JS built in object

Math object ：

// 1. Absolute value method 
console.log(Math.abs(1)); // 1
console.log(Math.abs(-1)); // 1
console.log(Math.abs('-1')); //  Implicit conversion   Will put the string type  -1  Convert to digital 
console.log(Math.abs('pink')); // NaN 

// 2. Three rounding methods 
// (1) Math.floor()  The floor   Rounding down   Take the minimum value 
console.log(Math.floor(1.1)); // 1
console.log(Math.floor(1.9)); // 1
// (2) Math.ceil() ceil  The ceiling   Rounding up   Take the maximum value 
console.log(Math.ceil(1.1)); // 2
console.log(Math.ceil(1.9)); // 2
// (3) Math.round()  rounding   All other numbers are rounded , however  .5  special   It's going big  
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)); //  The result is  -1

//3. Maximum 
Math.max()
console.log(Math.max(1, 99, 3)); // 99
console.log(Math.max(-1, -10)); // -1

//4. minimum value 
Math.min()
console.log(Math.max(1, 99, 'pink teacher ')); // NaN
console.log(Math.max()); // -Infinity

//5.PI
Math.PI	
console.log(Math.PI);

//6. random number 
// 1.Math Object random number method  random()  Returns a random decimal  0 =< x < 1
// 2.  There are no parameters in this method 
// 3.  Code validation  
console.log(Math.random());
// 4.  We want to get a random integer between two numbers   also   Include this 2 It's an integer 
// 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.  Random roll call  
var arr = [' Zhang San ', ' Zhang Sanfeng ', ' Zhang San is crazy ', ' Li Si ', ' Li Sisi ', 'pink teacher '];
console.log(arr[getRandom(0, arr.length - 1)]);

Date object

//Date()  The date object is a constructor , You have to use new To call the create date object 
//Date The object is based on 1970 year 1 month 1 Japan （ World standard time ） Milliseconds from 
//1. Use Date  No parameters , Returns the current time of the current system 
var date = new Date();
console.log(date)
//2. Returns the current year 
console.log(date.getFullYear());
//3. Returns the current month 1 Months , want +1
console.log(date.getMonth());
console.log(date.getMonth() + 1);
//4. Return the number 
console.log(date.getDate());
//5. Back to the week ,  Sunday - Saturday  0-6
console.log(date.getDay());
//6. return 
console.log(date.getHours());
//7. Return points 
console.log(date.getMinutes());
//8. Return seconds 
console.log(date.getSeconds());
//9. Return distance 1970 year 1 month 1 How many milliseconds have passed （ Time stamp , Never repeat ）
console.log(date.valueOf());
console.log(date.getTime());
var dat = +new Date();  //+new Date()  The most common way to write 
console.log(dat)
console.log(Date.now()) //H5 Newly added , The simplest 

//10. Common writing of parameters 
//- Digital 
var date1 = new Date(2019, 10, 1)
console.log(date1);  //Fri Nov 01 2019 00:00:00 GMT+0800 ( China standard time )
//- String type （ The main ）
var date2 = new Date('2019-10-1 8:8:8')
console.log(date2); //Tue Oct 01 2019 08:08:08 GMT+0800 ( China standard time )


// Format in minutes and seconds 
function getTimer() {
    
    var time = new Date();
    var h = time.getHours();
    h = h < 10 ? '0' + h : h;
    var m = time.getMinutes();
    m = m < 10 ? '0' + m : m;
    var s = time.getSeconds();
    s = s < 10 ? '0' + s : s;
    return h + ':' + m + ':' + s;
}

//  format  2019 year  5 month  1 Japan   Wednesday 
function getYMD() {
    
    var year = date.getFullYear();
    var month = date.getMonth() + 1;
    var dates = date.getDate();
    var arr = [' Sunday ', ' Monday ', ' Tuesday ', ' Wednesday ', ' Thursday ', ' Friday ', ' Saturday '];
    var day = date.getDay();
    return (' It's today ：' + year + ' year ' + month + ' month ' + dates + ' Japan  ' + arr[day]);
}

