How to express parent-child relationship in Javascript

Javascript, as a scripting language, greatly simplifies our front-end development process. Among them, expressing the parent-child relationship is a very important concept. Let's briefly introduce how to express the parent-child relationship in Javascript.

1. DOM tree structure

Let’s first talk about the most widely used DOM tree structure in Javascript. The HTML DOM tree is a hierarchical structure in which each HTML element becomes part of a parent, child, or sibling relationship, and all nodes originate from a root. The nodes of the tree structure have the following concepts:

1. Element node: represents the HTML element
2. Text node: represents the text content of the element
3. Attribute node: represents the attribute of the element
4. Document node: represents the entire HTML document
5. Comment node: represents the comment content in HTML

In the DOM tree, the element node is the closest point between the parent and child nodes. Common relationships. For example:

<div>
  <ul>
    <li>item 1</li>
    <li>item 2</li>
  </ul>
</div>

We can abstract a piece of HTML content into the above DOM structure, where the div tag is the parent element of the ul tag. The li tag is a child element of the ul tag and a grandson element of the div tag. And how is this father-son relationship expressed in Javascript?

2. Node Object

In Javascript, we use Node objects to represent these DOM nodes. The Node object is a base class for all DOM nodes, and all DOM nodes inherit the properties and methods of Node. The Node object has the following common attributes:

1. parentNode: indicates the parent node, and returns NULL if there is none.
2. childNodes: An array representing child nodes.
3. previousSibling: Represents the previous sibling node.
4. nextSibling: Indicates the next sibling node.

Taking the above HTML code as an example, we can obtain the relationship between parent and child nodes in the following ways:

const div = document.getElementById('parent');
const ul = div.firstElementChild;
const li1 = ul.firstElementChild;
const li2 = li1.nextElementSibling;

console.log(ul.parentNode === div); // true
console.log(li1.parentNode === ul); // true
console.log(li2.previousSibling === li1); // true
console.log(li1.nextSibling === li2); // true

In the above code, we used getElementByIdMethod to get div, and use the firstElementChild method to get its first child element ul. Next, we use the firstElementChild method to get the first child element li1 of ul, and the nextElementSibling method to get li1 The last sibling node of is li2. Finally, we use the properties of the Node objects to detect the parent-child relationship between them.

3. Parent-child operations in jQuery

In addition to using Node objects to represent the relationship between parent-child nodes, jQuery also provides convenient DOM operation functions to achieve the same function. Here are some simple examples:

1. parent(): This method returns the parent element of the matching element. You can pass in a selector to filter the results.

$('li').parent() // 返回行成父子关系的ul节点

2. children(): This method returns a collection of all child elements of the matching element collection. You can also pass in a selector to filter the results.

$('div').children() // 返回div下的所有子元素

3. siblings(): This method returns all sibling elements of the matching element. You can also pass in a selector to filter the results.

$('li').siblings() // 返回li的兄弟元素（不包括它自己）

In the above simple example, jQuery provides a convenient method to help us find and operate the parent-child relationship of DOM nodes, simplifying the process of operating DOM nodes with Javascript.

Conclusion

Node objects are used in Javascript to represent the parent-child relationship of DOM nodes. When using Node objects, we can directly access the properties of the node, such as parentNode, previousSibling, etc. to obtain the relationship between parent and child nodes. In addition, the jQuery library also provides simpler and more convenient functions that allow us to operate and find the parent-child relationship between DOM nodes. By cleverly using these functions, we can greatly quickly complete the front-end development work and improve the efficiency of front-end development.

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