What is an array?

Array: a window that stores a set or series of related data, equivalent to a combination of multiple variables.

• When multiple data are stored and processed, use an array. What if 5 or 50 data are stored?
• Array is a continuous space in memory. Compared with variables, it has higher and faster performance for storing or reading data

Array subscript: the index number that can identify a unique array space.

Subscript starts from 0, maximum subscript: length of array - 1

Element of array: the data stored in the array with unique index numbers is called element.

How do I declare arrays?

1. Literal method: []
2. Constructor method: new Array()

var arr = []; //Declare an empty array
var list = new Array(); //Declare an empty array

The difference between the two declaration methods?

1. In literal mode, no matter what data is stored, it represents array elements.
2. When there is only one positive integer in the constructor, it indicates the length of the array.
3. When there is a negative integer or decimal in the constructor, an error is reported directly
4. Represents an array element when there is one or more other types of data in the constructor.

var arr = [5]; // Indicates that one element in the array is 5 and the length is 1
var arr = new Array(5); //Indicates that the length of this array is 5 and can hold 5 elements. The default element is undefined,undefined,undefined,undefined,undefined
var arr = new Array(-5); //The length cannot be negative. The program reports an error
var arr = new Array(5.3); //The length cannot be decimal. The program reports an error
var arr = new Array('3'); //Indicates that an element in the array is'3',The length is 1
var arr = new Array(1,2,3,4,5);//Indicates that there are 5 elements in the array, 1 2 3 4 5 respectively ,The length is 5

How do I access arrays?

Array name [subscript]

var arr = [1,2,3,4,5]; //Declare an array containing five elements
arr[5] = 6; //Added an element 6 to the array with a subscript of 5
console.log(arr[0]); //The element corresponding to the 0 subscript in the access array is 1

Array properties

length  : Indicates the length of the array (that is, how much memory space is in the array)

var arr = [1,2,3,4];
console.log(arr.length); // 4 Represents the length of the array

Array method (add, delete, modify, cut, spell, compound, arrange, transfer)

Add:

Unshift (element, element, element,...)

• Function: add a new element to the head of the array.
• Return value: the length of the array after adding
• Affect original array: Yes

var arr = [5,6,7,8,9]; //Declare an array
console.log(arr.unshift(true,[1,2,3],false)); //unshift Method, 8 means that the length of the array after adding is 8
console.log(arr); // Output original array [true,[1,2,3],false,5,6,7,8,9]

Push (element, element, element,...)

• Function: add new elements at the end of the array.
• Return value: the length of the array after adding
• Affect original array: Yes

var arr = [5,6,7,8,9];
console.log(arr.push(true,[1,2,3],false)); //push Method, 8 means that the length of the array after adding is 8
console.log(arr); // Output original array [5, 6, 7, 8, 9, true, [1,2,3], false]

Delete:

shift()

• Function: delete an element at the head of the array (only one element can be deleted at a time)
• Return value: deleted element
• Affect original array: Yes

var arr = [5,6,7,8,9]; 
console.log(arr.shift()); //shift Method, 5 represents the element 5 deleted in the array
console.log(arr); // Output original array [6, 7, 8, 9]
//Delete all elements
var arr = [5,6,0,8,9];
while(arr.length){
    arr.shift();
}
console.log(arr);

pop()

• Function: delete an element at the end of the array (only one element can be deleted at a time)
• Return value: deleted element
• Affect original array: Yes

var arr = [5,6,7,8,9];
console.log(arr.pop()); //9 pop Method, 9 represents the element 9 deleted from the array
console.log(arr); // Output original array [5, 6, 7, 8]

Change:

Splice (start, del_, length, element, element,...)

