How to declare arrays in JavaScript and a summary of the usage of some common array functions

How to declare an array

The declaration of an array in s can be declared in several ways

var tmp = [];  // 简写模式
var tmp = new Array(); // 直接new一个
var tmp = Array();  // 或者new也可以

When creating a new array, you can pass in a parameter to represent the array. Initialization length

// new的时候传入一个参数表示初始化数组长度
var tmp = new Array(3); 
 
alert(tmp.length);  // 3

But if you want to create an array with only one element 3, then using the new method cannot be achieved, because the system will treat the 3 you pass in as the length of the array, unless you use quotes Caused as a string, such as

var tmp = new Array('3'); 
alert(tmp);  // 3

We can use the abbreviation mode to create an array, so that we can create an array with only one numeric element 3

var tmp = [3]
alert(typeof tmp[0]);  // number

We can also initialize multiple elements, and the elements The value can be of any type

// 简约模式创建数组
// 数组的元素可以是任意一种数据类型
var tmp = [3,true,8.5,{'name':'lizhong'},['a','b']];
alert(tmp.length); // 5

1. unshift Insert an element before the first element of the array

// 使用unshift在数组第一个元素前插入元素
// 返回数组长度
var tmp = ['a','b'];
var len = tmp.unshift('c');
alert(len); // 3
alert(tmp); // c,a,b

You can also insert multiple elements at once, in order from the left Arrange

// 使用unshift在数组第一个元素前插入元素
// 返回数组长度
var tmp = ['a','b'];
var len = tmp.unshift('c','d');
alert(len); // 4
alert(tmp); // c,d,a,b

2. Shift pops the first element of the array and returns the value of the popped element

Small example:

// 使用shift弹出数组第一个元素
// 返回被弹出的元素值
var tmp = ['a','b','c'];
var val = tmp.shift();
alert(val); // a
alert(tmp); // b,c

If it is a Empty array:

// 使用shift弹出数组第一个元素
// 返回被弹出的元素值
var tmp = [];
var val = tmp.shift();
alert(val); // undefined
alert(tmp); // 空

3. push adds elements to the end of the array

Contrary to unshift, push adds elements to the end of the array and returns the length of the array after adding the elements

// 使用push在数组末尾添加多个元素
// 返回数组最新长度
var tmp = ['a','b','c'];
var len = tmp.push('d');
alert(len); // 4
alert(tmp); // a,b,c,d

You can also add multiple elements at once

// 使用push在数组末尾添加多个元素
// 返回数组最新长度
var tmp = ['a','b','c'];
var len = tmp.push('d','e','f');
alert(len); // 6
alert(tmp); // a,b,c,d,e,f

4. The pop function deletes the elements at the end of the array

Contrary to shift, pop pops up the elements at the end of the array , returns the value of the popped element

// 使用pop弹出数组末尾元素
// 返回被弹出的元素值
var tmp = ['a','b','c'];
var val = tmp.pop();
alert(val); // c
alert(tmp); // a,b

If the array is empty, returns undefined

// 使用pop弹出数组末尾元素
// 返回被弹出的元素值
var tmp = [];
var val = tmp.pop();
alert(val); // undefined
alert(tmp); // 空

Using the above four functions, we can do some queue processing, and we will not write code for specific cases.
The push function can actually be implemented like this

var tmp = ['a','b','c'];
tmp[tmp.length] = 'd';
alert(tmp); // a,b,c,d

Note: The above four functions unshift, shift, pop, and push function operations will modify the array itself.

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