This article brings you relevant knowledge about javascript, which mainly introduces DOM and BOM and related issues about their differences and usage. Let’s take a look at them together. I hope it will be helpful to everyone. help.
[Related recommendations: javascript video tutorial]
I. Brief description: What is DOM and what is BOM?
At the beginning of the article, I would like to mention Generally speaking, what is DOM and what is BOM, because this article is ultimately intended for I am a friend who has a certain JavaScript foundation, but does not understand DOM and BOM, or even doesn't know it.
However, before talking about what is DOM and what is BOM, please allow me to show you the entire structure of Javascript:
In the picture above, we can see that there are four elements: JavaScript, ECMAScript, DOM and BOM. So what is the connection between the four of them? Use an equation to summarize the relationship between them:
JavaScript = ECMAscript BOM DOM
Let’s
compare them one by one To give an overview:
ECMAscript:
ECMAScript is a language adopted by ECMA International (formerly the European Computer Manufacturers Association) ECMA-262 standardized scripting programming language,It is the standard for JavaScript (JS for short), and the browser is to implement this standard.
ECMAscript is more like a regulation that stipulates how each browser executes the syntax of JavaScript, because we know that JavaScript is a scripting language that runs on the browser! There are regulations, but we still lack a way to interact with each element on the page. At this time, the following DOM was born!
DOM:
DOM (Document Object Model, Document Object Model) is a language independent, Application programming interface used to operate xml, html documents.
For JavaScript: In order to enable JavaScript to operate Html, JavaScript has its own DOM programming interface.
One sentence summary:
DOM provides a "method" for JavaScript to access and operate HTML elements(The reason why we don’t use the word interface is because we are afraid that some friends will see the interface It’s a scary situation, but in fact the most accurate description of is that it provides an interface for JavaScript)
##BOM:
BOM is
Browser Object Model, browser object model. BOM is an interface that appears to control the behavior of the browser. For JavaScript: In order to
allow JavaScript to control the behavior of the browser, JavaScript has its own BOM interface.
One sentence summary:
BOM provides JavaScript with a "method" to control browser behavior.
Finally, among the above three components of JavaScript,
ECMscript is a specification
, while the other two provide "methods", respectively Corresponding to HTML elements and browsers, so below we will give a systematic explanation for the latter two: DOM and BOM. Since it is for beginners, my following explanation will be as simple as possible It’s easy to understand, even if you don’t have a basic knowledge, you can eat it with confidence!
II. DOM and its related operations
First of all, let’s
explain the relevant knowledge of DOM
, I divided it into two parts ：
II.I DOM tree
Okay,
What is a DOM tree?
Maybe some beginners who have learned DOM will be a little unfamiliar with this word, but in fact, the DOM tree is not a particularly fantasy thing. On the contrary, for front-end staff,
DOM tree is the tree composed of HTML elements of the pages you deal with every day:
BOM tree
, Each node can have two identities: it can be a child node of the parent node, or it can be the parent node of other child nodes. Let's observe the following code:<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>DOM_demo</title>
</head>
<body>
<p>
<a href="https://blog.csdn.net/qq_52736131">跳探戈的小龙虾</a>
</p>
</body>
</html>
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DOM tree
should look like this: 
这时候有人要问了，你说这么半天的DOM树，跟操作html元素有关吗？
答案是非常有关，而且只有了解了文档的DOM树结构，才能准确而有效的操作DOM元素，否则会出现各种意料之外的bug。在我们操作一个页面的HTML元素之前，要对整个页面的DOM有一个清晰的绘图，即使在不实实在在绘图，也要在脑中有一个清晰的脉络结构。
II.II DOM的一些常见的操作元素方法
关于JavaScript中DOM的一些常见的操作html元素的方法，我又双叒叕分成几个子部分给大家归类介绍：
获取节点的DOM方法
//1.通过元素的id属性值来获取元素，返回的是一个元素对象
var element = document.getElementById(id_content)
//2.通过元素的name属性值来获取元素，返回的是一个元素对象的数组
var element_list = document.getElementsByName(name_content)
//3.通过元素的class属性值来获取元素，返回的是一个元素对象的数组
var element_list = document.getElementsByClassName(class_content)
//4.通过标签名获取元素，返回的是一个元素对象数组
var element_list = document.getElementsByTagName(tagName)
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获取/设置元素的属性值的DOM方法
//1.获取元素的属性值，传参自然地是属性名，例如class、id、href
var attr = element.getAttribute(attribute_name)
//2.设置元素的属性值，传参自然地是元素的属性名及要设置的对应的属性值
element.setAttribute(attribute_name,attribute_value)
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创建节点(Node)的DOM方法
//1.创建一个html的元素，传参是元素类型，例如p、h1-5、a，下以p为例
var element = document.createElement("p")
//2.创建一个文本节点，传参的是对应的文本内容(注意是文本节点，不是某个html元素)
var text_node = document.createTextNode(text)
//3.创建一个属性节点，传参是对应的属性名
var attr_node = document.createAttribute(attribute_name)
element.setAttributeNode(attr_node)Copy after login
特别注意第三个，创建属性节点这个方法，要搭配具体的元素，也就是你要先获取某个具体元素element，创建一个属性节点，最后对这个元素添加这个属性节点(setAttributeNode)。
