What is Virtual DOM?

Question

Recently, I looked at Facebook's React framework. It uses a concept called "the Virtual DOM," which I didn't really understand.

What is the Virtual DOM? What are the advantages?

Answer 1

React creates a tree of custom objects representing a part of the DOM. For example, instead of creating an actual DIV element containing a UL element, it creates a React.div object that contains a React.ul object. It can manipulate these objects very quickly without actually touching the real DOM or going through the DOM API. Then, when it renders a component, it uses this virtual DOM to figure out what it needs to do with the real DOM to get the two trees to match.

You can think of the virtual DOM like a blueprint. It contains all the details needed to construct the DOM, but because it doesn't require all the heavyweight parts that go into a real DOM, it can be created and changed much more easily.

Answer 2

Let's take an example — though a very naive one: If you have something messed up in a room in your home and you need to clean it, what will be your first step? Will you be cleaning your room which is messed up or the whole house? The answer is definitely that you will be cleaning only the room which requires the cleaning. That's what the virtual DOM does.

Ordinary JS traverses or renders the whole DOM instead of rendering only the part which requires changes.

So whenever you have any changes, as in you want to add another <div> to your DOM then the virtual DOM will be created which actually does not do any changes in the actual DOM. Now with this virtual DOM, you will be checking the difference between this and your current DOM. And only the part which is different (in this case the new <div>) will be added instead of re-rendering the whole DOM.

Answer 3

What is the virtual DOM?

The virtual DOM is an in-memory representation of the real DOM elements generated by React components before any changes are made to the page.

It’s a step that happens between the render function being called and the displaying of elements on the screen.

A component’s render method returns some markup, but it’s not the final HTML yet. It’s the in-memory representation of what will become real elements (this is step 1). Then that output will be transformed into real HTML, which is what gets displayed in the browser (This is step 2).

So why go through all this to generate a virtual DOM? Simple answer — This is what allows react to be fast. It does this by means of virtual DOM diffing. Comparing two virtual trees — old and new — and make only the necessary changes into the real DOM.

Source from Intro To React #2

Answer 4

A virtual DOM(VDOM) is not a new concept: https://github.com/Matt-Esch/virtual-dom.

VDOM is strategically to update DOM without redrawing all the nodes in a single page application. Finding a node in a tree structure is easy but the DOM tree for a SPA app can be drastically huge. Finding and updating a node/nodes in case of an event is not time efficient.

VDOM solves this problem by creating a high level abstraction of actual dom. The VDOM is a high level lightweight in-memory tree representation of actual DOM.

For example, consider adding a node in DOM; react keep a copy of VDOM in memory

1. Create a VDOM with a new state
2. Compare it with older VDOM using diffing.
3. Update only different nodes in real DOM.
4. Assign new VDOM as an older VDOM.

Answer 5

This is a brief description and reiteration of the Virtual DOM often mentioned alongside ReactJS.

The DOM (Document Object Model) is an abstraction of structured text, which means it is made of HTML code and css. These HTML elements become nodes in the DOM. There are limitations to the previous methods of manipulating the DOM. Virtual DOM is an abstraction of the literal HTML DOM created well before React was created or used, but for our purposes we will use it in concert with ReactJS. The Virtual DOM is lightweight and detached from the DOM implementation in the browser. The Virtual DOM is essentially a screenshot (or copy) of the DOM at a given time. A way to look at it from a developers perspective is the DOM is the production environment and the Virtual DOM is the local (dev) environment. Each time the data changes in a React app a new Virtual DOM representation of the user interface is created.

The most basic method needed in order to create a static component in ReactJS are:

You must return code from the render method. You must convert every class to className since class is reserved word in JavaScript. Aside from the more major changes there are minor differences between the two DOMs including three attributes appearing in the Virtual DOM but not in the HTML DOM (key, ref and dangerouslySetInnerHTML).

An important thing to understand when working with the Virtual DOM is the difference between ReactElement and ReactComponent.

ReactElement

• A ReactElement is a light, stateless, immutable, virtual representation of a DOM Element.
• ReactElement - This is the primary type in React and resides in the Virtual DOM.

• ReactElements can be rendered into HTML DOM

  var root = React.createElement('div'); ReactDOM.render(root, document.getElementById('example'));

• JSX compiles HTML tags into ReactElements

  var root = <div/>; ReactDOM.render(root, document.getElementById('example'));

ReactComponent

• ReactComponent - ReactComponent's are stateful components.
• React.createClass is considered a ReactComponent.
• Whenever state changes the component is rerendered.

Whenever a ReactComponent has a state change, we want as little change to the HTML DOM as possible so ReactComponent is converted to the ReactElement which can then be inserted to the Virtual DOM, compared and updated fast and easily.

