An array of JS foundations

Arrays: Arrays are values that use separate variable names to store a series of identical data types.

One-dimensional array: ① empty array: var variable name = new Array ();/var variable name = ""; var variable name =new Array (array length);

② has an array of values: var variable name =new Array ("1" "2");/var variable name = "Three-in-one";

Attention:
var arr1 = [3]; Arr1.length = = 1, arr1[0] = = 3
var arr2 = new Array (3); Arr2.length = = 3, arr2[0] = = undefined

Multidimensional array: var twoarray = new Array ();
TWOARRAY[0] = new Array ();//array index=0 and new array
TWOARRAY[1] = new Array ();//array index=1 and new array

Build Twoarray[0] Array
Twoarray[0][0] = "AA";
TWOARRAY[0][1] = "ba";
TWOARRAY[0][3] = "Ca";
Build Twoarray[1] Array
Twoarray[1][0] = "ACCA";
TWOARRAY[1][1] = "Bada";
TWOARRAY[1][3] = "Cdasa";

var twoarray = [["A0b0", "a0b1"],["a1b0", "A1b1"],["a2b0", "a2b1"]];

var twoarray = new Array ();
for (var i = 0; i < FirstIndex; i++) {
Twoarray[i] = new Array ();
for (var j = 0; J < LastIndex; J + +) {
TWOARRAY[I][J] = "Database BlaBla";
}
}

To iterate over an array:

For loop

var New Array ("1", "2", "3");     for　　　　　　　　　　　　　　　　　　　 (var i = 0;i<aa.length;i++) {       alert (aa[i]);   }


                              For-in

var New Array ("A", "B", "C");     for　　　　　　　　　　　　　　　　　　　　　　　　 (var in aa) {       alert (aa[b]);   }

　　 //Add a value to the array
var arr=new Array (1,2,3,4,5);
var len=arr.push (7,9);
Console.log (Len,arr);//array.push () adds a value at the end of the array, returning the added array length
var arr=new Array (1,2,3,4,5);
var len=arr.unshift (7,9);
Console.log (Len,arr);//array.unshift () adds a value at the beginning of the array, returning the added array length, as opposed to Array.push ()

//Delete the values in the array
var arr=new Array (1,2,3,4,5);
var val=arr.pop ();
Console.log (Val,arr);//delete the value from the end of the array to return the deleted value
var arr=new Array (1,2,3,4,5);
var val=arr.shift ();
Console.log (Val,arr);//delete the value from the beginning of the array returns the deleted value in contrast to the pop ()

//Array conversion string
var arr=new Array (1,2,3,4,5);
var str=arr.join ('-');//interval string, default to ","
Console.log (str);//1-2-3-4-5

//Array sorting
var arr=new Array (1,2,3,4,5);
arr.reverse ();//Reverse Sort
Console.log (arr)//[5, 4, 3, 2, 1]
var arr=new Array (1,2,3,4,5,59,6);
Arr.sort ();
Console.log (arr);//Output [1, 2, 3, 4, 5, 59, 6] Note: sort () By default, the conversion string is reordered so it is not sorted correctly by number size
//correct method to write comparison function
Arr.sort (function (A, b) {return-A-B});//Ascending
Console.log (arr);//Output [1, 2, 3, 4, 5, 6, +]
Arr.sort (function (A, b) {return b-a});//Descending
Console.log (arr);//Output [6, 5, 4, 3, 2, 1]

built-in operation functions for arrays:

//connection array
var arr1=[1,2,3];
var arr2=[4,5,6];
var arr3=[7,8,9];
var arr=arr1.concat (arr2,arr3,[10,11]);
Console.log (arr);//Output [1, 2, 3, 4, 5, 6, 7, 8, 9, ten, one]

returns the selected element in a//existing array
var arr=new Array (1,2,3,4,5);
var arr2=arr.slice (2);
The first parameter start selects the index subscript value 2 for the third value (0,1,2) Number 3 begins with an optional default to the end of the array
Console.log (ARR2);//Output [3, 4, 5]
//Note the end argument is that the parameter is the array at the end of the array fragment subscript 4 is the number 5 of the subscript after intercept 5, can also be truncated to end-1
var arr2=arr.slice (2,4);
Console.log (ARR2);//Output [3, 4] not [3,4,5]
//intercept starts with a negative number
var arr2=arr.slice ( -2,4);
//If it is a negative number, it specifies the position from the end of the array. In other words, 1 refers to the last element, 2 refers to the second-lowest element, and so on.
///You can also convert an array length (5) plus a negative value (-2) equivalent to Arr.slice (3,4);
Console.log (ARR2);//Output [4]

//splice () implement delete insert Replace array
//Delete
var arr=new Array (1,2,3,4,5);
var arr2=arr.splice;//delete starts with a value of 1 (2) begins with 2 values (2 and 3) to return the deleted value
Console.log (ARR,ARR2);//Output [1, 4, 5] [2, 3]
//Insert
var arr=new Array (1,2,3,4,5);
var arr2=arr.splice (1,0,11,111);//The value is inserted before the value of subscript 1 (2), the second of the parameter is 0 not deleted, insert 11,111
Console.log (ARR,ARR2);//[1, One, 111, 2, 3, 4, 5] [] arr2 returned as an empty array because it is not deleted
//Replace
var arr=new Array (1,2,3,4,5);
var arr2=arr.splice (1,2,11,111);//Replace the value before the value (2) of the subscript 1, the second of the parameter is 2 to remove 2 values, and then insert the 11,111
Console.log (ARR,ARR2);//[1, 11, 111, 4, 5] [2, 3]//delete and add

//Find the subscript where the value is located
var arr=new Array (1,2,3,4,5,6,7,3);
var index=arr.indexof (3);
Console.log (index);//returns 2 detects where the first 3 appears
var index=arr.indexof (3,4);//The second argument starts with a value
Console.log (index);//return 7 detect the position where the first 3 appears, starting with 4 (number 5)
var index=arr.indexof (ten);
Console.log (index);//returns-1 for lookup to return-1
start looking for LastIndexOf () at the end of//
var index=arr.lastindexof (3);
Console.log (index);//Return 7
//indexof () lastIndexOf () Low version browser incompatible

An array of JS foundations