增添节点的DOM方法
//1.向element内部的最后面添加一个节点，传入的参数是节点类型
element.appendChild(Node)
//2.向element内部某个已存在的节点的前面插入一个节点，仍然传入一个节点类型的参数
element.insertBefore(new_Node,existed_Node)
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注意，添加节点之前，你要先创建好节点，同时要选好父节点element，第二个方法甚至你还要找好插入位置后面的兄弟节点。
删除节点的DOM方法
//删除element内的某个节点，传参是节点类型参数
element.removeChild(Node)
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注意，删除时，要找到对应的父节点element才可以顺利删除。
DOM常见的一些属性
最后是一些常见的DOM属性：
//1.获取当前元素的父节点
var element_father = element.parentNode
//2.获取当前元素的html元素型子节点
var element_son = element.children
//3.获取当前元素的所有类型子节点，包括html元素、文本和属性
var element_son = element.childNodes
//4.获取当前元素的第一个子节点
var element_first = element.firstChild
//5.获取当前元素的前一个同级元素
var element_pre = element.previousSibling
//6.获取当前元素的后一个同级元素
var element_next = element.nextSibling
//7.获取当前元素的所有文本，包括html源码和文本
var element_innerHTML = element.innerHTML
//8.获取当前元素的所有文本，不包含html源码
var element_innerTEXT = element.innerText
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其中，第七个的意思是说把元素内的html代码和文本都转成文本，原先的html代码是执行的，转成文本相当于变成了普通的字符串！
III. BOM及其相关操作
III.I BOM总述
下面我们再讲一讲BOM，由于篇幅有限，BOM不做特别细致的讲解(实用性的确也没有DOM那么大)。我们回顾一下开始的时候关于BOM的介绍：
BOM给JavaScript提供用来操作浏览器的若干的"方法"
那么首先我们用一张图把整个BOM的结构给大家梳理一下，与DOM类似，BOM也有一个树状结构：
III.II BOM常见对象的介绍
window对象
从上面这张图上，我们可以看到：
window是整个BOM树食物链的顶端，因此每一个新打开的窗口，都被认为是一个window对象。
window对象有以下常见的属性和方法：
| Properties/Methods |
Meaning |
| opener | The parent window of the current window |
| length |
The number of frames in the window |
| document |
The document object currently displayed in the window |
| alert(string) |
Create a warning dialog box and display a message |
| close() |
Close the window |
| confirm() |
Create a dialog box that requires user confirmation |
| open(url,name,[options]) |
Open a new window and return the new window object |
| prompt(text,defaultInput) |
Create a dialog box to ask the user to enter information |
| setInterval(expression, milliseconds) |
Calculate an expression after a specified time interval |
| setInterval(function,millis enconds,[arguments]) |
Call a function after the specified time interval |
| setTimeout(expression,milli seconds) |
Calculate an expression after the timer expires |
| setTimeout(expression,milli seconds,[arguments]) |
Calculate an expression after the timer expires Function |
Among them, you can see that there is a function alert() on it, because when you learn JavaScript, the input and output streamsEveryoneMost of them use the alert() function pop-up window as their first demo, so when you see this you may ask:
At that timeUsed alert() When it comes to functions, it seems that window is not mentioned, so is the alert() here the same alert() that I learned before? The answer is this:
These two alert() are indeed the same function . The reason why window can be omitted is because all the properties and methods of window , can have two representation methods:
(1) window.property/window.method()
(2) direct property / Calling method ()
is not just alert(), all the above window properties and functions are established, and interested friends can try it by themselves.
location object
What is the location object?
The location object is a property of the window object, which provides information about the document loaded in the current window and also provides some navigation functions.
The location object has the following common properties and methods:
| Properties/Methods
|
Content
|
##host
| Host name: port number |
|
hostname
| Hostname |
|
href
| Entire URL |
| ##pathname
Pathname |
| port
Port number |
| protocol
Protocol part |
| search
Query string |
| reload()
Reload the current URL |
| repalce()
Replace the current page with the new URL |
|
In fact, if we carefully observe the structure diagram above, we will find:
The location object is not only an attribute of the window object, but also an attribute of the document object.
This means:
##window.location = location = document.location
history object
What is the history object?
The history object is an attribute of the window object. It saves the record of the user's Internet access. The timestamp of this record is calculated from the moment the window was opened.
The history object has the following common properties and methods:
| Properties/Methods
|
Description
|
length | The number of records in the history object |
back() | Go to the URL of the previous browser history entry, similar to back |
forward() | Go to the next URL of browser history entry, similar to forward |
go(num) | The browser moves forward or backward in the history object |
navigator object
Finally introduce the navigator object:
The navigator object is the one in the BOM that identifies the client browser. window attribute.
Some common properties related to navigator:
| Properties
|
Description
|
appName | Complete browser name and version information |
platform | The system platform where the browser is located |
plugins | Array of plug-in information installed in the browser |
userAgent | The browser’s user agent string |
userLanguage | The operating system’s default language |
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