Javascript basics about built-in objects

1. Introduction to built-in objects

JavaScript consists of: ECMAScript | DOM | BOM

ECMAScript： variables, functions, data types, flow control, built-in objects

Objects in js: custom objects, built-in objects, browser objects (not part of ECMAScript)

1.1 Math Object

Provides a series of math-related methods or properties (static | instance)

1.2 Methods in Math

• Math,PI --- Get the value of pi
• Math.random() --- Returns a random number between 0 and 1.

Flexible use:

a: Find a random number between two numbers, including these two numbers:

Math.floor( Math.random() * (max - min + 1) + min );

Example: Find a random integer between 1 and 10 [inclusive]

function getRondom(min, max) {
            return Math.floor(Math.random() * (max - min + 1) + min);
        }
        var i = getRondom(1,10);
        console.log(i);

b: When you want to sort randomly, you can use:

Math.random() - 0.5 precision. Using this weakness will randomly sort.

Example: Random sort

var arr1 = ["Lu Han", "Wang Junkai", "Cai Xukun", "Eddie Peng", "Jay Chou", "Andy Lau", "Zhao Benshan"];
        arr1.sort(function(){
            return Math.random() - 0.5;
        });
        console.log(arr1);
 

• Math.floor() --- Round down and return an integer less than the current number
• Math.ceil() --- Rounds up and returns an integer greater than the current number
• Math.round() --- Round to the nearest integer

Special cases:

console.log(Math.round(-1.5)) //The result is -1  

• Math.abs() --- Take the absolute value (return the absolute value of the current number, a positive integer)

console.log(Math.abs("1")); //The implicit conversion will convert the string 1 into a number console.log(Math.abs("fanfan")); //NaN

• Math.max() --- Returns the maximum value in a set of numbers (you can set multiple parameters and return the maximum value, the parameter cannot be an array)

console.log(Math.max(1,4,8,35,"fanfan")); //NaN
  console.log(Math.max()); //-Infinity 

• Math.min() --- Returns the minimum value in a set of numbers (multiple parameters can be set at the same time, the same effect as the maximum value)

console.log(Math.min()); //Infinity 

• Math.sin(x)
• Math.cos(x)
• Math.tan(x)
• Math.pow(x,y) --- Returns x raised to the power of y

1.3 Date Object

Usage 1: Empty constructor

var d = new Date(); //If there is no parameter, return the current time of the current system

Usage 2: Pass in a string in date format

 var d = new Date("1988-8-8 8:8:8")

Usage 3: Pass in numbers

 var d = new Date(year, month[day,time,second]); //You must set the year and month. The items in brackets are optional.

Note: Months start at 0, where 0 represents January

 var date1 = new Date(2019, 10, 01); //Actually November 1, 2019

Get the millisecond value of the current time: (the number of milliseconds since January 1, 1970)

Writing method 1:

d.valueOf()    

d.getTime() // Recommended

//Get the current millisecond value var date = new Date();
        console.log(date.valueOf());
        console.log(date.getTime());

Writing method 2 : (most commonly used)

 var date1 = +new Date(); // +new Date() returns the total number of milliseconds

Writing method three:

Date.now() //H5 new method is compatible

2. Methods in Date

• var d = new Date(); --- Date formatting method
• d.getSeconds() //Get seconds
• d.getMinutes() //Get minutes
• d.getHours() //Get hours
• d.getDay() //Return the current day of the week when there is no parameter (0 means Sunday) 0-6

How to get the day of the week:

var arr = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];
         var day = date.getDay();
         console.log(arr[day]);

• d.getDate() //Returns the current date when there is no parameter
• d.getMonth() //Returns the current month when there is no parameter (starting from 0) 0-11
• d.getFullYear() //Returns the current year when there is no parameter
• d.toString(); //Convert to string
• d.toDateString(); //Convert to date string
• d.toTimeString(); //Convert to time string
• d.toLocaleDateString(); //Return the local date format (different browsers have different effects)
• d.toLocaleTimeString(); //Return the local time format (different browsers have different effects)

