How to insert elements at the head or tail of JS array learning?

In the previous article "JS Array Learning - Creation and Access of One-Dimensional and Two-Dimensional Arrays", we introduced various methods of creating arrays. Now that we have an array, we need to operate the array, and the most common operation is to add or delete array elements. This time we will first introduce to you several ways to insert array elements, and then we will introduce to you how to delete array elements.

There are many ways to add array elements: you can add them at the beginning or end of the array, or you can add them at any position based on the array subscript. Today we will first introduce to you how to insert elements at the beginning or end of an array.

Insert elements at the beginning of the array

In JavaScript, we can use the unshift() function to insert elements at the beginning of the array. This function can Append one or more parameter values ​​to the head of the array:

array.unshift(元素1, 元素2, ..., 元素X)

The first parameter Element 1 is the new element 0 of the array, and the second parameter Element 2 is the new element 1, and so on, and finally returns the length of the array after adding the element.

Let’s take a closer look at the following example:

var a = [0];  //定义数组
console.log(a);  //返回[0]

a.unshift(1,2);  //一次性增加两个元素
console.log(a);  //返回[1,2,0]

var a = [0];
a.unshift(1);  //增加元素1
a.unshift(2);  //增加元素2
console.log(a);  //返回[2,1,0]

It can be seen that we use a.unshift(1,2) to press at the beginning of the a array Insert two values ​​​​1 and 2 sequentially; use a.unshift(1) to insert the value 1 at the beginning of the a array, and then use a.unshift(2) at the beginning (that is, before the value 1 ) insert the value 2. So the output is:

Insert an element at the end of the array

In JavaScript, we can use push() or concat() method to insert elements at the end of the array. Let’s introduce them one by one:

1. Use the push() method

The push() method can append one or more parameter values ​​to the end of the array, and Returns the length of the array after adding elements.

array.push(元素1, 元素2, ..., 元素X)

Let’s take a closer look at the following example:

var a = [0];  //定义数组
console.log(a);  //返回[0]

a.push(1,2);  //一次性增加两个元素
console.log(a);  //返回[0,1,2]

var a = [0];
a.push(1);  //增加元素1
a.push(2);  //增加元素2
console.log(a);  //返回0,1,2]

It can be seen that we use a.push(1,2) to press at the end of the a array Insert two values ​​​​1 and 2 sequentially; use a.push(1) to insert the value 1 at the end of the a array, and then use a.push(2) on this basis. Insert a value 2 at the end (that is, after the value 1). Therefore, the output result is:

2. Use the concat() method

1). The concat() method can connect two or multiple arrays, will add the elements of one or more arrays as parameters to the end of the specified array.

var a = [1,2,3];
var b = [4, 5, 6];
var c = [7,8];
var d = a.concat(b,c);
console.log(d);

concat() method will break up the one-dimensional array as a parameter and connect them to the end of the array as separate elements. Therefore, the output result is:

But the concat() method cannot completely break up the two-dimensional array. It can only connect the one-dimensional elements contained in the two-dimensional array to the end of the array.

var a = [1,2,3,4,5];  
var b = [[1,2,3], [4,5]];
var c = [[1,2,3], 3];
var d = a.concat(b,c);
console.log(d);

2) The concat() method can also insert one or more given elements, and can add all the parameters passed to the end of the array in order.

var a = [1,2,3,4,5];  //定义数组
var b = a.concat(6,7,8);  //为数组a连接3个元素
console.log(b);  //返回[1,2,3,4,5,6,7,8]

The output result is:

Description: The concat() method will create and return a new array instead of adding new elements based on the original one ;But the unshift() method will add elements to the original array.

Okay, that’s all. If you need it, you can read: javascript advanced tutorial

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