I gave a live-coding talk last week at the Manchester Web Meetup #4
The video of my talk is uploaded here
Here is the github repo
$ - when referring to real doms, e.g. $div, $el, $appv - when referring to virtual doms, e.g. vDiv, vEl, vAppVirtual DOMs usually refer to plain objects representing the actual DOM
The Document Object Model (DOM) is a programming interface for HTML documents.
For example, when you do this:
const $app = document.getElementById('app');
You will get the DOM for <div id="app"></div> on the page. This DOM will have some programming interface for you to control it. For example:
$app.innerHTML = 'Hello world';
To make a plain object to represent $app, we can write something like this:
const vApp = {
tagName: 'div',
attrs: {
id: 'app',
},
};
There is no strict rule of how the virtual DOM should look like. You can call it tagLabel instead of tagName, or props instead of attrs. As soon as it represents the DOM, it is a "virtual DOM".
Virtual DOM will not have any of those programming interface. This is what makes them lightweight comparing to actual DOMs.
However, keep in mind that since DOMs are the fundamental elements of the browser, most browsers must have done some serious optimisation to them. So actual DOMs might not be as slow as many people claim.
We start of by creating and going into our project directory.
$ mkdir /tmp/vdommm
$ cd /tmp/vdommm
We will then initiate the git repo, create the .gitignore file with gitignorer
$ git init
$ gitignore init node
$ npm init -y
Let's do out initial commit.
$ git add -A
$ git commit -am ':tada: initial commit'
Next, install Parcel Bundler
$ npm install parcel-bundler
(Fun fact: you no longer need to pass --save anymore.)
While this is installing, let's create some files in our project.
src/index.html
<html>
<head>
<title>hello world</title>
</head>
<body>
Hello world
<script src="./main.js"></script>
</body>
</html>
src/main.js
const vApp = {
tagName: 'div',
attrs: {
id: 'app',
},
};
console.log(vApp);
package.json
{
...
"scripts": {
"dev": "parcel src/index.html", // add this script
}
...
}
Now you can spawn the development server by doing:
$ npm run dev
> [email protected] dev /private/tmp/vdommm
> parcel src/index.html
Server running at http://localhost:1234
Built in 959ms.
Going to http://localhost:1234
Most virtual DOM implementation will have this function called createElement function, often referred as h. These functions will simply return a "virtual element". So let's implement that.
src/vdom/createElement.js
export default (tagName, opts) => {
return {
tagName,
attrs: opts.attrs,
children: opts.children,
};
};
With object destructuring we can write the above like this:
src/vdom/createElement.js
export default (tagName, { attrs, children }) => {
return {
tagName,
attrs,
children,
};
};
We should also allow creating elements without any options, so let's put some default values for out options.
src/vdom/createElement.js
export default (tagName, { attrs = {}, children = [] } = {}) => {
return {
tagName,
attrs,
children,
};
};
Recall the virtual DOM that we created before:
src/main.js
const vApp = {
tagName: 'div',
attrs: {
id: 'app',
},
};
console.log(vApp);
It now can be written as:
src/main.js
import createElement from './vdom/createElement';
const vApp = createElement('div', {
attrs: {
id: 'app',
},
});
console.log(vApp);
Go back to the browser and you should see the same virtual dom as we defined previously. Let's add an image under the div sourcing from giphy:
src/main.js
import createElement from './vdom/createElement';
const vApp = createElement('div', {
attrs: {
id: 'app',
},
children: [
createElement('img', {
attrs: {
src: 'https://media.giphy.com/media/cuPm4p4pClZVC/giphy.gif',
},
}),
],
});
console.log(vApp);
Go back to the browser and you should see the updated virtual DOM.
Object literals (e.g. { a: 3 }) automatically inherit from Object. This means that the object created by object literals will have methods defined in the Object.prototypehasOwnProperty, toString, etc.
We could make our virtual DOM a little bit "purer" by using Object.create(null). This will create a truly plain object that doesn't inherit from Object but null instead.
src/vdom/createElement.js
export default (tagName, { attrs, children }) => {
const vElem = Object.create(null);
Object.assign(vElem, {
tagName,
attrs,
children,
});
return vElem;
};
Now we got a function that generates virtual DOM for us. Next we need a way to translate our virtual DOM to real DOM. Let's define render (vNode) which will take in a virtual node and return the corresponding DOM.
src/vdom/render.js
const render = (vNode) => {
// create the element
// e.g. <div></div>
const $el = document.createElement(vNode.tagName);
// add all attributs as specified in vNode.attrs
// e.g. <div id="app"></div>
for (const [k, v] of Object.entries(vNode.attrs)) {
$el.setAttribute(k, v);
}
// append all children as specified in vNode.children
// e.g. <div id="app"><img></div>
for (const child of vNode.children) {
$el.appendChild(render(child));
}
return $el;
};
export default render;
The above code should be quite self-explanatory. I am more than happy to explain more tho if there is any request for it.
