Overview of js basic arrays, array length, array traversal

Brief introduction to the array in js, and getting the length of the js array, traversing the array in js, let us learn js Array!

1. Overview of Arrays An array represents a collection of ordered data.

Representation method of array: []. You can think that as long as you see square brackets, it is an array.
Each data in the array is separated by commas, and there is no comma in the last item.
array: Array

1 // 定义一个数组
2 var arr = [1,2,4,5];
3 console.log(arr[2]);

Use index (index), also called subscript, to accurately read and set an item in the array. Array subscripts start from 0.

1 arr[index];  //读取    
设置：arr[2] = 456;

Each item in the array can be a different data type. Usually we are used to writing the same data type as a set of arrays.

2. The length of the array
The data of the array is referenced to the data type.

1 // 数组的数据类型
2 console.log(typeof arr);

Reference types generally have their own properties and methods ;

Properties: characteristics of things. For example: a person's height, shortness, fatness or thinness.

Method: It is the behavior of things. For example: playing computer, sleeping.

We can use the dot method to call attributes.

1 Reference data. Attribute

The length of the array is represented by the length attribute. Call directly.

1 // 得到数组的长度
2 console.log(arr.length);

The last item of the array: the subscript is the array length minus 1.

1 arr[arr.length - 1];

If the subscript exceeds arr.length - 1, the value does not exist , the output undefined is undefined.

1 var arr = [1,23,45,2,56,1,67,89];
2 // 数组的最大下标是length -1,当超过这个下标时输出undefined
3 console.log(arr[9]);

We can assign a value to an item through a subscript. If we assign a value to an item whose subscript exceeds arr.length - 1, the assignment is forced and the length of the array is forcibly lengthened.

1 var arr = [1,23,45,2,56,1,67,89];
2 arr[20] = 20;
3 console.log(arr[20]);
4 // 现在的数组长度是多少
5 console.log(arr.length);

Although the length has been lengthened, the items that are not assigned a value in the middle are still undefined.

arr.length can also be forced to assign a value. If it exceeds the original length, the extra part is undefined if it is not assigned a value. If it is less than the original length, the extra subscript data will be deleted directly.

1 var arr1 = [1,2,45,67,2,5566,67,12];
2 console.log(arr1.length);
3 arr1.length = 5;
4 console.log(arr1.length);
5 console.log(arr1);

3. Array traversalWe can get each item in the array through a certain method. This is traversal.

We traverse the array through for loop.

1 var arr = [1,2,"号","",function(){},12,23,null,false];
2 // 遍历输出数组中每一项
3 for(var i = 0 ; i <= arr.length - 1 ; i ++){
4 console.log(arr);
5 }

Case: arr = [2,4,6,7,8]; Find the factorial of each item in the array and then sum it.

1 // 案例
2 var arr = [2,4,6,7,8];
3 // 求阶乘和
4 // 累加器
5 var sum = 0;
6 for(var i = 0 ; i <= arr.length - 1 ; i ++){
7 // 需要一个求阶乘的函数
8 sum += jiecheng(arr);
9 }
10 console.log(sum);
11 
12 
13 // 阶乘函数
14 // 累乘器
15 function jiecheng(a){
16 var cheng = 1;
17 for(var i = 1 ; i <= a ; i ++){
18 cheng *= i;
19 }
20 return cheng;
21 }

The above is an overview of js basic arrays, the length of arrays, and all the traversal of arrays. It briefly describes how to obtain the length and traverse of js arrays

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