Array operations in JS

Declare objects 、 New elements 、 Remove elements 、 Additional elements 、 Modifying elements 、 clear list

1. Declare objects

var arrayObject=[];
var arrayObject=new Array();
var arrayObject=new Array(size);
var arrayObject=new Array(element0, element1, ..., elementn);

2. New elements

2.1 unshift() Method to add one or more elements to the beginning of an array , And returns the new length .

grammar

arrayObject.unshift(newelement1,newelement2,....,newelementX)
 Parameters 	      describe 
newelement1	  It's necessary . The first element added to the array .
newelement2	  Optional . The second element added to the array .
newelementX	  Optional . Several elements can be added .

explain
unshift() Method will insert its parameters into arrayObject The head of , And move the existing elements to the higher subscript successively , To make room for . The first parameter of this method will be the new element of the array 0, If there's a second parameter , It's going to be a new element 1, And so on .
Please note that ,unshift() Method does not create a new array , Instead, modify the original array directly .unshift() Methods cannot be used in Internet Explorer To work correctly in ！

2.2 push() Method to add one or more elements to the end of an array , And returns the new length .

grammar

arrayObject.push(newelement1,newelement2,....,newelementX)

explain
push() Method to add its parameter order to arrayObject Tail of . It directly modifies arrayObject, Instead of creating a new array .push() Methods and pop() Method to use the advanced post stack function provided by array .
Tips ： This method changes the length of the array .

3. Remove elements

3.1 shift() Method is used to remove the first element of an array , And returns the value of the first element .

grammar

arrayObject.shift()

Return value
The value of the first element of the original array .

explain
If the array is empty , that shift() Method will do nothing , return undefined value . Please note that , This method does not create a new array , But directly modify the original arrayObject.
notes ：
This method changes the length of the array .

3.2 pop() Method is used to delete and return the last element of the array .

grammar

arrayObject.pop()

Return value
arrayObject Last element of .

explain
pop() Method will delete arrayObject Last element of , Reduce the length of the array by 1, And return the value of the element it deleted . If the array is already empty , be pop() Don't change the array , And back to undefined value .

4. Convert numeric values to strings

4.1 join() Method is used to put all elements in an array into a string .

The elements are separated by the specified separator .

grammar

arrayObject.join(separator)

separator	 Optional . Specifies the separator to use . If this parameter is omitted , Then use comma as separator .

Return value
Returns a string . The string is created by putting arrayObject Each element of is converted to a string , And then connect these strings , Insert... Between two elements separator String .

4.2 toString() Method converts an array to a string , And return the result .

grammar

arrayObject.toString()

Return value
arrayObject String representation of . The return value is the same as that without parameters join() Method returns the same string .

5. splice() Method

Definition and Usage
splice() Method direction / Add from the array / Delete the project , Then return the deleted item .

notes ：
This method changes the original array .

grammar

arrayObject.splice(index,howmany,item1,.....,itemX)

 Parameters        	         describe 
index	             It's necessary . Integers , Prescribed addition / Delete project location , Use negative numbers to specify the position from the end of the array .
howmany   	         It's necessary . Number of items to delete . If set to  0, The item will not be deleted .
item1, ..., itemX	 Optional . New items added to array .

Return value
Array： New array containing deleted items , If any .

explain
splice() Method can be deleted from index Zero or more elements starting at , And replace the deleted elements with one or more values declared in the parameter list .
If from arrayObject Element... Is removed from , Then it returns an array of deleted elements .

notes ：
Please note that ,splice() Methods and slice() The function of methods is different ,splice() Method will directly modify the array .

6. slice() Method

Definition and Usage
slice() Method to return selected elements from an existing array .

grammar

arrayObject.slice(start,end)

Parameters and descriptions
start It's necessary .
Specify where to start selecting . If it's a negative number , So it specifies the position from the end of the array . in other words ,-1 Last element ,-2 The second last element , And so on .
end Optional .
Specify where to end the selection . This parameter is the array subscript at the end of the array fragment . If the parameter is not specified , So the sharded array contains from start All elements to the end of the array . If this parameter is negative , So it specifies the elements starting from the end of the array .

Return value
Returns a new array , Contains from start To end （ This element is not included ） Of arrayObject The elements in .

explain
Please note that , This method does not modify the array , Instead, it returns a sub array . If you want to delete an element in an array , Should use method Array.splice().

Tips and notes
notes ： You can use negative values to select elements from the end of the array .
notes ： If end Not regulated , that slice() Method will be selected from start All elements to the end of the array .

7. concat() Method

Definition and Usage
concat() Method to connect two or more arrays .
This method does not change the existing array , Only one copy of the connected array will be returned .

grammar

arrayObject.concat(arrayX,arrayX,......,arrayX)

 Parameters 	 describe 
arrayX	 It's necessary . This parameter can be a specific value , It can also be an array object . It could be any number .

Return value
Returns a new array . The array is created by putting all arrayX Parameters added to arrayObject Generated in the . If you want to do concat() The parameters of the operation are arrays , Then you add the elements in the array , Not an array .
In this case , We created two arrays , And then use concat() Connect them ：

<script type="text/javascript">

var arr = new Array(3)
arr[0] = "George"
arr[1] = "John"
arr[2] = "Thomas"

var arr2 = new Array(3)
arr2[0] = "James"
arr2[1] = "Adrew"
arr2[2] = "Martin"

document.write(arr.concat(arr2))

</script>

Output results

George,John,Thomas,James,Adrew,Martin

Tips ：
At this time arr It hasn't changed , It is arr.concat(arr2) The return value of is a list containing both .

Please refer to the link for details : JavaScript Array object https://www.w3school.com.cn/jsref/jsref_obj_array.asp