// The countdown effect 
function countDown(time) {
    
    var nowTime = +new Date(); //  Returns the total number of milliseconds of the current time 
    var inputTime = +new Date(time); //  Returns the total number of milliseconds of user input time 
    var times = (inputTime - nowTime) / 1000; // times Is the total number of seconds remaining  
    var d = parseInt(times / 60 / 60 / 24); //  God 
    d = d < 10 ? '0' + d : d;
    var h = parseInt(times / 60 / 60 % 24); // when 
    h = h < 10 ? '0' + h : h;
    var m = parseInt(times / 60 % 60); //  branch 
    m = m < 10 ? '0' + m : m;
    var s = parseInt(times % 60); //  Current seconds 
    s = s < 10 ? '0' + s : s;
    return d + ' God ' + h + ' when ' + m + ' branch ' + s + ' second ';
}
console.log(countDown('2019-5-1 18:00:00'));
var date = new Date();
console.log(date);

Array objects

// Check if it's an array 
// 1.instanceof
var arr = []
console.log(arr instanceof Array);  //true
//2. Array.isArray( Parameters ) H5 Newly added ,ie9 Above version support 
console.log(Array.isArray(arr));    //true

// Add or remove array elements 
//1.push()  Add one or more array elements to the end of the array   Returns the length of the new array 
var arr = [1, 2, 3]
// arr.push(4, 'Pink')
console.log(arr.push(4, 'Pink'));  //5
console.log(arr);   //(5) [1, 2, 3, 4, "Pink"]
//2.unshift('red')  Add a new element to the array header   Returns the length of the new array 
console.log(arr.unshift(0, 'youyouo'));  //7
console.log(arr);   //[0, "youyouo", 1, 2, 3, 4, "Pink"]

// Remove elements 
//1.pop  Delete the last element   Returns the element to be deleted 
console.log(arr.pop())
console.log(arr);
//2.shift()  Delete first element   Returns the element to be deleted 
console.log(arr.shift())
console.log(arr);

// Array sorting 
//1. Flip array 
var arr = ['red', 'pink', 'green']
arr.reverse()
console.log(arr);

//2. Array sorting （ Bubble sort ）
var arr = [3, 9, 7, 8]
arr.sort()
console.log(arr);  //[3, 7, 8, 9]

var arr = [3, 43, 2, 18]
arr.sort()
console.log(arr);  //[18, 2, 3, 43]  Here is a comparison of 

var arr = [3, 43, 2, 18]
arr.sort(function (a, b) {
    
    return a - b // Ascending  [2, 3, 18, 43]
    // return b - a // Descending  [43, 18, 3, 2]
})
console.log(arr);


//  Array index  
// indexOf  Returns the index number of the first element of the array , When the element cannot be found , return -1
var arr = ['red', 'pink', 'green', 'red']
console.log(arr.indexOf('red'));  //0
// lastIndexOf
console.log(arr.lastIndexOf('red'));  //3

// Array weight removal 
function unique(arr) {
    
    var newArr = []
    for (var i = 0; i < arr.length; i++) {
    
        if (i === arr.indexOf(arr[i])) {
    
            newArr.push(arr[i])
        }
    }
    return newArr
}
var re = unique([2, 3, 5, 67, 2, 3, 4, 5, 7])
console.log(re);

// Array to string 
//1.toString() 
var arr = ['red', 'pink', 'green', 'red']
console.log(arr.toString());  //red,pink,green,red
//2. Separator 
var arr = ['red', 'pink', 'green', 'red']
console.log(arr.join());  //red,pink,green,red
console.log(arr.join('-')); //red-pink-green-red
console.log(arr.join('/'));//red/pink/green/red

Basic packaging type

//  Basic packaging type 
var str = 'andy';
console.log(str.length);

object Only then Properties and methods Complex data types Properties and methods
Why simple data types have length Attribute? ？
Basic packaging type ： Is to put Simple data type Packaging has become Complex data type
Three basic packaging types ：String,Number,Boolean

// so ：
//  Basic packaging type 
var str = 'andy';
console.log(str.length);

