Array method in JS 2021.09.18

Basic data type ：number、string、boolean、undefined、null

quote （ complex ） data type ：Array、function, Object

Conclusion ： Simple types store the value itself , Complex types store addresses , If you assign the first object to another variable , At this point, the two variables will point to the same object .

Stack memory ： Storage address ;      Heap memory ： Store the value pointed to by the address ;

Different types of data Compare Conversion required . Specific rules See the table below ：

https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Equality_comparisons_and_sameness#%E4%B8%A5%E6%A0%BC%E7%9B%B8%E7%AD%89

Methods of array operation ：

splice : Delete or add elements anywhere in the array

splice(start, deletedCount)   Remove elements

splice(start, deletedCount , item) Delete + add to ,   The third parameter is to add several new elements to the original deleted position

splice(start, 0 , item)   Is to add a new element at a certain position

array.push( Elements );// Add elements from behind , Returns the of the new array length
array.pop();// Remove elements from the back of the array , Return the deleted element
array.unshift( Elements );// Add elements from the front of the array , Returns the length of the new array
array.shift();// Remove the element from the front of the array , Return the deleted element

-- All that is added is the return length
-- All the deleted elements are returned to the deleted elements

Array splicing ：concat： Array merge , It will not affect the original array , Will return a new array .

Sort of array ：array.sort();    // Sort of array , The default in accordance with the Letter / First character Order  

Inversion of array ：array.reverse();   // Flip array

Array and string conversion ：array.join( Separator )    // effect ： Concatenate the values of the array into strings , And return string

split: Split a string into arrays （ Very often ）

New array method ：

indexOf   Find the position where an element first appears in the array ;

grammar ：arr.indexOf( Elements ,[ The starting subscript of the start search ]);  # Return value ： If you find it , Just return the subscript of this element in the array , If not found , Just go back to -1;

lastIndexOf( value )     Get the last position of the corresponding subscript through the value of the array

every(function( value , Subscript , Array ){})      Determine whether each value in the array meets the requirements

some(function( value , Subscript , Array ){})      Determine whether at least one value in the array meets the requirements

forEach   For traversing arrays ; This method does not return a value , Pure read operation , It will not change the value of the original array .

grammar ：

arr.forEach(function( value , Subscript , Current array ){
    // Code segment
});

var arr = ['a', 'b', 'c'];
// forEach Yes 2 Parameters ：
// callback :  Support 3 Parameters 
// thisArg :  Specifies the name of the callback function this Point to 
arr.forEach(function callback(item, index, arr) {
   console.log(this == arr); // true 
   console.log(item, index);
}, arr);

map   Traversal array , And pass each element back to the function for processing , Return a new array ,.（ This method is mainly used to process each value in the array with the same rules , And form a new array to return ）

grammar ：

arr.map(function( value , Subscript , Current array ){
    // Yes v Process and return each after processing v Array of components
});

// map Also comes with traversal , During traversal   You need to return a new value   As an element in the new array 
        var newArr = arr.map(function (item, index, arr) {
            //  Callback function   Return value required 
            // return 1;
            //  Based on the original value   modify , Returns the new value 
            return item.toUpperCase();
        });
        console.log(newArr);

filter   Traversal array , According to the filter conditions , Filter out the elements that meet the conditions in the array , Form a new array and return .（ The use method and function are the same as map Methods like , But the running rules are different .map Functions in methods , Used to return a new element , and filter Functions in methods , According to return true or false To filter elements ）

grammar ：

arr.filter(function( value , Subscript , Current array ){
    // filter
});

reduce   Merger , Where the call back function , There are two arguments in the fallback function , The first parameter is the return value of the last operation , The second parameter is from the second element to the last element .

grammar ：

arr.reduce(function(prev,next){
    // Logic code
});

Common pseudo arrays ：arguments、nodeList、htmlCollection

Bubble sort

Compare two adjacent elements , Use a loop to arrange the numbers in an array in ascending or descending order .

Selection sort

Descending ： First find the maximum value , On the far left , Find the second largest value , To the left ..., Already arranged , No longer participate in the comparison