Detailed explanation of several JavaScript array definition methods

How much do you know about the method of array definition? Generally speaking, it is divided into implicit creation, direct instantiation, creating an array and specifying the length. Later, in order to facilitate the direct use of json format to define the array, this article will share with you several detailed methods of javascript array definition. I hope it can help you.

1. What is an array?

An array is a collection of data.
Its expression is a continuous memory address in the memory.
Array name In fact, it is the first address of the continuous memory address

2. About the characteristics of arrays in js

There is no need to specify the data type when defining an array
You don’t need to specify when defining an array Array length
Arrays can store data of any data type (for example, one element stores integer type and one element stores string type. This is possible in JS)

Syntax for creating arrays ：

var arr=[value 1, value 2, value 3];                                                                                                  ; //Direct instantiation

var array=new Array(size); //Create an array and specify the length

//方法一
var arr1 = [1,3,5,7,9];
document.write(arr1[2] + &#39;<br>&#39;);
 
//方法二
var arr2 = new Array(2,4,6,8,10);
document.write(arr2[3] + &#39;<br>&#39;);
 
//方法三
var arr3 = new Array(3);//固定数组长度为3
arr3[0] = 1;
arr3[1] = 2;
arr3[2] = 3;
document.write(arr3[2] + &#39;<br>&#39;);

() indicates function execution

{} indicates syntax simulation, Indicates simulating an instance of the Object class (=new Object())
//Indicates syntax simulation (regular object), indicates simulating an instance of the RegExp class (=new RegExp())

Array object.length

In js, each array object can call the length attribute, which indicates how many array elements there are under the array object

Example:

var row = ['zhangsan','lisi','wangwu'];
doucument.write(&#39;共有&#39;+row.length+&#39;个人<br>&#39;);

var length = row.length;//对数组进行遍历
for (var i=0;i<length;i++){
doucument.write(row[i]+&#39;<br>&#39;);
}

In js, arrays are not data Type, the data type of the array is actually the object

The For...in statement in Js can traverse all the properties of an object

You can also use for...in Statement to traverse all elements of an array

Syntax:

for(var i in array){

}

Principle: There are For several elements, the for..in statement will be executed in a loop as many times as possible

Every time it is executed, the subscript of the current array element will be stored in the variable i

var row = ['zhangsan','lisi','wangwu','xiaoqiang'];
for (var i in row){
 document.write(i + &#39;:&#39; + row[i] + &#39;<br>&#39;);
}

Result:

0:zhangsan

2:wangwu
3:xiaoqiang

Format:

arr['key'] = value;

In js, the array elements of the text subscript are not included in the array length

Added to the array in the form of text subscript, actually added to the array object in the form of attributes

var arr = [1,2,3];
arr['first'] = 'zhangsan';
arr['second'] = 'lisi';

document.write(arr.length + &#39;<br>&#39;);
document.write(arr.first + &#39;<br>&#39;);
document.write(arr.second + &#39;<br>&#39;);

Result:

　3

lisi

Traverse the array with text subscripts:

var arr = [1,2,3];
arr['first'] = 'zhangsan';
arr['second'] = 'lisi';

for(var i in arr){
 document.write(i + &#39;:&#39; + arr[i] + &#39;<br>&#39;);
}

Result:

0 :1

　2:3
　First:zhangsan
　Second:lisi

var arr = [
  [10,'zhangsan','male'],
  [11,'lisi','female'],
  [12,'wangwu','male']
 ];
for (var i in arr){
 for(var j in arr[i]){
  document.write(arr[i][j]);
 }
 document.write(&#39;<br>&#39;);
}

var myArray1 = new Array(); //不定义元素个数
  myArray1[0] = "脚";
  myArray1[1] = "本";
  myArray1[2] = "之";
 myArray1[3] = "家";
var myArray2 = new Array(4); //定义元素的个数
  myArray1[0] = "脚";
  myArray1[1] = "本";
  myArray1[2] = "之";
 myArray1[3] = "家";

var　myArray3 = new Array("foot","本","之","家");

//You need to specify a numerical or logical value in the array, then the variable type should be a numerical variable or a Boolean variable, not a character variable

var myArray4 = [
   {"num":"1001","name":"jack","age":16},
   {"num":"1002","name":"Mary","age":17},
   {"num":"1003","name":"Tom","age":19}
   ];
//注意中括号和大括号的位置

Students who don’t understand can check this article: http://www.jb51.net/article/20688.htm

Json: It is a method used to store data in javascript.

Array format: var arr=[1,2,3];

1. The subscript of Json is a string. For example, when calling the first data, use: Json.a. The subscript of the array is a number such as arr[0]

Use Json to store data. If you need to loop, you need to use for (element variable in variable name) {}; Because Json does not have a length, a for in loop must be used. The for in loop can be used for Json and arrays

for(i in Json){

};

When to use for loop and when to use for in loop?