When React knows the diff - it's converted to the low-level (HTML DOM) code, which is executed in the DOM.

Answer 6

It’s a neat concept: instead of manipulating the DOM directly, which is error prone and relies on mutable state, you instead output a value called the Virtual DOM. The Virtual DOM is then diffed with the current state of the DOM, which generates a list of DOM operations that would make the current DOM look like the new one. Those operations are applied quickly in a batch.

Taken from here.

Answer 7

Virtual DOM is an abstraction of the HTML DOM that selectively renders subtrees of nodes based upon state changes. It does the least amount of DOM manipulation possible in order to keep your components up to date.

Answer 8

All the answers are great. I just came up with an analogy which probably can give a real-world metaphor.

The real DOM is like your room, nodes are the furniture in your room. The virtual DOM is like we draw a blueprint of this current room.

We all have the experience of moving furniture, it's very tiring (Same concept as updating views in computers). Therefore, whenever we want to change the position/add furniture (nodes), we want to only do the very necessary change.

Blueprint came for the rescue to achieve it. We draw a new blueprint and compare the difference with the original one. This lets us know what part has been changed and what part stays the same. We then do the necessary change to the real room (update the changed nodes on the real DOM). Hurray.

(Some might think, why do we have to rely on the virtual one and don't directly compare the real DOM? Well, in the analogy, comparing the real DOM means you have to create another real room and compare it with your original one. It's just too costly.)

Answer 9

Virtual Dom is created one copy of Dom. Virtual dom is compared to dom, and virtual dom update only that part in dom which changed. it's not rendering the whole dom it's just changed the updated part of dom in dom. It's very time consuming and from this functionality, our app works fast.

Answer 10

React's structural unit is component. Each component has a state. Whenever the state of a component is changed, React modifies the V-DOM tree. Thereafter latest version of the V-DOM is compared with previous version of the V-DOM. After this calculation(diffing) when React knows which V-DOM objects have been changed it modifies only those objects in the R-DOM.

In layman's terms,

Say I have added a div element in DOM, React creates a copy of V-DOM without changing the whole R-DOM. This newly created V-DOM is compared with older V-DOM. It updates only different nodes in real DOM. Now the newly created V-DOM is regarded as previous version for upcoming V-DOM.

P.S. 1. So Unlike plain js Whole new version of V-DOM is created and R-DOM is partly updated. 2. React does not update every single change in state, rather updates to the R-DOM are sent in batches.

Answer 11

According to React doc: https://reactjs.org/docs/faq-internals.html#what-is-the-virtual-dom

'In React world, the term “virtual DOM” is usually associated with React elements since they are the objects representing the user interface. '

import React, { Component } from 'react'; //You need to do this inside a module to import

class App extends Component{
   render(){
       return (
       <button>Hi</button> //This returns a virtual DOM
       )
   }
}

The code inside return is actually a call to function React.createElement:

//render can be rewritten like this:
render(){
   return [
            React.createElement(
                'button',
                {
                    key: null,
                    ref: null,           
                },
                'Hi',
            )
   ]
}

which returns something like this:

{
  $$typeof: Symbol.for('react.element'), 
  type: "button", 
  key: null, 
  ref: null, 
  props: { 
     children: 'Hi',
  }
}

and this is virtual DOM. It's an JavaScript object which costs much less to manipulate than the actual DOM element created by

document.createElement('button');

which is also a JavaScript object looks like this:

