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Kamesh Sethupathi
Kamesh Sethupathi

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KReact: Building your own React & Virtual DOM

#react #preact #rollup #virtualdom

In this blog we will build a library similar to react and understand how react works internally. This blog is written in a way that even if you don't know react or want to learn the internal working of react, you would definitely find it useful.

Link to the GitHub Repository: https://github.com/git-kamesh/kReact

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[REQ] Requirements & Use-cases : What we want to build?

  • [COMPONENT] Able to split complicated UI's into smaller reuseable components (Button, DropDown, ImageSlider, etc ).
  • [JSX] Able to write HTML template in JavaScript itself. Lets call this JSX.
  • [LOGIC] JSX should allows to embed JavaScript expressions and logically controlled .
  • [PROPS] Able to pass data/information to component from outside, lets call this props.
  • [STATE] Component can have its own data/information without passing to it from outside, lets call this state.
  • [RE-RENDER] Whenever props or state gets modified, the changes should be reflected to the UI automatically. Lets call this re-render.
  • [PREVENT] Should be explicitly able to prevent re-render whenever required.
  • [LIFE-CYCLE] Know components life-cycle events like,
    • [1] before component mounted into DOM (birth of component)
    • [2] after component mounted into DOM
    • [3] component's props gets changed
    • [4] before component is updated
    • [5] after component is updated
    • [6] before component is unmounted from the DOM (death of component)
  • [DOM-OPERATION] Should handle DOM operations itself.
  • [DIFF] On re-render should find difference between the old DOM and new Virtual DOM, and only update that part to DOM. Lets call this patch.

Lets understand

  • Before jumping, we have to understand few things/concepts like React, DOM, Virtual DOM, JSX, Components, State, JSX Transpiler, Reconsiliation.
    • Don't know? don't worry we will see it here itself.

React

  • React is a component based UI library.
  • Component can be anything like Button, DropDown, ImageSlider, PasswordInput, etc.
  • Here, components are building blocks of UI and will respond to data change.
  • Components allows reusability thus providing development speed, consistancy, seperation of concern, easy to maintain and unit testable.
  • Also alows both,
    • Building an entire application (or)
    • Part of an application/Feature.

DOM

  • DOM stands for Document Object Model
  • Its an object representation of the parsed HTML document
  • We could update the dom through DOM APIs (e.g: )
  • When an DOM object is updated, browser run two expensive operations
    • Reflow - Calculates dimension and position of every element and its children.
    • Repaint - Determines visual changes (like color, opacity, visibility) and applies them.

Virtual DOM

  • Virtual DOM is nothing but an lightweight in-memory javascript object representation of the actual DOM.
  • It basically mimics as an actual DOM.

JSX 

const element = <h1 className="clrR">Hello, world!</h1>;
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  • The above tag syntax is neither a string nor HTML. It is JSX.
  • JSX stands for JavaScript XML. It is used to define our virtual DOM.
  • Just like HTML used for buildinng actual DOM, JSX is used for building virtual DOM.
  • JSX in most simple word is how React allows us to write HTML in JavaScript.
  • JSX is a syntax extension for JavaScript and it is not valid JavaScript, web browsers cant read it directly.
  • So, if JavaScript files contains JSX, that that file will have to be transpiled. That means that before the file gets to the web browser, a JSX compiler will translate any JSX into regular JavaScript.
  • After compilation, JSX expressions become regular JavaScript function calls and evaluate to JavaScript objects.
  • The above JSX example will be compiled smiliar to below. 
React.createElement('h1', { className: 'clrR'}, 'Hello, world!');
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Components

Component Fn

Reconsiliation

  • Whenever component's state or props gets updated, the component gets re-rendered and builds a new virtual DOM.
  • Then react runs the diffing algorithm to calculate what changes should be applied to real DOM. This process is know as reconsiliation.

Terms to understand

  • Rendering: Process of converting virtual dom into dom
  • Mounting: Process of injecting rendered dom into target dom
  • Patching: Process of comparing the virtual dom and actual dom, updating the nodes which are changed

Theories over lets play with code 😍

Rendering Logic 

render(vnode, parent):
      IF vnode IS class component:
         CREATE NEW INSTANCE FOR component --> componentInstance
         GENERATE component VNODE BY INVOKING componentInstance.render() --> VNode
         RENDER VNODE BY PASSING VNODE INTO render FUNCTION --> DOMNode
     ELSE IF vnode IS function component:
         GENERATE VNODE BY EXECUTING vnode --> VNODE
         RENDER VNODE BY PASSING VNODE INTO render FUNCTION --> DOMNode
     ELSE IF vnode IS OBJECT:
         CONVERT vnode OBJECT INTO DOMNode
         RECURSIVELY APPLY render FUNCTION on vnode children
         ASSIGN PROPS AS DOMNode attributes 
         MOUNT DOMNode ON parent
     RETURN DOMNode
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Patching Logic 

patch(dom, vnode, parent):
     IF dom AND vnode DIFFED:
         RENDER vnode --> DOMNode
         REPLACE dom WITH DOMNode
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Full source code

