2022.7.29 Array

1. Basic operations of arrays

1. Definition: An array is a list object, and its prototype provides operations related to traversing and modifying elements. The length and element type of JS arrays are non-fixed (putting the data together is an array)

2. Declaration method:

//Literal form (commonly used, convenient)var list1 = [1,2,true,'hello']//constructor declarationvar list2 = new Array(1,2,true,'hello')

3. Access elements of an array

 var list = [1,2,3,4]console.log(list[2]) //Through the index (subscript), get 3var list = [1,2,3,4]console.log(list.length) //Get the length of the array, get 4

4. Query array subscript

 var a = ["a", "b", "c"];var b = a.indexOf("c");console.log(b); // 2

Second, the basic method of operating arrays, adding, deleting, modifying and checking

 var list = [1, 2, 3, 4];// incrementlist[4] = 5; // [1, 2, 3, 4, 5]list[6] = 7; // [1, 2, 3, 4, 5, empty, 7]list.push(8); // [1, 2, 3, 4, 5, empty, 7, 8] add data at the endlist.unshift(0); // [0, 1, 2, 3, 4, 5, empty, 7, 8] add data to the front// changelist[2] = "a"; // [0, 1, 'a', 3, 4, 5, empty, 7, 8]// deletedelete list[2]; // [0, 1, Empty, 3, 4, 5, Empty, 7, 8] Generally not used, because it will produce Empty.with splicelist.pop(); // [0, 1, empty, 3, 4, 5, empty, 7] delete the last datalist.shift(); // [1, empty, 3, 4, 5, empty, 7] delete the first data// checkconsole.log(list[2]);

Three, advanced array operation method

1.splice

Role: delete or replace elements; the function has a return value, which returns the deleted element; this method will change the original array.

Usage scenarios: replace elements in an array; delete part of an array; empty an array.

 var list = ["a", "b", "c", "d", "e"];list.splice(0, 3, 2);console.log(list);

The first parameter is to control the deletion or replacement from the first position (inclusive) (depending on whether the third parameter has a value), the second parameter is to control the number of deletions, and the third parameter will be deletedElements are replaced, separated by commas.

 var list = ["a", "b", "c", "d", "e"];list.splice(0, 3, 2);console.log(list); //[2, 'd', 'e']

If there is only one parameter, it means to delete all values ​​after the first digit.

2.join

Function: Convert the data of the array type into a string, the difference from toString is that you can separate the elements with custom symbols.

 var list = [1, 2, 3, 4, 5];var a = list.join("-");console.log(a); // 1-2-3-4-5

3.concat

Effect: Concatenates two or more arrays, does not change the existing array, but returns a new array containing the values ​​of the concatenated arrays.

 var a = [1, 2];var b = [3, 4];var ab = b.concat(a);console.log(ab); // [3, 4, 1, 2]