UiQuestions/docs/element-properties.mdx

---
id: element-properties
title: Element attributes and properties
---

import Admonition from "../components/Admonition"
import Accordion from "../components/Accordion"
import InternalLink from "../components/InternalLink"
import MDN from "../components/MDNBadge"

### 1. tagName

`Element.tagName : string`

`.tagName` is a read-only property on any element, which returns the tag name in uppercase form. This is useful to identify type of a element.

Ex. For a `<div/>` element, tagName is `"DIV"`. 

```js
ele = document.querySelector("img");
ele.tagName // "IMG"
```

:::note Text nodes don't have `tagName`

Text nodes are not element, 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" />&nbsp;

  <MDN title="nodeType" i url="https://developer.mozilla.org/en-US/docs/Web/API/Node/nodeType" />&nbsp;
</span>
 

## Children

:::note Types of Children

All elements will *generally* have 2 types of children - 1. element and 2. text node.
You can think of elements as other tags and text node as the text within a tag. 

```jsx
<div>
  This is  // <- 1. text node
  <span>   // <- 2. element
    some text // 2.1. <span> contains text node
  </span>
</div>
```

If you recall, *Element* extends from *Node* - so all elements are nodes. That means, every child is technically a node.  
This is important because some properties return **only child elements**, whereas others return **all child nodes** (both element and text node). 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).
:::

### 2. children

`Element.children: HTMLCollection`

This read-only property returns a live `HTMLCollection` containing all the **child elements** of the node. If there are no child elements, it returns empty HTMLCollection.

This property is available on `Document`, `Element`, and `DocumentFragment`, but not on text node.

`HTMLCollection` can be consumed easily by converting it to an array, using `Array.from()` or `[...ele.children]`. HTMLCollection is also a iterable and 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
})

```

<MDN title="children" url="https://developer.mozilla.org/en-US/docs/Web/API/ParentNode/children" />

### 3. childNodes

`Element.childNodes: NodeList`

This read-only property returns a live `NodeList` containing all the **child nodes** of the current element, including text nodes. If there are no child nodes, it returns a empty NodeList.

This property is available on all types of `Node` - like Element, text node, etc.

`NodeList` can be converted to an array easily, using `Array.from()` or `[...ele.children]`. NodeList is also a iterable and has a `.length` property.  
Here, the NodeList is **live**.

```jsx
<div id="parent">
  Some text
  <span>some element</span>
  and more text
</div>

const childNodes = parent.childNodes;
childNodes // NodeList [text, span, text]

[...childNodes].forEach(node => {
  // do something
})
```

<MDN title="childNodes" url="https://developer.mozilla.org/en-US/docs/Web/API/Node/childNodes" />

### 4. firstChild + nextSibling

`Element.firstChild: Node`  


This property returns the element's **first child** node in the tree. If the node has no children, then 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>

var eleA = document.getElementByID("A")

var eleB = eleA.firstChild // <div id="B">
var 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 variant for all these properties 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
var queue = [root]
while (queue.length > 0) {
  const item = queue.shift();
  console.log(item);
  if (item.hasChildNodes()) {
    queue = Array.from(item.childNodes).concat(queue)
  }
}

// 2. LCRS approach
function traverse(item) {
  if (item === null) {
    return;
  }

  console.log(item);
  traverse(item.firstChild)
  traverse(item.nextSibling)
}
traverse(root.firstChild)
```
:::

</Accordion>


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