JS basic knowledge collation - array

Recently, I feel that I don't have a solid grasp of basic knowledge , If you encounter an interception and replacement, you have to check , Forget it after a long time , Find out the notes you used to take after watching the video tutorial , You can also review while sorting .

One 、 Array

use [ ] A group of data packed together is called an array

He has an index length It has its own attribute （ So called ergodic loop , Is the use for loop , Realize the traversal of the array ）

Array public methods （14 individual ）

Any of our arrays can call , There is Array Prototype prototype Common methods on , Because every array is Array An instance of the array class ;

1、 Array of “ Additions and deletions ” The original array changes
1）push The return value is the length of the array after adding a new item

var ary=[1,2,3,4,5];
var res=ary.push(4,5,6,8);
console.log(ary,res);

Tips

ary[ary.length]="10";
console.log(ary);

adopt ary.length The index implementation adds a new entry to the array

2）unshifit The return value is the length of the array after adding a new item
Add items to the beginning of the array

var ary=[1,2,3,4,5];
var res=ary.unshift(0);
console.log(ary,res);

3）pop
Delete the last item The return value is delete item
Tips ： utilize length Property to delete the last item of the array ary.length-=1; ary.length–; Both have the same effect

4）shift
Delete the beginning of the array The return value is delete item
var ary=[1,2,3,4,5];
var res=ary.shift();
console.log(ary,res);

5）splice Return a crop item to form a new array
Crop the array
grammar ：ary.splice( Crop index , How many items to cut , What should be used to replace the crop item )

var ary = [1, 2, 3, 4, 5, 6, 7, 8, 9];
var res = ary.splice(3, 2, 0);
console.log(ary, res);

var ary = [1, 2, 3, 4, 5, 6, 7, 8, 9];
var res = ary.splice(3);// From the index 3 Start deleting all subsequent items 
console.log(ary, res);

Delete the last item ary.splice(ary.length-1,1); ary.splice(ary.length-1);

Add... At the end var res=ary.splice(ary.length,0,"a");
Delete 0 term add to “a” Return an empty array Because I didn't delete anything

Add at the beginning var res=ary.splice(0,0," Opening item ")

Replacement and addition of intermediate items
Replace ary.splice(0,1,"a"); Delete replace 1 term
add to ary.splice(3,2,"c","d"); Delete and replace multiple add to “c” “d”

Increase is to increase the index in front of the current crop
var ary = [1, 2, 3, 4, 5, 6, 7, 8, 9]; var
res=ary.splice(1,0," I "," yes "," Two ");
console.log(ary);

ary.splice(0) Clone array
ary.splice() Pass nothing on Create an empty array , Return an array instance

6）slice（n,m） Search index n Start to index m Before the item
grammar ：ary.slice( Index at the beginning , The index at the end but does not contain the index entry at the current end )
stay slice Searching , The original array does not change ,slice The return value of the function is a new array composed of each found item ;

Pass only one parameter
ary.slice(0) It is equivalent to copying an array ary.slice(0,ary.length) Clone array
ary.slice(1) Returned array It is equivalent to deleting the beginning item
ary.slice(0,ary.length-1) Returned array It is equivalent to deleting the last item

（ From the index 0 Start To the last Because it does not contain m The length is larger than the index 1 So the last one will stay Delete the last item ）

From n term , Index correspondence （n-1）, Find the first m term （m-1）, Include m term , Index correspondence （m）

ary.splice() Pass nothing on and concat equally Equivalent to cloning an array

7）concat Array merge
ary.concat() We can pass one , Or multiple contents , The data type of content can be arbitrary , If it's an array , It is equivalent to merging two arrays , If it's not an array , It is equivalent to adding

The original array remains the same Return a new array after merging

ary.concat("e",1,2,3) If the argument is not an array , Is to add to the end

ary.concat() Pass nothing on , It is equivalent to cloning the original array

ary.concat("e",1,2,3,{name:" Monkey seven "}) Merge multiple items

Array of “ Convert to string ”
1）join The original array remains unchanged The returned result is a string separated by the specified separator , If you do not add the specified separator , The default is separated by commas

2）toString Directly convert the array into a string

characteristic ： What is called is the method on the object prototype （ Arrays are also object data types So you can call methods on the object prototype ）

Find an item in the array
1）indexOf The return value is , From the index 0 Start , Find the first position of the item in the array

2）lastindexOf The return value is , Start with the end index , Find the first position of the item in the array

Array of “ Loop traversal ” （ There are methods on the prototype for Loops are not methods that exist on prototypes ）
1）forEach
Format ：forEach （ Callback function ）

var ary=["a","b","c"];
ary.forEach(function ( Each specific item found , The index of each item ) {
    
    console.log(n, m);
})

2）map Traverse each item in the array , You can modify every item ; Then put each item after processing , Combine into a new array and return ; Our original array does not change

Format ：

ary.map(function (item){
    
           
})

var ary=[1,2,3,4];
var res=ary.map(function (item) {
    
    return item+"a"// One is string   String splicing 
});
console.log(ary, res);// ary[1, 2, 3, 4] res["1a", "2a", "3a", "4a"]

The callback function needs to use return Get the return value , Make changes to each item and return the modified results

var ary=[1,2,3,4,5];
var res=ary.map(function (item) {
    
    return item*5;   A return value is required 
});
console.log(ary, res);//--->(5) [1, 2, 3, 4, 5] (5) [5, 10, 15, 20, 25]

1）sort Sort the array Sort the original array , Our return value is also an ordered array
Through formal parameters a b Compare , To achieve sorting ：a b It represents every item of our array , That is, take the return value of each comparison of the array in the callback function to sort Method , Then sort the essence

var ary = [10, 22, 53, 68, 71, 83, 19];
var res=ary.sort(function (a,b) {
      <--- Callback function 
    return a-b;
});
console.log(ary, res);

return a-b; a-b It is to compare each item to get an array from small to large
return b-a; b-a It is to compare each item to get an array from large to small

ary.sort() Call directly , Without transferring the callback function , Yes, only right 10 The numbers within are sorted from small to large

According to English 26 Letters can also be sorted

> var ary=["az","ec","d5","b6","a5"]; 
> ary.sort();
>  console.log(ary);

1）reverse Invert an inverted array

var ary=[1,2,3,4,5,6];
var res=ary.reverse();
console.log(ary, res);

User information of five people , Put the present 5 An array of individuals , Sort by age

a b Represents each item in the array use a.age To get the age first a yes 12