JavaScript – built-in objects

Built-in objects

  • foreword
  • 1. MDN document
    • MDN documentation link:
    • How to learn about objects in MDN
  • Two, Math object
    • 2.1 Overview of Math
    • 2.2 Random number method Random()
  • 3. Date object
    • 3.1 Overview of Date
    • 3.2 Use of the Date() method
    • 3.3 Date formatting
    • 3.4 Get the total milliseconds of the date
  • 4. Array object
    • 4.1 Creation of array objects
    • 4.2 Check if it is an array
    • 4.3 Methods of adding and deleting array elements
    • 4.4 Array sorting
    • 4.5 Array Indexing Methods
    • 4.6 Convert array to string
    • 4.7 Other methods
  • 5. String object
    • 5.1 String properties
    • 5.2 Return location based on content
    • 5.3 Return characters by position
    • 5.4 String manipulation methods

Foreword

1. There are three types of objects in JavaScript: custom objects, built-in objects, and browser objects. The first two objects are the basic content of JS and belong to ECMAScript; the third browser object belongs to our JS unique
2. Built-in objects refer to some objects that come with the JS language. These objects are used by developers and provide some commonly used or the most basic and necessary functions (properties and methods)
3. The biggest advantage of built-in objects is to help us develop quickly
4. JavaScript provides multiple built-in objects: Math, Date, Array, String, etc.

1. MDN documentation

MDN documentation link:

MDN documentation: https://developer.mozilla.org/zh-CN/

How to learn about objects in MDN

  1. Check out what this method does
  2. Check the meaning and type of the parameters inside
  3. See the meaning and type of the return value
  4. Test with demo

2. Math object

2.1 Math Overview

The Math object is not a constructor, it has properties and methods for mathematical constants and functions. Operations related to mathematics (absolute value, rounding, maximum value, etc.) can use the members of Math

The code is as follows (example):

Math.PI // Pi
Math.floor() // round down
Math.ceil() // round up
Math.round() // The rounded version is rounded to the nearest integer. Note that -3.5 results in -3
Math.abs() // absolute value
Math.max() // Find the maximum value
Math.min() // Find the minimum value

Note: The above method must have parentheses

2.2 Random number method Random()

The random() method can return a decimal at random, and its value range is [0, 1), left closed and right open 0 <= x < 1

Example: Get a random integer between two numbers, including the two numbers.
The code is as follows (example):

function getRandom(min, max) {<!-- -->
  return Math. floor(Math. random() * (max - min + 1)) + min;
}

3. Date object

3.1 Date Overview

  • The Date object is different from the Math object, it is a constructor, so we need to instantiate it before using it
  • Date instance for working with dates and times

3.2 Use of Date() method

Getting the current time must be instantiated

Code example:

var now = new Date();
console. log(now);

Arguments to the Date() constructor
If there is a time in the parentheses, the time in the parameter is returned.
For example, the date format string is ‘2019-5-1’, which can be written as

new Date('2019-5-1') or new Date('2019/5/1')

If Date() does not write parameters, it will return the current time
If the parameter is written in Date(), it will return the time entered in the brackets

3.3 Date formatting

Method name Description Code
getFullYear() Get the current year Date.getFullYear()
getMonth() Get the month of the month, (0-11) Date.getMonth()
getDate() Get the date of the day Date.getDate()
getDay() Get the day of the week, (Sunday: 0 to Saturday: 6) Date.getDay()
getHours() Get the current hour Date.getHours()
getMinutes() Get the current minute Date.getMinutes()
getSeconds() Get the current number of seconds Date.getSeconds()

3.4 Get the total milliseconds of the date

Date object is based on the number of milliseconds since January 1, 1970 (UTC)

Code example:

// Instantiate the Date object
var now = new Date();
// 1. Used to get the original value of the object
console. log(date. valueOf())
console. log(date. getTime())
// 2. Simple writing can do this
var now = + new Date();
// 3. The method provided in HTML5 has compatibility issues
var now = Date. now();

4. Array object

4.1 Creation of array objects

Literal way:

var arr = [10,4,2,19,7,4];

new Array() method:

var arr = new Array();

4.2 Check if it is an array

  • The instanceof operator can determine whether an object belongs to a certain type
  • Array.isArray() is used to judge whether an object is an array, isArray() is a method provided in HTML5

Code example:

var arr = [1, 23];
var obj = {<!-- -->};
console.log(arr instanceof Array); // true
console.log(obj instanceof Array); // false
console.log(Array.isArray(arr)); // true
console.log(Array.isArray(obj)); // false

4.3 Methods of adding and deleting array elements

Method name Description Return value
push(parameter 1, parameter 2,…) Adding one or more elements at the end of the array will modify the original array Return the new length of the array
pop() Deleting the last element of the element will modify the original array Return the deleted element
unshift(parameter 1,parameter 2,…) adding one or more elements at the beginning of the array will modify the original array return the new array Length
shift() Deleting the first element of the array will modify the original array Return the value of the deleted element

4.4 Array sorting

Method name Description Whether to modify the original array
reverse Reversing the order of the elements in the array will change the original array and return a new array
sort Sorting the elements of the array will change the original array and return a new array

Code example:

var arr = [1, 64, 9, 6];
arr. sort(function(a, b) {<!-- -->
    return b - a; // descending order
    // return a - b; // ascending order
});

4.5 Array index method

Method name Description Return value
indexOf() Find the first index of a given element in the array If it exists, return the index number, otherwise, return -1
lastIndexOf() Find the last index of a given element in the array If it exists, return the index number, otherwise, return -1

4.6 Convert array to string

Method name Description Return value
toString() Convert the array into a string, comma separated each element Return a string
Join (“delimiter”) Convert all elements in the array into a string Return a string

4.7 Other methods

5. String object

5.1 String features

The immutability of the string: refers to the immutability of the content. After reassigning the variable of the string type, a new address is actually opened up. All string methods will not modify the string itself (string is immutable), and a new string will be returned after the operation is completed.

5.2 Return location based on content

Method name Description Return value
concat() Connects two or more arrays without affecting the original array returns a new array
slice () Array interception slice(begin,end) Return a string
splice() array deletion, splice (start position, number of deletions), affects the original array returns a new array
Method name Description
indexOf(‘The character to look for ‘,’The position to start searching’) Returns the position of the specified content in the string, if not found, returns -1
lastIndexOf(‘to The searched character’) Search from the back to the front, return the first found position, return -1 if not found

5.3 Return characters according to position

Method name Description Use
charAt(index) Returns the character at the specified position (index is the index value of the string) str.charAt(index)
charCodeAt(index) Returns the ASCII code of the character at the specified position string.charCodeAt(index)
str[inde] Returns the character at the specified position (index is the index value of the string) stri[index] HTML, IE8 + support

5.4 String manipulation methods

Method name Description
replace(‘replaced character’ ,’To replace the character’) Replace the corresponding character in the string
split(‘delimiter’) Split String, split the string into an array, return a new array
concat(parameter 1,parameter 2,…) Connect two or more string, same effect as +, recommend +
substr(start,length) to intercept the string, starting from start, intercepting length characters
slice(start,end) Intercept the string, starting from start and ending at end does not contain the end character
substring(start,end) Intercept the string, starting from start and ending at end does not contain the end character, and does not support negative numbers
toUpperCase() Convert all characters of the string to uppercase
toLowerCase() Convert all characters of the string to lowercase