,red console.log (arr1.join (' -')); / / green-blue-red console.log (arr1.join ('&')) / / green&blue&red method name indicates that the return value concat () connects two or more arrays without affecting the original array returning a new array slice () Array intercepting slice (begin,end) returning a new array of intercepted items splice () Array Delete splice (the number to be deleted) Note, this will affect the original array.

The purposes of slice () and splice () are basically the same

String object

1 basic packing type

To facilitate the manipulation of basic data types, JavaScript also provides special reference types on the: String, Number, and Boolean.

The basic wrapper type is to wrap a simple data type as a complex data type so that the basic data type has properties and methods.

/ / basic wrapper type var str = 'andy'; console.log (str.length); / / objects have properties and methods only complex data types have properties and methods / / Why do simple data types have length attributes? / / basic wrapper type: the simple data type is called complex data type / / (1) the simple data type is wrapped as the complex data type var temp = new String ('andy'); / / (2) the value of the temporary variable is given to str str = temp;// (3) destroy the temporary variable temp = null

2 the immutability of string

It means that the value inside is immutable, although it seems that the content can be changed, but in fact, the address has changed and a new memory space has been opened up in memory.

Immutability of / / string var str = 'andy'; console.log (str); str =' red'; console.log (str) / / when the str is re-assigned, the constant 'andy' will not be modified, but will still be assigned to the string in memory / / and will re-open up space in memory. This characteristic is that the string is immutable / / due to the immutability of the string. There will be a problem of efficiency when concatenating a large number of strings / / because our strings are immutable, so do not var str a large number of strings. For (var I = 1; I max) {max = o [k]; ch = k;}} console.log (max); / / 4 console.log ('most characters are' + ch); / / most characters are o

5 string operation method

The method name indicates that the concat (str1,str2,str3...) concat () method is used to concatenate two or more strings. Concatenation string, equivalent to +, + more commonly used substr (start,length) starts from the start position (index number), the number of slice (start,end) taken by length starts from the start position, intercepts to the end position, the end cannot get it (both of them are index numbers) substring (start,end) starts from the start position, intercepts to the end position, and end cannot get it, which is basically the same as slice. But do not accept negative value / / string manipulation method / / 1.concat ('string 1' string 2'.) Var str='andy'; console.log (str.concat ('red')); / / 2. Substr (' start position of interception', 'intercept a few characters'); var str1=' reform spring breeze blowing all over the ground; console.log (str1.substr (2d2)); / / the first 2 is the index number 2 from which to start the second 2 is to take several characters / / 1. The replacement character replace ('replaced character', 'replaced character') it will only replace the first character var str = 'andyandy'; console.log (str.replace (' averse,'b')); / / there is a string 'abcoefoxyozzopp' that requires that all o in it be replaced with * var str1 =' abcoefoxyozzopp' While (str1.indexOf ('o')! = =-1) {str1 = str1.replace ('oasis,' *');} console.log (str1); / / 2. Convert characters to an array split ('delimiter') join converts an array to a string var str2 = 'red,pink,blue'; console.log (str2.split (',')); var str3 = 'red&pink&blue'; console.log (str3.split (' &'))

ToUpperCase () / / convert capitalization

ToLowerCase () / / convert lowercase

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