Introduction to JavaScript built-in objects

1. Built-in objects
2. Math Object

1. Use of Math objects
2. Generate random numbers in a specified range

3. Date Object

1. Use of Date() method
2. Use of date objects
3. Get timestamp

4. Array Object

1. Creation of array objects
2. Check whether it is an array
3. Methods for adding and deleting array elements
4. Array sorting
5. Array indexing method
6. Array to string conversion

5. String Object

1. Return position according to character
2. Return characters based on position
3. String operation methods
4. split() method

1. Built-in objects

Built-in objects refer to some objects that come with the JS language. These objects are for developers to use and provide some commonly used or most basic and necessary functions (properties and methods). The biggest advantage of built-in objects is that they help us develop quickly. JavaScript provides multiple built-in objects: Math , Date , Array , String , etc.

2. Math Object

Math object is not a constructor, it has properties and methods for mathematical constants and functions. Mathematical operations (such as absolute value, integer rounding, maximum value, etc.) can use members in Math.

1. Use of Math objects

       Math.abs(x): Returns the absolute value of xMath.pow(x,y): Returns x raised to the power of yMath.sqrt(x): Returns the square root of xMath.random(): Returns a pseudo-random number (between 0.0 and 1.0)
    
    Math.round(x): Returns the nearest integer after rounding x. Math.floor(x): Returns the largest integer less than or equal to x. (Round down)
    
    Math.ceil(x): The function returns the smallest integer greater than or equal to x (rounded up)
    
    Math.max(): Returns the maximum value of the parameters. Math.min(): Returns the minimum value of the parameters.

Note: The above method must be enclosed in parentheses

As shown below:

console.log('Math.abs(-2)='+Math.abs(-2));
        console.log('Math.ceil(1.7)='+Math.ceil(1.7));
        console.log('Math.floor(1.7)='+Math.floor(1.7));
        console.log('Math.pow(2,3)='+Math.pow(2,3));
        console.log('Math.sqrt(3)='+Math.sqrt(3));
        console.log('Math.round(3.4)='+Math.round(3.4));
        console.log('Math.round(3.6)='+Math.round(3.6));
        console.log('Math.max(1,2)='+Math.max(1,2));
        console.log('Math.min(1,2)='+Math.min(1,2));

The print result is:

2. Generate random numbers in a specified range

Encapsulate a function that generates a random number between 1 and 10:

The code is as follows:

function random(min,max){
    return Math.floor(Math.random()*(max-min+1))+min;
}
for(var i =1;i<=10;i++){
    console.log('The result of the '+i+'th print is: '+random(1,10));
}

The random printout results are:

Among them, min and max are the ranges of generated random numbers.

3. Date Object

The Date object is different from the Math object. It is a constructor, so we need to instantiate it before we can use it.
Date instances are used to process dates and times

1. Use of Date() method

To get the current time, you must instantiate:

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

Parameters of the Date() constructor:

If there is a time in the brackets, the time in the parameter is returned. For example, if the date format string is '2019-5-1', it can be written as new Date('2019-5-1') or new Date('2019/5/1')
If Date() does not write parameters, it returns the current time
If you write parameters in Date(), it returns the time entered in the brackets.

For example:

       var date1 = new Date()
        console.log('date1='+date1);
        var date2 = new Date(2021,11,08,20,51);
        console.log('date2='+date2);
        date3 = new Date('2021-11-08 20:54')
        console.log('date3='+date3);

The output is:

2. Use of date objects

Use of get method:

For example:

 var date1 = new Date()
        console.log('date1='+date1);
        console.log('This year is:' + date1.getFullYear() + 'year');
        console.log('This month is:' + date1.getMonth() + 'Month');
        console.log('Today is:' + date1.getDate() + 'number');
        console.log('Now is: '+date1.getHours()+'');
        console.log('Now is:' + date1.getMinutes() + 'minutes');

The result is:

Use of set method:

For example:

 var date1 = new Date()
        console.log(date1);
        date1.setFullYear(2020)
        console.log(date1);

The printed result is:

3. Get timestamp

Through the valueof() or getTime() method of the date object:

var date1 = new Date();
console.log(date1.valueOf()); 
console.log(date1.getTime());

Use the "+" operator to convert to a numeric type:

var date2 = +new Date();
console.log(date2);

Use the new Date.now() method in HTML5 :

console.log(Date.now());

4. Array Object

1. Creation of array objects

There are two ways to create array objects:

Literal method
new Array()

2. Check whether it is an array

instanceof operator can determine whether an object is of a certain type.
Array.isArray() is used to determine whether an object is an array. isArray() is a method provided in HTML5

As shown below:

 var arr = [1,2,3,4];
        var obj = {};
        console.log(arr instanceof Array);
        console.log(obj instanceof Array);
        console.log(Array.isArray(arr));
        console.log(Array.isArray(obj));

3. Methods for adding and deleting array elements

Note: The return value of push() and unshift() methods is the length of the new array, while the pop() and shift() methods return the array element that was moved out.