Link to the GitHub Repository: https://github.com/git-kamesh/kReact

Follow me on twitter: @kamesh_koops

export class Component {
    constructor( props = {}) {
        this.props = props;
        this.state = null;
    }

    setState(nextState) {
        const isCompat = isObject(this.state) && isObject(nextState);
        const commitState = ()=> this.state = isCompat? Object.assign({}, this.state, nextState) : nextState;
        const prevState = isObject(this.state)? Object.assign({}, this.state) : this.state;

        if( runHook(this, 'shouldComponentUpdate') && this.base ) {
            runHook(this, 'componentWillUpdate', this.props, nextState);
            commitState();
            patch(this.base, this.render());
            runHook(this, 'componentDidUpdate', this.props, prevState);
        } else commitState();
    }

    static render(vnode, parent) {
        if( isClassComponent(vnode) ) {
            let instance = new vnode.type( combineChildrenWithProps( vnode ) );
            runHook(instance, 'componentWillMount');
            instance.base = render( instance.render(), parent);
            instance.base.instance = instance;
            runHook(instance, 'componentDidMount');
            return instance.base;
        } else return render( vnode.type(combineChildrenWithProps( vnode )), parent );
    }

    static patch(dom, vnode, parent=dom.parentNode) {
        if (dom.instance && dom.instance.constructor == vnode.type) {
            runHook(dom.instance, 'componentWillReceiveProps', combineChildrenWithProps( vnode ) );
            dom.instance.props = combineChildrenWithProps( vnode );
            return patch(dom, dom.instance.render(), parent);
        } else if ( isClassComponent(vnode.type) ) {
            const newdom = Component.render(vnode, parent);
            return parent ? (replace(newdom, dom, parent) && newdom) : (newdom);
        } else if ( !isClassComponent(vnode.type) ) return patch(dom, vnode.type( combineChildrenWithProps( vnode ) ), parent);
    }
}

export const createElement = (type, props, ...children ) => ({ type, props: props || {}, children });

export function render(vnode, parent) {
    if( isObject(vnode) ) {
        let dom = isFunction(vnode.type) ? Component.render(vnode, parent) : document.createElement( vnode.type );
        vnode.children.flat(1).map((child)=> render(child, dom));
        !isFunction(vnode.type) && Object.keys(vnode.props).map((key)=> setAttribute(dom, key, vnode.props[key]));
        return mount( dom, parent );
    } else return mount( document.createTextNode(vnode || ''), parent );
}

function patch(dom, vnode, parent=dom.parentNode) {
    if( isObject(vnode) ) {
        if( isTextNode(dom) ) return replace( render(vnode, parent), dom, parent );
        else if( isFunction(vnode.type) ) return Component.patch( dom, vnode, parent);
        else {
            let dom_map = Array.from(dom.childNodes) // Build a key value map to identify dom-node to its equivalent vnode
                .reduce((prev, node, idx)=> ({...prev, [node._idx || `__${idx}`]: node}), {});

            vnode.children.flat(1).map((child, idx)=> {
                let key = (child.props && child.props.key) || `__${idx}`;
                mount( dom_map[key]? patch(dom_map[key], child, dom) : render(child, dom) );
                delete dom_map[key]; // marks dom-vnode pair available by removing from map
            });

            Object.values(dom_map).forEach(element => { // Unmount DOM nodes which are missing in the latest vnodes
                runHook( element.instance, 'componentWillUnmount');
                element.remove();
            });

            !isFunction(vnode.type) && Object.keys(vnode.props).map((key)=> setAttribute(dom, key, vnode.props[key]));
        }
    }
    else if( isTextNode(dom) && dom.textContent != vnode ) return replace( render(vnode, parent), dom, parent );
}

function setAttribute(dom, key, value) {
    if( key.startsWith('on') && isFunction(value) ) delegateEvent(dom, key, value);
    else if( key == 'ref' && isFunction( value ) ) value( dom );
    else if( ['checked', 'value', 'className', 'key'].includes(key) ) dom[key=='key'? '_idx' :key] = value;
    else dom.setAttribute(key, value);
}

// Utils
const isFunction = ( node ) => typeof node == 'function';
const isObject = ( node ) => typeof node  == 'object';
const isTextNode = ( node ) => node.nodeType == 3;
const replace = (el, dom, parent)=> (parent && parent.replaceChild(el, dom) && el);
const mount = (el, parent)=> parent? parent.appendChild( el ) : el;
const isClassComponent = ( node ) => Component.isPrototypeOf( node.type );
const runHook = (instance, hook, ...args) => isFunction(instance && instance[hook]) ? instance[hook]( ...args) : true;
const delegateEvent = (dom, event, handler)=> {
    event = event.slice(2).toLowerCase();
    dom._evnt = dom._evnt || {};
    dom.removeEventListener(event, dom._evnt[ event ]);
    dom.addEventListener(event, dom._evnt[ event ] = handler);
}
const combineChildrenWithProps = ({ props, children })=> Object.assign({}, props, { children });
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Top comments (1)

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temaovchinnikov profile image
TemaOvchinnikov •

Good work. I tried it too, but it didn't work.
codepen

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