In real DOM, there are 8 types of nodes
ElementNode, such as <div> and <img>TextNode, plain textsOur virtual element structure, { tagName, attrs, children }, only represents the ElementNode in the DOM. So we need some representation for the TextNode as well. We will simply use String to represent TextNode.
To demonstrate this, let's add some text to our current virtual DOM.
src/main.js
import createElement from './vdom/createElement';
const vApp = createElement('div', {
attrs: {
id: 'app',
},
children: [
'Hello world', // represents TextNode
createElement('img', {
attrs: {
src: 'https://media.giphy.com/media/cuPm4p4pClZVC/giphy.gif',
},
}), // represents ElementNode
],
}); // represents ElementNode
console.log(vApp);
As I mentioned, we are considering two types of nodes. The current render (vNode) only only renders ElementNode. So let's extend render so that it supports rendering of TextNode too.
We will first rename our existing function renderElem as it is what it does. I will also add object destructuring to make the code looks nicer.
src/vdom/render.js
const renderElem = ({ tagName, attrs, children}) => {
// create the element
// e.g. <div></div>
const $el = document.createElement(tagName);
// add all attributs as specified in vNode.attrs
// e.g. <div id="app"></div>
for (const [k, v] of Object.entries(attrs)) {
$el.setAttribute(k, v);
}
// append all children as specified in vNode.children
// e.g. <div id="app"><img></div>
for (const child of children) {
$el.appendChild(render(child));
}
return $el;
};
export default render;
Let's redefine render (vNode). We just need to check if vNode is a String. If it is then we can use document.createTextNode(string) to render the textNode. Otherwise, just call renderElem(vNode).
src/vdom/render.js
const renderElem = ({ tagName, attrs, children}) => {
// create the element
// e.g. <div></div>
const $el = document.createElement(tagName);
// add all attributs as specified in vNode.attrs
// e.g. <div id="app"></div>
for (const [k, v] of Object.entries(attrs)) {
$el.setAttribute(k, v);
}
// append all children as specified in vNode.children
// e.g. <div id="app"><img></div>
for (const child of children) {
$el.appendChild(render(child));
}
return $el;
};
const render = (vNode) => {
if (typeof vNode === 'string') {
return document.createTextNode(vNode);
}
// we assume everything else to be a virtual element
return renderElem(vNode);
};
export default render;
Now our render (vNode) function is capable of rendering two types of virtual nodes:
createElement functionvApp!Now let's try to render our vApp and console.log it!
src/main.js
import createElement from './vdom/createElement';
import render from './vdom/render';
const vApp = createElement('div', {
attrs: {
id: 'app',
},
children: [
'Hello world',
createElement('img', {
attrs: {
src: 'https://media.giphy.com/media/cuPm4p4pClZVC/giphy.gif',
},
}),
],
});
const $app = render(vApp);
console.log($app);
Go to the browser and you would see the console showing the DOM for:
<div id="app">
Hello world
<img src="https://media.giphy.com/media/cuPm4p4pClZVC/giphy.gif">
</div>
We are now able to create our virtual DOM and render it to real DOM. Next we would need to put our real DOM on the page.
Let's first create a mounting point for our app. I will replace the Hello world on the src/index.html with <div id="app"></div>.
src/index.html
<html>
<head>
<title>hello world</title>
</head>
<body>
<div id="app"></div>
<script src="./main.js"></script>
</body>
</html>
What we want to do now is to replace this empty div with our rendered $app. This is super easy to do if we ignore Internet Explorer and Safari. We can just use ChildNode.replaceWith
Let's define mount ($node, $target). This function will simply replace $target with $node and return $node.
src/vdom/mount.js
export default ($node, $target) => {
$target.replaceWith($node);
return $node;
};
Now in our main.js simply mount our $app to the empty div.
src/main.js
import createElement from './vdom/createElement';
import render from './vdom/render';
import mount from './vdom/mount';
const vApp = createElement('div', {
attrs: {
id: 'app',
},
children: [
'Hello world',
createElement('img', {
attrs: {
src: 'https://media.giphy.com/media/cuPm4p4pClZVC/giphy.gif',
},
}),
],
});
const $app = render(vApp);
mount($app, document.getElementById('app'));
Our app will show on the page now and we should see a cat on the page.