For example:
There is an array [1500, 1200, 2000, 2100, 1800]. It is required to delete the items with more than 2000 in the array and put the remaining items into a new array. The code is as follows:

var arr = [1500, 1200, 2000, 2100, 1800];
        var newArr = [];
        for(var i=0;i<arr.length;i++){
            if(arr[i]<2000){
                newArr.push(arr[i]);
            }
        }
        console.log(newArr);

The printed result is:

4. Array sorting

method	Functional Description
reverse()	Reverse the position of elements in an array. This method changes the original array and returns a new array.
sort()	Sort the elements of an array. This method will change the original array and return a new array.

For example:

Reverse an array:

var arr = [1500, 1200, 2000, 2100, 1800];
        console.log(arr);
        console.log(arr.reverse());

Sort an array:

var arr = [1500, 1200, 2000, 2100, 1800];
        console.log(arr);
        var newArr = arr.sort(function(a,b){
            return ab; // ascending order return ba; // descending order })
        console.log(newArr);

The printed result is:

5. Array indexing method

Method Name	illustrate	Return Value
indexOf()	Find the first index of a given element in an array	If it exists, it returns the index number, if it does not exist, it returns -1
lastIndexOf()	The last index in the array	If it exists, it returns the index number, if it does not exist, it returns -1

For example:

var arr = [1500, 1200, 2000,1500, 2100, 1500,1800];
        console.log('arr.indexOf(1500)：' + arr.indexOf(1500));
        console.log('arr.lastIndexOf(1500):'+arr.lastIndexOf(1500));

The printed result is:

Array deduplication example:

Given an array ['c', 'a', 'z', 'a', 'x', 'a', 'x', 'c', 'b'], we need to remove duplicate elements from the array.
Case Study:
Goal: Select the non-duplicate elements in the old array and put them into the new array, keep only one duplicate element and put it into the new array to remove duplicates.
Core algorithm: traverse the old array, and then use the old array elements to query the new array. If the element does not appear in the new array, we add it, otherwise we do not add it.

The code is as follows:

 var arr = ['c', 'a', 'z', 'a','x', 'a', 'x', 'c', 'b'];
        var newArr = [];
        for(var i =0;i<arr.length;i++){
            if (newArr.indexOf(arr[i]) === -1)
            {
                newArr.push(arr[i]);
            }    
        }
        console.log(newArr);

The printed result is:

6. Array to string conversion

Method Name	illustrate	Return Value
toString()	Convert the array to a string, separating each item with a comma	Returns a string
join('delimiter')	Method is used to convert all elements in an array into a string.	Returns a string

For example:

var arr = ['a', 'b', 'c'];
console.log(arr);
console.log(arr.toString()); // Output: a,b,c
// Using join()
console.log(arr.join()); // Output: a,b,c
console.log(arr.join('')); // Output: abc
console.log(arr.join('-')); // Output: abc

5. String Object

1. Return position according to character

Method Name	illustrate
indexOf()	Returns the position of the specified content in the original string. If it is not found, it returns -1. The starting position is the index number.
lastIndexOf()	Search from back to front, only find the first matching

For example: It is required to find the position and number of occurrences of all specified elements in a set of strings. The string is ' Hello World, Hello JavaScript '.

The code is as follows:

var str = 'Hello World, Hello JavaScript';
console.log(str);
var index = str.indexOf('o');
var num = 0;
while (index != -1) {
  console.log(index); // Output in sequence: 4, 7, 17
  index = str.indexOf('o', index + 1);
  num++;
}
console.log('The number of times o appears is: ' + num); // The number of times o appears is: 3

The printed result is:

2. Return characters by position

For example:

var str = 'Apple';
console.log(str.charAt(3)); // Output: 1
console.log(str.charCodeAt(0)); // Output: 65 (ASCII code of character A is 65)
console.log(str[0]); // Output: A

For example: Use charAt() method to count the most common characters and times in a string through a program

var str = 'Apple';
// Step 1, count the number of occurrences of each character var o = {};
for (var i = 0; i < str.length; i++) {
  var chars = str.charAt(i); // Use chars to save each character in the string if (o[chars]) { // Use the properties of the object to facilitate the search for elements o[chars]++;
  } else { o[chars] = 1; }
}
console.log(o);

The print result is:

3. String operation methods

var str = 'HelloWorld';
str.concat('!'); // Concatenate characters at the end of the string, result: HelloWorld!
str.slice(1, 3); // Extract the content from position 1 to position 3. Result: el
str.substring(5); // Extract the content from position 5 to the end. Result: World
str.substring(5, 7); // intercept the content from position 5 to position 7, result: Wo
str.substr(5); // Intercept the content from position 5 to the end of the string, result: World
str.toLowerCase(); // Convert the string to lowercase, result: helloworld
str.toUpperCase(); // Convert the string to uppercase, result: HELLOWORLD
str.split('l'); // Use "l" to split the string, result: ["He", "", "oWor", "d"]
str.replace('World', '!'); // Replace string, result: "Hello!"

4. split() method

split() method is used to split a string. It can split a string into an array. After the segmentation is completed, a new array is returned.
For example:

var str = 'a,b,c,d';
console.log(str);
console.log(str.split(',')); // Returns an array [a, b, c, d]

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