3. Classic case: countdown effect:

function countTime(time) {
            var nowTime = +new Date();
            console.log(nowTime);
            var timer = +new Date(time);
            console.log(timer);
            var times = (timer - nowTime) / 1000;
            var d = parseInt(times / 60 / 60 / 24);
            d = d < 10 ? "0" + d : d;
            var h = parseInt(times / 60 / 60 % 24);
            h = h < 10 ? "0" + h : h;
            var m = parseInt(times / 60 % 60);
            m = m < 10 ? "0" + m : m;
            var s = parseInt(times % 60);
            s = s < 10 ? "0" + s : s;
            return d + "day" + h + "hour" + m + "minute" + s + "second"
 
        }
        var i = countTime("2021-11-11 18:00:00");
        console.log(i);

4. Array object

4.1 Array creation

Using array literals:

var arr = [1,2,3];

Using new Array():

var arr1 = new Array(); //Create an empty array
var arr1 = new Array(2); //This 2 means the length of the array is 2 and there are 2 empty array elements

var arr1 = new Array(2,3); // equivalent to [2,3] This means there are 2 array elements, 2 and 3.

4.2 Common methods in arrays

Determine whether a variable is an array:

• Array.isArray(ary) //H5 new attribute IE9 and above support
• Parameter instanceof Array

toString() --- Convert an array to a string, using commas to separate the values.

valueOf() --- Returns the array object itself

ary.pop() [Common] --- Delete the last word in the array and modify the length of the array. Note: No parameters are included in ().

ary.shift() --- Delete the first element in the array and modify the length of the array. Note: There is no parameter inside ().

ary.push() [Common] --- This method has a return value, which indicates the latest length of the array. Multiple parameters can be set in this method. Add one or more

ary.unshift() --- Add a value to the beginning of an array

reverse() --- Reverse an array

indexOf(content[,index]) --- Method to return the index of an array element. When searching from the beginning, it only returns the first index number that meets the conditions.
lastIndexOf() --- Search from the end of the array and return the index if found, or -1 if not found

join(分隔符) --- Connect each element in the array to a block through a character (array to string) Change the delimiter between characters, the default is ","

Sorting:

arr.sort() --- Sort by digits sort(function(){})

arr.sort(function(a,b){
    return ab; // ascending order})
arr.sort(function(a,b){
    return ba; //Descending order})

concat() --- concatenates two arrays together and returns a new array

slice(startindex, endindex) --- Extract a new array from the current array. The first parameter represents the starting index position, and the second parameter represents the ending index position.

splice(startindex, deletCont, options) --- Delete or replace certain values ​​in an array
The first parameter indicates where to start deleting
The second parameter represents how many to delete in total
The third parameter represents the value to be replaced

5. String

1. charAt(index) --- Get the character at the specified position

2. str[index] --- Get the character at the specified position (method in H5)

3. charCodeAt(index) --- Returns the ASCII value of the character at the corresponding index number to determine which key the user pressed

4. concat() --- concatenates strings, equivalent to +

5. slice(strat,end) --- Start from the specified position and intercept the string to the end position. The end value can be

6. substring(start,end) --- Start from the specified position and intercept the string to the end position. The end value cannot be obtained, but the start value can be obtained.

7. substr('Starting position of interception', 'How many characters to intercept') //Starting from the specified position, intercept length characters

8. indexOf(character) --- Returns the position of a character in a string [first time]

9. lastIndexOf(character) --- Search from back to front, only find the first matching character [last]

10. trim() --- can only remove leading and trailing spaces in a string

11. toLocaleUpperCase() //Convert to uppercase
toLocaleLowerCase() //Convert to lowercase

12. replace(a,b) --- replace a with b
split() --- Split a string into an array using a separator (string to array)

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