Detailed explanation of sample code of javascript DOM

This article mainly introduces the detailed explanation of javascript DOM and relevant information about example codes. Friends in need can refer to

javascript DOM Summary

I have always thought that DOM (DocumentObjectModel) is the simplest part of JS. It is undeniable that it is indeed very simple, because the thinking mode of DOM is a bit fixed, and you only need to simply remember some fixed methods, so DOM can be said to be the starting point for all js (here refers to client js).

Recently, when I was doing some useful DOM exercises, I found that my DOM knowledge was very fragmented (I always thought I had a good grasp of it). Many friends may think that DOM is just a matter of calling Call a few methods, or I use jQuery directly, without having to consider the details of the DOM at all. Yes, that’s right. jQuery’s encapsulation of DOM is unprecedented, but just like growing up, you must be able to walk before you can run, so I think you must have a more detailed understanding of DOM, so I summarized the following about DOM Some usage methods.

In the W3C summary of DOM specifications, there are some that are very commonly used and some that are not very useful. Here we mainly discuss some commonly used DOM operations (related to DOM The basic concepts of will not be introduced here):

Node level

The so-called node level refers to the html document# There are nodes (such as tags) with their own characteristics, data, and methods in ##, and the relationship between nodes constitutes a hierarchy (in fact, it can be imagined as a tree structure). A NODE interface is defined in the W3C's DOM level 1 specification. This interface has some methods that are very useful:

　Node.ELEMENT_NODE;(Element node)

　Node.TEXT_NODE;(Text node)

　Node.DOCUMENT_NODE (document node)

Each node has its own node type flag, which is the NodeType attribute, for example, the nodeType of the element node == ' 1';The nodeType of the text node == '3';The nodeType of the document node == '9';

　1. Document node

 The document node is in a document It is represented by the document object, and its basic characteristics are as follows:

console.log(document.nodeType); // 9 
console.log(document.nodeName); // #document 
console.log(document.nodeValue); // null 
console.log(document.parentNode); // null(它已经是最顶层的节点，树的根节点)

Tip one (child node of the document):

1.Document.documentElement can get the html object, and it can also be obtained through document. Get childNodes[0] and document.firstchild. However, documentElement provides faster, more direct access to elements.

tip two (document related information):

1. Get the document title: document.title;

2. Get the complete url: document.URL;

3. Get the domain name (ip): document.domain;

4. Get the URL of the page: document.referrer;

tip three (document search element):

　1. By id: document.getElementById("xxx"); Generally, the page ids will be different. If there are multiple identical ids, the first element with that id will be fetched.

2. Through tagName: document.getElementsByTagName("xxx"); Return the collection of elements with the tag name "xxx"!

3. Through name: document.getElementByName();

It is very simple to understand the document object, and the compatibility is also relatively advanced.

　2. Element node

The element node provides access to the element tag name, sub-nodes and attributes. Its basic characteristics are as follows:

var ele = document.getElementById("element"); //通过document取到一个标签对象 
console.log(ele.nodeType); // 1 
console.log(ele.nodeName); // 返回元素的标签名 
console.log(ele.nodeValue); //永远返回null!

tip one (html element):

　<p id="myp" class="bd" title="Body text" lang="en" dir="ltr"></p>
　　　　var p = document.getElementById("p");
　　　　1. console.log(p.id); // "myp"
　　　　2. console.log(p.className); // "bd"
　　　　3. console.log(p.title); // "Body text"
　　　　4. console.log(p.lang); // "en"
　　　　5. console.log(p.dir); // "ltr"

tip two (get, set and delete attributes):

　1.p.getAttribute("id"); // "myp"
　　　　2.p.setAttribuye("id","yourp"); // id已变动
　　　　3.p.removeAttribute("id"); //id已删除

It should be noted: In IE7 and below versions, there are some abnormal behaviors in the three methods. Setting the class and style attributes through set, especially when

event processing program, has no effect, and the same is true for get. . Therefore, in general development, the above three methods should be avoided, and it is recommended to set characteristics through attributes.

tip three (child node of the element):

What I want to focus on is here, the following code:

<ul id="myList"> 
  <li>Item 1</li> 
  <li>Item 2</li> 
  <li>Item 3</li> 
</ul> 
 
var mL = document.getElementById("myList"); 
//很明显mL对象有三个元素节点，我们也知道用childNodes属性去找到节点数，然而陷阱随之而来 
 
console.log(mL.childNodes); // 7 
//?!怎么会有7个? 
 
//原因在于childNodes中不仅包含了元素节点，还有4个文本节点。因此需要过滤 
 
for(var i=0,len=ml.childNodes.length;i<len;i++){ 
   if(ml.childNodes[i].nodeType == "1"){ // 利用元素节点的特性 
     // .... 
   }   
}<br>//最好的方法可以这么做<br>//如果元素对象内部标签名是一样的<br>var items = ml.getElementsByTagName("li"); //能得到三个li节点对象 
　　

3. Text node

Text nodes contain plain text content that can be interpreted literally. Plain text can contain escaped HTML characters, but cannot contain HTML codes. The basic characteristics of text nodes are as follows:

<p id="myp">123</p> 
 
var myp = document.getElementById("myp") //取到元素节点 
var tx = myp.childNodes[0] //前面也提过childNodes的特性，这次取到了文本节点 
 
console.log(tx.nodeType) // 3 
console.log(tx.nodeName) // 所有文本节点的nodeName都是"#text"; 
console.log(tx.nodeValue) // 123(节点包含的文本)，注意元素节点是不能取到它包含的文本节点的文本的 
 
//所以其父节点显然是个元素节点.

Tip one:

Two methods to create text nodes: document.createTextNode(),document.createText();

Create Once completed, it will not be directly embedded in the document and will need to be quoted.

var a = document.createElement("p");

　　　　var b = document.createTextNode("123");

　　　　a.appendChild(b);

　　　　document.body.appendChild(a);

In this way, a tag like

123

will appear at the end of the body

Personally, I think DOM is definitely the entry point for learning js, but it also takes a long time to polish. I have read the DOM no less than three times, it is only the DOM level 1 specification, and there is something new every time. If you learn DOM, you must pay attention to some pitfalls and spend more time thinking about them.

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