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411 lines
11 KiB
Plaintext
411 lines
11 KiB
Plaintext
---
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id: element-child-content
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title: Child and Content
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---
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import Admonition from "../components/Admonition"
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import Accordion from "../components/Accordion"
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import InternalLink from "../components/InternalLink"
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import MDN from "../components/MDNBadge"
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### 1. tagName
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`Element.tagName: string`
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This read-only property on any element returns the **tag name** in uppercase form. This is useful to identify type of an element.
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Ex. `.tagName` of `<div/>` element is `"DIV"`.
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```js
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<img id="someImage" src="test.jpg" />
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someImage.tagName // "IMG"
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```
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:::note Text nodes don't have `tagName`
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Text nodes are not elements, so they don't have any tagName. To identify the type of node, use `.nodeType` property. For text nodes, `nodeType` is `3` - same as the constant `Node.TEXT_NODE`.
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:::
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<span>
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<MDN title="tagName" i url="https://developer.mozilla.org/en-US/docs/Web/API/Element/tagName" />
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<MDN title="nodeType" i url="https://developer.mozilla.org/en-US/docs/Web/API/Node/nodeType" />
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</span>
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### 2. parentNode
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`Node.parentNode: Element | Document | DocumentFragment | null`
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This property returns **the parent node** of the current node. If there is no parent, it returns null. In case of Document and DocumentFragment, `.parentNode` is always null.
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The topmost parentNode in DOM is `document`. This can be used to check if an element is detached from the DOM.
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```js
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<div id="parent">
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<div id="child"/>
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</div>
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child.parentNode // <div id="parent>...</div>
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```
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<Accordion title="Related">
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<Admonition type="note" title="Node.parentElement : Element | null">
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This property returns ** the parent element ** of the current node. If there is no parent, it returns null.
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The topmost parentElement in DOM is the `<html/>` element, which is also available as `document.documentElement`.
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```js
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let newEle = document.createElement("div");
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newEle.parentNode // null
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newEle.parentElement // null
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// In case of appending to an element
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document.body.append(newEle)
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newEle.parentNode // <body>...</body>
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newEle.parentElement // <body>...</body>
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//In case of appending to a document fragment, say, df
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df.append(newEle)
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newEle.parentNode // document-fragment
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newEle.parentElement // null
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```
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</Admonition>
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</Accordion>
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<span><MDN title="parentNode" url="https://developer.mozilla.org/en-US/docs/Web/API/Node/parentNode" /></span>
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## Children
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:::note Types of Children
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All elements will *generally* have 2 types of children - an element or a text node.
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You can think of elements as other tags and text node as the text within it.
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```jsx
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<div>
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This is // <- 1. text node
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<span> // <- 2. element
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some text // 2.1. text node
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</span>
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</div>
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```
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If you recall, *Element* extends from *Node* - so element and text node are both actually Node.
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This is important because some properties return **only child elements**, whereas some return **all child nodes** (including text nodes).
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For ex, `ele.childNodes` returns all child nodes, but `ele.children` returns only child elements.
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**Always use the one which is more suitable for your case**. Remember, even if you haven't added any "text" between a tag, it can still have **invisible text nodes** due to spaces and newline characters.
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```jsx
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<div id="container">
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<div></div>
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</div>
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container.childNodes // [text, div, text]
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// extra text nodes contain newline and space
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```
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Read more about [other types of nodes](https://developer.mozilla.org/en-US/docs/Web/API/Node/nodeType) and how [whitespace works in html](https://developer.mozilla.org/en-US/docs/Web/API/Document_Object_Model/Whitespace).
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:::
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### 3. children
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`Element.children: HTMLCollection`
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This property returns a live `HTMLCollection` containing all the **child elements** of the node. If there are no child elements, it returns empty HTMLCollection.
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`HTMLCollection` can be converted to an array using `Array.from()` or `[...ele.children]`. HTMLCollection also has a `.length` property.
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Here, the HTMLCollection is **live** - meaning if new child elements are added, they will automatically appear in this collection.
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```jsx
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<div id="parent">
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Some text
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<span>some element</span>
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</div>
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const children = parent.children;
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children // HTMLCollection [span]
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// 1. For-loop using .length
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for (let i = 0; i < children.length; i++) {
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console.log(children[i].tagName);
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}
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// 2. Convert to array
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[...children].forEach(ele => {
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// do something
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})
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```
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This property is also available on `Document` and `DocumentFragment`.
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:::note Counting number of child elements
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`Element.childElementCount: number`
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It returns the **number of child elements** of the given element. This is equivalent to `ele.children.length` value.
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:::
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<MDN title="children" url="https://developer.mozilla.org/en-US/docs/Web/API/ParentNode/children" />
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### 4. childNodes
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`Node.childNodes: NodeList`
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This property returns a live `NodeList` containing all the **child nodes** of the current element. If there are no child nodes, it returns a empty NodeList.
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`NodeList` can be converted to a array using `Array.from()` or `[...node.childNodes]`. NodeList also has a `.length` property.
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Here, the NodeList is **live** - so the content of NodeList will auto-update.
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```jsx
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<div id="parent">
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Some text
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<span>some element</span>
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and more text
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</div>
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const {childNodes} = parent;
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childNodes // NodeList [text, span, text]
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[...childNodes].forEach(node => {
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// do something
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})
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```
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This property is available on all types of `Node` - like Element, text node, etc.
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:::note Check if there is any child node
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`Node.hasChildNodes() : boolean`
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This method checks whether the given node **has child nodes or not** and returns a boolean value. This is equivalent to checking `node.childNodes.length > 0`.