Now let's make our app more interesting. We will wrap our vApp in a function called createVApp. It will then take in a count which then the vApp will use it.
src/main.js
import createElement from './vdom/createElement';
import render from './vdom/render';
import mount from './vdom/mount';
const createVApp = count => createElement('div', {
attrs: {
id: 'app',
dataCount: count, // we use the count here
},
children: [
'The current count is: ',
String(count), // and here
createElement('img', {
attrs: {
src: 'https://media.giphy.com/media/cuPm4p4pClZVC/giphy.gif',
},
}),
],
});
let count = 0;
const vApp = createVApp(count);
const $app = render(vApp);
mount($app, document.getElementById('app'));
Then, we will setInterval to increment the count every second and create, render and mount our app again on the page.
src/main.js
import createElement from './vdom/createElement';
import render from './vdom/render';
import mount from './vdom/mount';
const createVApp = count => createElement('div', {
attrs: {
id: 'app',
dataCount: count, // we use the count here
},
children: [
'The current count is: ',
String(count), // and here
createElement('img', {
attrs: {
src: 'https://media.giphy.com/media/cuPm4p4pClZVC/giphy.gif',
},
}),
],
});
let count = 0;
const vApp = createVApp(count);
const $app = render(vApp);
let $rootEl = mount($app, document.getElementById('app'));
setInterval(() => {
count++;
$rootEl = mount(render(createVApp(count)), $rootEl);
}, 1000);
Note that I used $rootEl to keep track of the root element. So that mount knows where to mount our new app.
If we go back to the browser now, we should see the count increment every second by 1 and works perfectly!
We now gain the power to declaratively create our application. The application is rendered predictably and is very very easy to reason about. If you know how things are done in the JQuery way, you will appreciate how much cleaner this approach is.
However, there are a couple of problems with re-rendering the whole application every second:
<input> will lose their focus whenever the application re-mount to the page. See live demo hereWe will solve these problems in the next section.
Imagine we have a function diff (oldVTree, newVTree) which calculate the differences between the two virtual trees; return a patch function that takes in the real DOM of oldVTree and perform appropriate operations to the real DOM to make the real DOM looks like the newVTree.
If we have that diff function, then we could just re-write our interval to become:
src/main.js
import createElement from './vdom/createElement';
import render from './vdom/render';
import mount from './vdom/mount';
import diff from './vdom/diff';
const createVApp = count => createElement('div', {
attrs: {
id: 'app',
dataCount: count, // we use the count here
},
children: [
'The current count is: ',
String(count), // and here
createElement('img', {
attrs: {
src: 'https://media.giphy.com/media/cuPm4p4pClZVC/giphy.gif',
},
}),
],
});
let count = 0;
let vApp = createVApp(count);
const $app = render(vApp);
let $rootEl = mount($app, document.getElementById('app'));
setInterval(() => {
count++;
const vNewApp = createVApp(count)
const patch = diff(vApp, vNewApp);
// we might replace the whole $rootEl,
// so we want the patch will return the new $rootEl
$rootEl = patch($rootEl);
vApp = vNewApp;
}, 1000);
So let's try to implement this diff (oldVTree, newVTree). Let's start with some easy cases:
newVTree is undefined
$node passing into the patch then!$node with render(newVTree).$node with render(newVTree).oldVTree.tagName !== newVTree.tagName
$node with render(newVTree).Two elements of different types will produce different trees.
src/vdom/diff.js
import render from './render';
const diff = (oldVTree, newVTree) => {
// let's assume oldVTree is not undefined!
if (newVTree === undefined) {
return $node => {
$node.remove();
// the patch should return the new root node.
// since there is none in this case,
// we will just return undefined.
return undefined;
}
}
if (typeof oldVTree === 'string' ||
typeof newVTree === 'string') {
if (oldVTree !== newVTree) {
// could be 2 cases:
// 1. both trees are string and they have different values
// 2. one of the trees is text node and
// the other one is elem node
// Either case, we will just render(newVTree)!
return $node => {
const $newNode = render(newVTree);
$node.replaceWith($newNode);
return $newNode;
};
} else {
// this means that both trees are string
// and they have the same values
return $node => $node;
}
}
if (oldVTree.tagName !== newVTree.tagName) {
// we assume that they are totally different and
// will not attempt to find the differences.