accessKey: ""
ariaAtomic: null
ariaAutoComplete: null
ariaBusy: null
ariaChecked: null
ariaColCount: null
ariaColIndex: null
ariaColSpan: null
ariaCurrent: null
ariaDescription: null
ariaDisabled: null
ariaExpanded: null
ariaHasPopup: null
ariaHidden: null
ariaKeyShortcuts: null
ariaLabel: null
ariaLevel: null
ariaLive: null
ariaModal: null
ariaMultiLine: null
ariaMultiSelectable: null
ariaOrientation: null
ariaPlaceholder: null
ariaPosInSet: null
ariaPressed: null
ariaReadOnly: null
ariaRelevant: null
ariaRequired: null
ariaRoleDescription: null
ariaRowCount: null
ariaRowIndex: null
ariaRowSpan: null
ariaSelected: null
ariaSetSize: null
ariaSort: null
ariaValueMax: null
ariaValueMin: null
ariaValueNow: null
ariaValueText: null
assignedSlot: null
attributeStyleMap: StylePropertyMap {size: 0}
attributes: NamedNodeMap {length: 0}
autocapitalize: ""
autofocus: false
baseURI: "http://localhost:3000/"
childElementCount: 0
childNodes: NodeList []
children: HTMLCollection []
classList: DOMTokenList [value: ""]
className: ""
clientHeight: 0
clientLeft: 0
clientTop: 0
clientWidth: 0
contentEditable: "inherit"
dataset: DOMStringMap {}
dir: ""
disabled: false
draggable: false
elementTiming: ""
enterKeyHint: ""
firstChild: null
firstElementChild: null
form: null
formAction: "http://localhost:3000/"
formEnctype: ""
formMethod: ""
formNoValidate: false
formTarget: ""
hidden: false
id: ""
innerHTML: ""
innerText: ""
inputMode: ""
isConnected: false
isContentEditable: false
labels: NodeList []
lang: ""
lastChild: null
lastElementChild: null
localName: "button"
name: ""
namespaceURI: "http://www.w3.org/1999/xhtml"
nextElementSibling: null
nextSibling: null
nodeName: "BUTTON"
nodeType: 1
nodeValue: null
nonce: ""
offsetHeight: 0
offsetLeft: 0
offsetParent: null
offsetTop: 0
offsetWidth: 0
onabort: null
onanimationend: null
onanimationiteration: null
onanimationstart: null
onauxclick: null
onbeforecopy: null
onbeforecut: null
onbeforepaste: null
onbeforexrselect: null
onblur: null
oncancel: null
oncanplay: null
oncanplaythrough: null
onchange: null
onclick: null
onclose: null
oncontextmenu: null
oncopy: null
oncuechange: null
oncut: null
ondblclick: null
ondrag: null
ondragend: null
ondragenter: null
ondragleave: null
ondragover: null
ondragstart: null
ondrop: null
ondurationchange: null
onemptied: null
onended: null
onerror: null
onfocus: null
onformdata: null
onfullscreenchange: null
onfullscreenerror: null
ongotpointercapture: null
oninput: null
oninvalid: null
onkeydown: null
onkeypress: null
onkeyup: null
onload: null
onloadeddata: null
onloadedmetadata: null
onloadstart: null
onlostpointercapture: null
onmousedown: null
onmouseenter: null
onmouseleave: null
onmousemove: null
onmouseout: null
onmouseover: null
onmouseup: null
onmousewheel: null
onpaste: null
onpause: null
onplay: null
onplaying: null
onpointercancel: null
onpointerdown: null
onpointerenter: null
onpointerleave: null
onpointermove: null
onpointerout: null
onpointerover: null
onpointerrawupdate: null
onpointerup: null
onprogress: null
onratechange: null
onreset: null
onresize: null
onscroll: null
onsearch: null
onseeked: null
onseeking: null
onselect: null
onselectionchange: null
onselectstart: null
onstalled: null
onsubmit: null
onsuspend: null
ontimeupdate: null
ontoggle: null
ontransitionend: null
onvolumechange: null
onwaiting: null
onwebkitanimationend: null
onwebkitanimationiteration: null
onwebkitanimationstart: null
onwebkitfullscreenchange: null
onwebkitfullscreenerror: null
onwebkittransitionend: null
onwheel: null
outerHTML: "<button></button>"
outerText: ""
ownerDocument: document
parentElement: null
parentNode: null
part: DOMTokenList [value: ""]
prefix: null
previousElementSibling: null
previousSibling: null
scrollHeight: 0
scrollLeft: 0
scrollTop: 0
scrollWidth: 0
shadowRoot: null
slot: ""
spellcheck: true
style: CSSStyleDeclaration {alignContent: "", alignItems: "", alignSelf: "", alignmentBaseline: "", all: "", …}
tabIndex: 0
tagName: "BUTTON"
textContent: ""
title: ""
translate: true
type: "submit"
validationMessage: ""
validity: ValidityState {valueMissing: false, typeMismatch: false, patternMismatch: false, tooLong: false, tooShort: false, …}
value: ""
willValidate: true

You can learn more about virtual DOM and React at https://indepth.dev/inside-fiber-in-depth-overview-of-the-new-reconciliation-algorithm-in-react/

Answer 12

React keeps a lightweight representation of the real DOM in the memory, and that is known as the virtual DOM. When the state of an object changes, virtual DOM changes only that object in the real DOM, rather than updating all the objects.

Answer 13

let's make order and sense in this matter. React (or any other library) are a "layer" on javascript.

There is no such thing as virtual dom, there is unattached dom .

let me explain in simple javascript :

 let vDom = {};     // this is a object that will be used to hold the elements

 let d = document.createElement('div');
 d.innerHTML = 'hi, i am a new div';

 vDom['newDiv'] = d;

at this point we have created a Div which is not shown on the dom , because it has not attached

but we can access it, add attributes, values, change etc..

once we call : (for ex,add it to body)

    document.body.appendChild(vDom['newDiv'])

then we will see it;

 for one how saw javascript libs come and go , i suggest to any one 
 to do one simple thing : master JAVAscript, not layers :)