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:::
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<MDN title="childNodes" url="https://developer.mozilla.org/en-US/docs/Web/API/Node/childNodes" />
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### 5. firstChild + nextSibling
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`Element.firstChild: Node`
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This property returns the element's **first child** node in the tree. If there is no children, it returns `null`.
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`Element.nextSibling: Node`
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This property returns the node immediately **after the current one** in their parent's childNodes.
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```jsx
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<div id="A">
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<div id="B"/>
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<div id="C"/>
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</div>
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let eleA = document.getElementById("A")
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let eleB = eleA.firstChild // <div id="B">
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let eleC = eleB.nextSibling // <div id="C"/>
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eleC.nextSibling // null
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```
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There is also a similar `previousSibling` property which returns the previous node. Also, there are element-only variants like `firstElementChild`, `nextElementSibling`, etc.
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<Accordion title="Why is this useful?">
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:::note Easier traversal
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This allows accessing the DOM tree in [Left-child right-sibling](https://en.wikipedia.org/wiki/Left-child_right-sibling_binary_tree) (LCRS) form, which is more memory efficient for usual traversal.
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#### How do you implement dom traversal ?
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The typical approach is to use a queue, push all children to the queue, pop them one-by-one and visit them. This requires **extra memory** because of the queue data structure which holds all the *next items to visit*.
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But with the LCRS form, you always have access to the next item **without using extra space**. If current element has child, visit `.firstChild`. Then visit next node using `.nextSibling`.
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```js
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// 1. Typical approach
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let queue = [root]
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while (queue.length > 0) {
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const item = queue.shift();
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console.log(item);
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if (item.hasChildNodes()) {
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queue.push(Array.from(item.childNodes));
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}
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}
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// 2. LCRS approach
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function traverse(item) {
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if (item === null) {
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return;
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}
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console.log(item);
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traverse(item.firstChild)
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traverse(item.nextSibling)
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}
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traverse(root.firstChild)
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```
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:::
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</Accordion>
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<span>
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<MDN i title="firstChild" url="https://developer.mozilla.org/en-US/docs/Web/API/Node/firstChild" />
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<MDN i title="nextSibling" url="https://developer.mozilla.org/en-US/docs/Web/API/Node/nextSibling" />
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</span>
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## Content
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### 6. innerText
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`Element.innerText: string [= value]`
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Returns the **text content** of the element and its descendants **"as rendered"**.
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Can be set, to **replace the element's children** with the given string. It creates a *text node* for every normal text range, but replaces each line break with a `<br/>` element.
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**Example 1 -**
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`.innerText` tries to return a string which represents the actual visible text, not the text literally written in html. Notice how <br/> tags and the css affects the output -
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```jsx
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<span id="text">
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Some text <br>then a newline<br>and another.
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</span>
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<style>#text{ text-transform: uppercase; }</style>
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console.log(text.innerText)
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// SOME TEXT
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// THEN A NEWLINE
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// AND ANOTHER.
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```
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:::note
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On the other hand, `node.textContent` concatenates the actual text nodes used in the html and returns that string.
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```jsx
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console.log(text.textContent)
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// Some text then a newline and another.
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```
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:::
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**Example 2 -**
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As setter, it replaces `\n` and `\r\n` in the input string with <br/> tags and wraps the rest in text nodes.
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```jsx
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<span id="text"></span>
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text.innerText = "new text \n and a newline"
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text.childNodes
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// NodeList [
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// text "new text ", br, text " and a newline."
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// ]
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// result html -
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// <span id="text">new text <br/> and a newline</span>
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```
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<span>
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<MDN title="innerText" i url="https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/innerText" />
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<MDN title="textContent" i url="https://developer.mozilla.org/en-US/docs/Web/API/Node/textContent" />
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</span>
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### 7. innerHTML
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`Element.innerHTML: string [= value]`
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Returns the **html markup contained within the element** as a string.
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Can be set, to replace the **contents of the element with nodes parsed** from the given string. The existing content will be removed.
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**Example 1-**
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```jsx
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<div id="parent">
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<div id="c1">text1</div>
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<div id="c2">text2</div>
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</div>
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console.log(parent.innerHTML)
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// <div id="c1">text1</div>
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// <div id="c2">text2</div>
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```
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**Example 2-**
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```jsx
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<div id="parent"/>
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parent.innerHTML = `
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<div id="c1">text1</div>
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<div id="c2">text2</div>
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`
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// result html-
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<div id="parent">
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<div id="c1">text1</div>
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<div id="c2">text2</div>
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</div>
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```
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:::note To remove all children
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To clear the content of any element, you can use `ele.innerHTML = ""`.
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:::
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<MDN title="innerHTML" url="https://developer.mozilla.org/en-US/docs/Web/API/Element/innerHTML" />
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### 8. outerHTML
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`Element.outerHTML: string [= value]`
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Returns the html markup of the **element and its contents** as a string.
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Can be set, to **replace the element** itself with nodes parsed from the given string.
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**Example 1-**
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```jsx
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<div id="parent">
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<div id="c1">text1</div>
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</div>
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console.log(parent.outerHTML)
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// <div id="parent">
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// <div id="c1">text1</div>
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// </div>
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```
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**Example 2-**
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Replacing `parent2` with 2 new nodes -
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```jsx
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<div id="parent1">
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<div id="parent2">parent 2</div>
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<div id="parent3">parent 3</div>
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</div>
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parent2.outerHTML = `
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<div>child 1</div>
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<div>child 2</div>
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`
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// result html-
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<div id="parent1">
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<div>child 1</div>
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<div>child 2</div>
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<div id="parent3">parent 3</div>
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</div>
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```
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<MDN title="outerHTML" url="https://developer.mozilla.org/en-US/docs/Web/API/Element/outerHTML" />
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