// simply render the newVTree and mount it.
return $node => {
const $newNode = render(newVTree);
$node.replaceWith($newNode);
return $newNode;
};
}
// (A)
};
export default diff;
If the code reaches (A), it implies the following:
oldVTree and newVTree are both virtual elements.tagName.attrs and children.We will implement two functions to deal with the attributes and children separately, namely diffAttrs (oldAttrs, newAttrs) and diffChildren (oldVChildren, newVChildren), which will return a patch separately. As we know at this point we are not going to replace $node, we can safely return $node after applying both patches.
src/vdom/diff.js
import render from './render';
const diffAttrs = (oldAttrs, newAttrs) => {
return $node => {
return $node;
};
};
const diffChildren = (oldVChildren, newVChildren) => {
return $node => {
return $node;
};
};
const diff = (oldVTree, newVTree) => {
// let's assume oldVTree is not undefined!
if (newVTree === undefined) {
return $node => {
$node.remove();
// the patch should return the new root node.
// since there is none in this case,
// we will just return undefined.
return undefined;
}
}
if (typeof oldVTree === 'string' ||
typeof newVTree === 'string') {
if (oldVTree !== newVTree) {
// could be 2 cases:
// 1. both trees are string and they have different values
// 2. one of the trees is text node and
// the other one is elem node
// Either case, we will just render(newVTree)!
return $node => {
const $newNode = render(newVTree);
$node.replaceWith($newNode);
return $newNode;
};
} else {
// this means that both trees are string
// and they have the same values
return $node => $node;
}
}
if (oldVTree.tagName !== newVTree.tagName) {
// we assume that they are totally different and
// will not attempt to find the differences.
// simply render the newVTree and mount it.
return $node => {
const $newNode = render(newVTree);
$node.replaceWith($newNode);
return $newNode;
};
}
const patchAttrs = diffAttrs(oldVTree.attrs, newVTree.attrs);
const patchChildren = diffChildren(oldVTree.children, newVTree.children);
return $node => {
patchAttrs($node);
patchChildren($node);
return $node;
};
};
export default diff;
Let's first focus on the diffAttrs. It is actually pretty easy. We know that we are going to set everything in newAttrs. After setting them, we just need to go through all the keys in oldAttrs and make sure they all exist in newAttrs too. If not, remove them.
const diffAttrs = (oldAttrs, newAttrs) => {
const patches = [];
// setting newAttrs
for (const [k, v] of Object.entries(newAttrs)) {
patches.push($node => {
$node.setAttribute(k, v);
return $node;
});
}
// removing attrs
for (const k in oldAttrs) {
if (!(k in newAttrs)) {
patches.push($node => {
$node.removeAttribute(k);
return $node;
});
}
}
return $node => {
for (const patch of patches) {
patch($node);
}
return $node;
};
};
Notice how we create a wrapper patch and loop through the patches to apply them.
Children would be a little bit more complicated. We can consider three cases:
oldVChildren.length === newVChildren.length
diff(oldVChildren[i], newVChildren[i])
where i goes from 0 to oldVChildren.length.oldVChildren.length > newVChildren.length
diff(oldVChildren[i], newVChildren[i])
where i goes from 0 to oldVChildren.length.newVChildren[j] will be undefined for j >= newVChildren.lengthdiff can handle diff(vNode, undefined)!oldVChildren.length < newVChildren.length
diff(oldVChildren[i], newVChildren[i])
where i goes from 0 to oldVChildren.length.newVChildren.slice(oldVChildren.length).To conclude, we loop through oldVChildren regardless and we will call diff(oldVChildren[i], newVChildren[i]).
Then we will render the additional children (if any), and append them to the $node.
const diffChildren = (oldVChildren, newVChildren) => {
const childPatches = [];
oldVChildren.forEach((oldVChild, i) => {
childPatches.push(diff(oldVChild, newVChildren[i]));
});
const additionalPatches = [];
for (const additionalVChild of newVChildren.slice(oldVChildren.length)) {
additionalPatches.push($node => {
$node.appendChild(render(newVChildren));
return $node;
});
}
return $parent => {
// since childPatches are expecting the $child, not $parent,
// we cannot just loop through them and call patch($parent)
$parent.childNodes.forEach(($child, i) => {
childPatches[i]($child);
});
for (const patch of additionalPatches) {
patch($parent);
}
return $parent;
};
};
I think it is a little bit more elegant if we use the zip function.
