Sharing several ways to define arrays in JavaScript

This article mainly introduces several methods of defining JavaScript arrays. Generally speaking, they are divided into implicit creation, direct instantiation, creating an array and specifying the length. Later, for convenience, the array is directly defined in json format. Here is the Script Home The editor will introduce it to you. Friends who need it can refer to it

1. What is an array

An array is a collection of data
its expression It is a continuous memory address in the memory
The array name is actually the first address of the continuous memory address

2. About the characteristics of arrays in js

Array definition There is no need to specify the data type
There is no need to specify the array length when defining the array
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 an array:

var arr=[value 1, value 2, value 3]; arr=new Array(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;);

The meaning of symbols in JS:

() means function execution

[] means syntax simulation, which means simulating an instance of the Array class (=new Array( ))

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

3. About array length

Array object.length

In js, each array object can call the length attribute, which represents the array There are several array elements under the 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;);
}

4, for...in statement

In js, arrays are not data types. The data type of arrays is actually objects.

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

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

Syntax:

for( var i in array){

}

Principle: There are several elements in the array, and 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 is 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

1:lisi

2:wangwu
3:xiaoqiang

5 , Text subscript

Format:

arr['key'] = value;

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

is added to the array in the form of a text subscript. It is actually added to the array object in the form of an attribute.

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

zhangsan

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

　1:2

　2:3

　First:zhangsan
　Second:lisi


6. Multidimensional array

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;);
}

The following is the supplement of Script House

1. Define first and then assign value

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";

2. Assign a value when defining the array

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

3. Definition of json format

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 definition The difference between usage and array

Json: It is a method used to store data in javascript. Json format: var Json={a:1, b:2, c:3};

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

Json The difference between two arrays:

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]2. Json does not have length, and the array has lengtht

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){

Statement

};

When to use for loop and when to use for in loop?
Array: Use for loop for(i=0,iJson: Use for in loop for(i in Json){};

4. Note

· The array only has one attribute, the array length · If no value is assigned to the array, the result is undefined, and the print is ""

· The array length is not enough and can be automatically expanded, it does not exist Cross-border issues

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