import InternalLink from "../components/InternalLink"
import MDN from "../components/MDNBadge"
### 1. tagName
`Element.tagName: string`
This read-only property on any element returns the **tag name** in uppercase form. This is useful to identify type of an element.
Ex. `.tagName` of `<div/>` element is `"DIV"`.
```js
<img id="someImage" src="test.jpg" />
someImage.tagName // "IMG"
```
:::note Text nodes don't have `tagName`
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`.
:::
<span>
<MDN title="tagName" i url="https://developer.mozilla.org/en-US/docs/Web/API/Element/tagName" />
<MDN title="nodeType" i url="https://developer.mozilla.org/en-US/docs/Web/API/Node/nodeType" />
</span>
### 2. parentNode
`Node.parentNode: Element | Document | DocumentFragment | null`
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.
The topmost parentNode in DOM is `document`. This can be used to check if an element is detached from the DOM.
```js
<div id="parent">
<div id="child"/>
</div>
child.parentNode // <div id="parent>...</div>
```
<Accordion title="Related">
<Admonition type="note" title="Node.parentElement : Element | null">
This property returns ** the parent element ** of the current node. If there is no parent, it returns null.
The topmost parentElement in DOM is the `<html/>` element, which is also available as `document.documentElement`.
```js
let newEle = document.createElement("div");
newEle.parentNode // null
newEle.parentElement // null
// In case of appending to an element
document.body.append(newEle)
newEle.parentNode // <body>...</body>
newEle.parentElement // <body>...</body>
//In case of appending to a document fragment, say, df
All elements will *generally* have 2 types of children - an element or a text node.
You can think of elements as other tags and text node as the text within it.
```jsx
<div>
This is // <- 1. text node
<span> // <- 2. element
some text // 2.1. text node
</span>
</div>
```
If you recall, *Element* extends from *Node* - so element and text node are both actually Node.
This is important because some properties return **only child elements**, whereas some return **all child nodes** (including text nodes).
For ex, `ele.childNodes` returns all child nodes, but `ele.children` returns only child elements.
**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.
```jsx
<div id="container">
<div></div>
</div>
container.childNodes // [text, div, text]
// extra text nodes contain newline and space
```
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).
:::
### 3. children
`Element.children: HTMLCollection`
This property returns a live `HTMLCollection` containing all the **child elements** of the node. If there are no child elements, it returns empty HTMLCollection.
`HTMLCollection` can be converted to an array using `Array.from()` or `[...ele.children]`. HTMLCollection also has a `.length` property.
Here, the HTMLCollection is **live** - meaning if new child elements are added, they will automatically appear in this collection.
```jsx
<div id="parent">
Some text
<span>some element</span>
</div>
const children = parent.children;
children // HTMLCollection[span]
// 1. For-loop using .length
for (let i = 0; i < children.length; i++) {
console.log(children[i].tagName);
}
// 2. Convert to array
[...children].forEach(ele => {
// do something
})
```
This property is also available on `Document` and `DocumentFragment`.
:::note Counting number of child elements
`Element.childElementCount: number`
It returns the **number of child elements** of the given element. This is equivalent to `ele.children.length` value.
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.
`NodeList` can be converted to a array using `Array.from()` or `[...node.childNodes]`. NodeList also has a `.length` property.
Here, the NodeList is **live** - so the content of NodeList will auto-update.
```jsx
<div id="parent">
Some text
<span>some element</span>
and more text
</div>
const {childNodes} = parent;
childNodes // NodeList[text, span, text]
[...childNodes].forEach(node => {
// do something
})
```
This property is available on all types of `Node` - like Element, text node, etc.
:::note Check if there is any child node
`Node.hasChildNodes() : boolean`
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`.
This property returns the element's **first child** node in the tree. If there is no children, it returns `null`.
`Element.nextSibling: Node`
This property returns the node immediately **after the current one** in their parent's childNodes.
```jsx
<div id="A">
<div id="B"/>
<div id="C"/>
</div>
let eleA = document.getElementById("A")
let eleB = eleA.firstChild // <div id="B">
let eleC = eleB.nextSibling // <div id="C"/>
eleC.nextSibling // null
```
There is also a similar `previousSibling` property which returns the previous node. Also, there are element-only variants like `firstElementChild`, `nextElementSibling`, etc.
<Accordion title="Why is this useful?">
:::note Easier traversal
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.
#### How do you implement dom traversal ?
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*.
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`.
```js
// 1. Typical approach
let queue = [root]
while (queue.length > 0) {
const item = queue.shift();
console.log(item);
if (item.hasChildNodes()) {
queue = queue.concat([...item.childNodes]);
}
}
// 2. LCRS approach
function traverse(item) {
if (item === null) {
return;
}
console.log(item);
traverse(item.firstChild)
traverse(item.nextSibling)
}
traverse(root.firstChild)
```
:::
</Accordion>
<span>
<MDN i title="firstChild" url="https://developer.mozilla.org/en-US/docs/Web/API/Node/firstChild" />
<MDN i title="nextSibling" url="https://developer.mozilla.org/en-US/docs/Web/API/Node/nextSibling" />
</span>
## Content
### 6. innerText
`Element.innerText: string [= value]`
Returns the **text content** of the element and its descendants **"as rendered"**.
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.
**Example 1 -**
`.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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Lakshya Thakur