import render from './render';
const zip = (xs, ys) => {
const zipped = [];
for (let i = 0; i < Math.min(xs.length, ys.length); i++) {
zipped.push([xs[i], ys[i]]);
}
return zipped;
};
const diffChildren = (oldVChildren, newVChildren) => {
const childPatches = [];
oldVChildren.forEach((oldVChild, i) => {
childPatches.push(diff(oldVChild, newVChildren[i]));
});
const additionalPatches = [];
for (const additionalVChild of newVChildren.slice(oldVChildren.length)) {
additionalPatches.push($node => {
$node.appendChild(render(additionalVChild));
return $node;
});
}
return $parent => {
// since childPatches are expecting the $child, not $parent,
// we cannot just loop through them and call patch($parent)
for (const [patch, $child] of zip(childPatches, $parent.childNodes)) {
patch($child);
}
for (const patch of additionalPatches) {
patch($parent);
}
return $parent;
};
};
src/vdom/diff.js
import render from './render';
const zip = (xs, ys) => {
const zipped = [];
for (let i = 0; i < Math.min(xs.length, ys.length); i++) {
zipped.push([xs[i], ys[i]]);
}
return zipped;
};
const diffAttrs = (oldAttrs, newAttrs) => {
const patches = [];
// setting newAttrs
for (const [k, v] of Object.entries(newAttrs)) {
patches.push($node => {
$node.setAttribute(k, v);
return $node;
});
}
// removing attrs
for (const k in oldAttrs) {
if (!(k in newAttrs)) {
patches.push($node => {
$node.removeAttribute(k);
return $node;
});
}
}
return $node => {
for (const patch of patches) {
patch($node);
}
return $node;
};
};
const diffChildren = (oldVChildren, newVChildren) => {
const childPatches = [];
oldVChildren.forEach((oldVChild, i) => {
childPatches.push(diff(oldVChild, newVChildren[i]));
});
const additionalPatches = [];
for (const additionalVChild of newVChildren.slice(oldVChildren.length)) {
additionalPatches.push($node => {
$node.appendChild(render(additionalVChild));
return $node;
});
}
return $parent => {
// since childPatches are expecting the $child, not $parent,
// we cannot just loop through them and call patch($parent)
for (const [patch, $child] of zip(childPatches, $parent.childNodes)) {
patch($child);
}
for (const patch of additionalPatches) {
patch($parent);
}
return $parent;
};
};
const diff = (oldVTree, newVTree) => {
// let's assume oldVTree is not undefined!
if (newVTree === undefined) {
return $node => {
$node.remove();
// the patch should return the new root node.
// since there is none in this case,
// we will just return undefined.
return undefined;
}
}
if (typeof oldVTree === 'string' ||
typeof newVTree === 'string') {
if (oldVTree !== newVTree) {
// could be 2 cases:
// 1. both trees are string and they have different values
// 2. one of the trees is text node and
// the other one is elem node
// Either case, we will just render(newVTree)!
return $node => {
const $newNode = render(newVTree);
$node.replaceWith($newNode);
return $newNode;
};
} else {
// this means that both trees are string
// and they have the same values
return $node => $node;
}
}
if (oldVTree.tagName !== newVTree.tagName) {
// we assume that they are totally different and
// will not attempt to find the differences.
// simply render the newVTree and mount it.
return $node => {
const $newNode = render(newVTree);
$node.replaceWith($newNode);
return $newNode;
};
}
const patchAttrs = diffAttrs(oldVTree.attrs, newVTree.attrs);
const patchChildren = diffChildren(oldVTree.children, newVTree.children);
return $node => {
patchAttrs($node);
patchChildren($node);
return $node;
};
};
export default diff;
Our current app doesn't really make full use of the power of our virtual DOM. To show how powerful our Virtual DOM is, let's make our app more complicated:
src/main.js
import createElement from './vdom/createElement';
import render from './vdom/render';
import mount from './vdom/mount';
import diff from './vdom/diff';
const createVApp = count => createElement('div', {
attrs: {
id: 'app',
dataCount: count, // we use the count here
},
children: [
'The current count is: ',
String(count), // and here
...Array.from({ length: count }, () => createElement('img', {
attrs: {
src: 'https://media.giphy.com/media/cuPm4p4pClZVC/giphy.gif',
},
})),
],
});
let vApp = createVApp(0);
const $app = render(vApp);
let $rootEl = mount($app, document.getElementById('app'));
setInterval(() => {
const n = Math.floor(Math.random() * 10);
const vNewApp = createVApp(n);
const patch = diff(vApp, vNewApp);
// we might replace the whole $rootEl,
// so we want the patch will return the new $rootEl
$rootEl = patch($rootEl);
vApp = vNewApp;
}, 1000);
Our app now will generate a random number n between 0 and 9 and display n cat photos on the page. If you go into the dev tools, you will see how we are "intelligently" inserting and removing <img> depending on n.
If you read all the way up to here, I would like to thank you for taking the time to read the whole thing. It is a very very long read! Please leave a comment if you actually read the whole thing. Love you!