• start: from which subscript position
• del_length: delete several elements
• Element: new element
• Function: you can add, delete and change at any position of the array.
• Return value: array of deleted elements
• Affect original array: Yes

var arr = [5,6,7,8,9];
//When there is only one parameter, it means to delete from the specified subscript to the end of the array
console.log(arr.splice(1)); // [6, 7, 8, 9] Return value
console.log(arr); // [5] Original array

var arr1 = [5,6,7,8,9];
//With two parameters, it means that the element of the specified length is deleted from the specified subscript position
console.log(arr1.splice(1,2)); // [6, 7]
console.log(arr1); // [5, 8, 9]

var arr2 = [5,6,7,8,9];
//Add a new element at the deleted location. Delete 2 elements from subscript 1 and add new elements at the position of subscript 1 true,false Two elements
console.log(arr2.splice(1,2,true,false)); // [6, 7]
console.log(arr2); // [5, true, false, 8, 9]

var arr3 = [5,6,7,8,9];
//Delete 0 elements at subscript 1 and add two elements at subscript 1 true,false
console.log(arr3.splice(1,0,true,false)); // []
console.log(arr3); // [5, true, false, 6, 7, 8, 9]

Cut:

slice(start,end)

• start: from which subscript position
• End: to which subscript position to end (excluding the end position)
• Function: intercept the elements of the specified range in the array
• Return value: the intercepted element array
• Affect original array: no

var arr = [5,6,7,8,9];
// Intercept from the subscript 1 position in the original array to the end of the array, and return the intercepted new array
console.log(arr.slice(1)); // [6, 7, 8, 9] Return value
console.log(arr); // [5,6,7,8,9] Original array

var arr1 = [5,6,7,8,9];
//Intercept from the position of subscript 1 in the original array to before subscript 3 (excluding subscript 3)
console.log(arr1.slice(1,3)); // [6, 7] Return value
console.log(arr1); // [5,6,7,8,9] Original array

var arr2 = [5,6,7,8,9];
//Intercept from the subscript 3 position in the original array to before the subscript 1 (because it can only be intercepted to the right, all elements cannot be intercepted)
console.log(arr2.slice(3,1)); // [] Return value
console.log(arr2); // [5,6,7,8,9] Original array

var arr3 = [5,6,7,8,9];
//Subscripts from the original array-3(The last element subscript is-1，-3 The element corresponding to the subscript is 7) and is intercepted from the position-1 Before subscript (excluding-1 Subscript)
console.log(arr3.slice(-3,-1)); // [7, 8] Return value
console.log(arr3); // [5,6,7,8,9] Original array

Spelling:

Concat (element, element,...)

• Function: splice new elements behind the specified array (if the new element is an array, remove the outermost [] and splice the inner elements)
• Return value: new array after splicing
• Affect original array: no

var arr = [5,6,7,8,9];
//If the spliced elements are multidimensional arrays, the elements in the outermost array are spliced to the back of the specified array as they are
console.log(arr.concat(1,[true,[1,2]],3)); // [5, 6, 7, 8, 9, 1, true, [1,2], 3] Return value
console.log(arr); // [5,6,7,8,9] Original array

Reply:

function copyArray(arr){
    return arr.slice(0);
}
function cloneArray(arr){
    return arr.concat();
}

Row:

reverse()

• Function: store the elements in the array in reverse order
• Return value: returns the array in reverse order
• Affect original array: Yes

var arr = [3,2,10,4,5];
console.log(arr.reverse()); // [5, 4, 10, 2, 3] Return value
console.log(arr); // [5, 4, 10, 2, 3] Original array

sort()

• Function: sort the elements in the array from small to large according to the character code
• Return value: returns the array in reverse order
• Affect original array: Yes

var arr = [3,2,10,4,5];
//Compare as a string-Sort: 10 vs. 2 compare 1 < 2,So 10 < 2
console.log(arr.sort()); // [10, 2, 3, 4, 5] Return value
console.log(arr); // [10, 2, 3, 4, 5] Original array

sort(function(a,b){return a-b;}): sort the elements in the array from small to large. sort(function(a,b){return b-a;}): sort the elements in the array from large to small according to numbers.

var arr = [3,2,10,4,5];
// From small to large by number
console.log(arr.sort(function(a,b){return a-b;})); //[2, 3, 4, 5, 10]
// Press numbers from large to small
console.log(arr.sort(function(a,b){return b-a;})); //[10, 5, 4, 3, 2]

Turn:

join('connector ')

• Function: convert the array into a string connected with the specified connector
• Return value: returns the converted string

Affect original array: no

var arr = [1,2,3,4];
// Converts an array to a string concatenated with an empty string
console.log(arr.join('')); //'1234'