// (1)  Wrapping simple data types into complex data types  
var temp = new String('andy');
// (2)  Put the value of the temporary variable   to  str
str = temp;
// (3)  Destroy this temporary variable 
temp = null;

String object

//  The immutability of strings 
//  It means that the values in it are immutable 
//  Although it seems that the content can be changed , But the address has changed , A new memory space has been opened up in memory .
//  So don't splice a lot of strings 


// Return position according to character 
// String name .indexOf(' Characters to find ', Starting position )  Return if not found -1
var str = ' The spring breeze of reform blows the earth , Spring is coming '
console.log(str.indexOf(' In the spring '));  //2  first ' In the spring '
console.log(str.indexOf(' In the spring ', 3));  //8
// String name .lastIndexOf(' Characters to find ')  Look backwards , Returns the first matching character 
console.log(str.lastIndexOf(' In the spring '));  //8


//  Find string "abcoefoxyozzopp" All in o Where and how often 
//  The core algorithm ： Find the first one first o Position of appearance 
//  then   as long as indexOf  The result returned is not  -1  Just keep looking back 
//  because indexOf  You can only find the first , So the following search , It must be the current index plus 1, To keep looking for 
var str = "oabcoefoxyozzopp";
var index = str.indexOf('o');
var num = 0;
// console.log(index);
while (index !== -1) {
    
    console.log(index);
    num++;
    index = str.indexOf('o', index + 1);
}
console.log('o The number of times is : ' + num);

// Returns the character according to the position 
//1.chatAt(index)
//  Traverse all the characters 
for (var i = 0; i < str.length; i++) {
    
    console.log(str.charAt(i));
}
// 2. charCodeAt(index)  Returns the character of the corresponding index number ASCII value   Purpose ：  Judge that the user pressed that key  
console.log(str.charCodeAt(0)); // 97
// 3. str[index] H5  Newly added 
console.log(str[0]); // a

//  Judging a string  'abcoefoxyozzopp'  The most frequently used character in , And count the times .
// o.a = 1
// o.b = 1
// o.c = 1
// o.o = 4
//  The core algorithm ： utilize  charAt(）  Traversing this string 
//  Store every character in the object ,  If the object does not have this property , for 1, If it exists  +1
//  Traversing objects , Get the maximum value and the character 
var str = 'abcoefoxyozzopp';
var o = {
    };
for (var i = 0; i < str.length; i++) {
    
    var chars = str.charAt(i); // chars  yes   Every character in a string 
    if (o[chars]) {
     // o[chars]  What you get is the attribute value 
        o[chars]++;
    } else {
    
        o[chars] = 1;
    }
}
console.log(o);
// 2.  Traversing objects 
var max = 0;
var ch = '';
for (var k in o) {
    
    // k  Get is   Property name 
    // o[k]  What you get is the attribute value 
    if (o[k] > max) {
    
        max = o[k];
        ch = k;
    }
}
console.log(max);
console.log(' The most common characters are ' + ch);


//  String splicing  concat(' character string 1',' character string 2'....)
var str = 'andy';
console.log(str.concat('red'));

//  String interception  substr(' Intercept start position ', ' Intercept a few characters ');
var str1 = ' The spring breeze of reform is sweeping the ground ';
console.log(str1.substr(2, 2)); //  first 2  It's the index number 2  From the first few   the second 2  Is to take a few 

//  Replace character  replace(' Replaced characters ', ' Replace with the new character ')  It will only replace the first character 
var str = 'andyandy';
console.log(str.replace('a', 'b'));
//  There's a string  'abcoefoxyozzopp'  Ask for all the inside  o  Replace with  *
var str1 = 'abcoefoxyozzopp';
while (str1.indexOf('o') !== -1) {
    
    str1 = str1.replace('o', '*');
}
console.log(str1);

//  Character to array  split(' Separator ')  We learned about  join  Convert an array to a string 
var str2 = 'red, pink, blue';
console.log(str2.split(','));
var str3 = 'red&pink&blue';
console.log(str3.split('&'));

// Convert a capital 
toUpperCase()

// Convert lowercase 
toLowerCase()