Built-in objects

Built-in objects refer to some objects in THE JS language, which are used by developers and provide some common or basic and necessary functions (attributes and methods).

JS objects fall into three categories:

• Custom objects: belong to ECMAScript
• Built-in objects: belong to ECMAScript,Math,Date,Array,String.
• Browser object: JS unique, JS API learning

The Math object

The Math object

Math is not a constructor and does not need to be called by new; it uses properties and methods inside. MDN Math document

Example: Encapsulate the Math object

var myMath = {
    PI:2.141592653.max:function(){
        var max=arguments[0];
        for (var i=1; i<=arguments.length; i++){if(arguments[i]>max){
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Math uses built-in properties and methods

I get a random integer between two numbers

This example returns a random integer between the specified values. This value is not less than min (or an integer up if min is not an integer) and is less than (not equal to) Max.

function getRandomInt(min, max) {
  min = Math.ceil(min);
  max = Math.floor(max);
  return Math.floor(Math.random() * (max - min)) + min; // No maximum value, no minimum value
}
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Get a random integer between two numbers, including two numbers

function getRandomIntInclusive(min, max) {
  min = Math.ceil(min);
  max = Math.floor(max);
  return Math.floor(Math.random() * (max - min + 1)) + min; // Contain maximum value, contain minimum value
}
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The date object

The Date() object is a constructor that must be called with new to create our Date object, which needs to be instantiated before it can be used.

• If no arguments are entered, the Date constructor creates a Date object based on the current time set by the system.

var arr = new Array(a);var  obj = new Object(a);var date = new Date(a);var date1 = new Date(2019.10.1);
console.log(date1);// Return November instead of October
var date2 = new Date('the 2019-10-1 8:8:8');
console.log(date2);// The return is correct
// Tue Oct 2019 08:08:08 GMT+0800
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Date formatting

// Encapsulates a function that returns the current time
function getTime() {
    var time = new Date(a);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;
}
console.log(getTime())
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Four methods to obtain the total number of milliseconds of Dtae (timestamp)

• The Date object is based on the number of milliseconds from January 1, 1970.

//1.valueOf getTime
var time = new Date(a);console.log(date.valueOf());
console.log(date.getTime());
//2.
var date1 = +new Date(a);console.log(date1);
//3. H5 additions get the total number of milliseconds
console.log(Date.now());
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Case study: Countdown

function countDown(time) {
    var nowTime = +new Date(a);// Total milliseconds of the current time
    var inputTime +new Date(time);// User input time total good description
    var times = (inputTime - nowTime); / / 1000
    var d = parseInt(times / 60 / 60 / 24);
    var h = parseInt(times / 60 / 60 % 24);
    var m = parseInt(times / 60 % 60);
    var s = parseInt(times % 60);
    return d + 'day' + h + 'when' + m + 'points' + s + '秒';
}
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The array object

Juejin. Cn/post / 700776…

String object

Basic packing type

For ease of operation, JS provides three special reference types: String, Number, and BOOlean. A primitive wrapper type is one that wraps a simple data type into a complex data type, so that the primitive data type has properties and methods.

The following code can be executed:

var str = 'andy';
console.log(str.length);
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Even though strings are simple data types, we can still use the attribute length. Why is that? Because JS wraps string types as complex data types:

//1. Generate temporary variables to wrap simple types into complex data types
var temp = new String('andy');
//2. Assign to the character variable we declared
str = temp;
//3. Destroy temporary variables
temp = null;
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Immutable string

The value inside the string is immutable, although it looks like it can change the content, but in fact the address is changed, and a new memory space is created in memory.

var str = 'abc';
str = 'hello';
// When STR is reassigned, the constant 'ABC' is not modified and remains in memory.
// Since strings are immutable, there are efficiency problems when concatenating large numbers of strings:
var str = ' ';
for(var i = 0; i < 100000; i++){ str += i; }console.log(str);// This result takes a lot of time to display because new space is constantly being created
// So don't reassign or concatenate strings too much
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Commonly used method

All string methods do not modify the string itself and return a new string after the operation.

Find the position and number of occurrences of a character

var str = "ofsjhfofsddfofsodfdofdsfoofd";
var idnex = str.indexOf('o');
var num = 0;
while(index ! = = -1) {
    console.log(index);
    num ++;
    index = str.indexOf('o',index+1);
}
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Return character by position (emphasis)

//1.charAt(index)
var str = 'andy';
console.log(str.charAt(3));

for (var i = 0; i < str.length; i++) {
    console.log(str.charAt(i));
}
//2.charCodeAt(index)
console.log(str.charCodeAt(0));//97 Purpose: Determine which key the user presses
//3. str[index]
console.log(str[0]);//a

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Determine the most characters in a string and count the number of occurrences.

var str = 'afkdofeooofdjfsa';
var o = {};
for (var i = 0; i < str.length; i++) {
    var chars = str.charAt(i);
    if (o[chars]) {
       o[chars]++;
    } else {
        o[chars] = 1; }}console.log(o);
var max = 0;
var ch = ' ';// Which character occurs the most times in the global variable
for (var k in o) {
    if(o[k] > max) { max = o[k]; ch = k; }}console.log(max);// The maximum number of occurrences
console.log(ch)
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Put all of the values in a stringoReplace with*

var str1 = 'r4jfroffopfdoofdofsd';
while (str1.indexOf('o')! = =1) {
    str1 = str1.replace('o'.The '*');
}
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Convert strings to arrays

var str = 'red, pink, blue';
console.log(str.split(', '));
var str1 = 'red&pink&blue';
console.log(str2.split('&'));
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reference

Developer.mozilla.org/zh-CN/docs/…