Build a Tiny React Chpdating vDOM

This tutorial is based on this tutorial, but with JSX, typescript and an easier approach to implement. You can checkout the notes and code on my GitHub repo.

Now let's talk about the reactivity.

Save the Old Fiber

We need to save the old fiber so that we can compare it with the new fiber. We can do this by adding a field to the fiber. We also need a committed field- which will be useful later.

export interface Fiber {
  type: string
  props: VDomAttributes
  parent: Fiber | null
  child: Fiber | null
  sibling: Fiber | null
  dom: HTMLElement | Text | null
  alternate: Fiber | null
  committed: boolean
}

Then we set the committed state here,

function commit() {
    function commitChildren(fiber: Fiber | null) {
        if(!fiber) {
            return
        }
        if(fiber.dom && fiber.parent?.dom) {
            fiber.parent.dom.appendChild(fiber.dom)
            fiber.committed = true
        }
        if(fiber.dom && fiber.parent && isFragment(fiber.parent.vDom) && !fiber.committed) {
            let parent = fiber.parent
            // find the first parent that is not a fragment
            while(parent && isFragment(parent.vDom)) {
                // the root element is guaranteed to not be a fragment has has a non-fragment parent
                parent = parent.parent!
            }
            parent.dom?.appendChild(fiber.dom!)
            fiber.committed = true
        }
        commitChildren(fiber.child)
        commitChildren(fiber.sibling)
    }
    commitChildren(wip)
    wipParent?.appendChild(wip!.dom!)
    wip!.committed = true
    wip = null
}

We also need to save the old fiber tree.

let oldFiber: Fiber | null = null

function commit() {
    function commitChildren(fiber: Fiber | null) {
        if(!fiber) {
            return
        }
        if(fiber.dom && fiber.parent?.dom) {
            fiber.parent.dom.appendChild(fiber.dom)
            fiber.committed = true
        }
        commitChildren(fiber.child)
        commitChildren(fiber.sibling)
    }
    commitChildren(wip)
    wipParent?.appendChild(wip!.dom!)
    wip!.committed = true
    oldFiber = wip
    wip = null
}

Now, we need to compare the old fiber with the new fiber during iteration. This is called the reconciliation process.

Reconciliation

We need to compare the old fiber with the new fiber. We first put the old fiber in the initial work.

export function render(vDom: VDomNode, parent: HTMLElement) {
    wip = {
        parent: null,
        sibling: null,
        child: null,
        vDom: vDom,
        dom: null,
        committed: false,
        alternate: oldFiber,
    }
    wipParent = parent
    nextUnitOfWork = wip
}

Then we separate the creation of the new fiber into a new function.

function reconcile(fiber: Fiber, isFragment: boolean) {
    if (isElement(fiber.vDom)) {
        const elements = fiber.vDom.children ?? []
        let index = 0
        let prevSibling = null

        while (index < elements.length) {
            const element = elements[index]
            const newFiber: Fiber = {
                parent: isFragment ? fiber.parent : fiber,
                dom: null,
                sibling: null,
                child: null,
                vDom: element,
                committed: false,
                alternate: null,
            }

            if (index === 0) {
                fiber.child = newFiber
            } else {
                prevSibling!.sibling = newFiber
            }
            prevSibling = newFiber
            index++
        }
    }
}

function performUnitOfWork(nextUnitOfWork: Fiber | null): Fiber | null {
    if(!nextUnitOfWork) {
        return null
    }
    const fiber = nextUnitOfWork
    const isFragment = isElement(fiber.vDom) && fiber.vDom.tag === '' && fiber.vDom.kind === 'fragment'

    if(!fiber.dom && !isFragment) {
        fiber.dom = createDom(fiber.vDom)
    }

    reconcile(fiber, isFragment)

    if (fiber.child) {
        return fiber.child
    }
    let nextFiber: Fiber | null = fiber
    while (nextFiber) {
        if (nextFiber.sibling) {
          return nextFiber.sibling
        }
        nextFiber = nextFiber.parent
    }
    return null
}

However, we need to mount the old fiber onto the new one.

function reconcile(fiber: Fiber, isFragment: boolean) {
    if (isElement(fiber.vDom)) {
        const elements = fiber.vDom.children ?? []
        let index = 0
        let prevSibling = null

        let currentOldFiber = fiber.alternate?.child ?? null
        while (index < elements.length) {
            const element = elements[index]
            const newFiber: Fiber = {
                parent: isFragment ? fiber.parent : fiber,
                dom: null,
                sibling: null,
                child: null,
                vDom: element,
                committed: false,
                alternate: currentOldFiber,
            }

            if (index === 0) {
                fiber.child = newFiber
            } else {
                prevSibling!.sibling = newFiber
            }
            prevSibling = newFiber
            currentOldFiber = currentOldFiber?.sibling ?? null
            index++
        }
    }
}

Now we have the old fiber mounted to the new fiber. But we don't have anything to trigger the re-rendering- for now, we manually trigger it by adding a button. Since we don't yet have state yet, we use props for mutating the vDOM.

import { render } from "./runtime";
import { createElement, fragment, VDomAttributes, VDomNode } from "./v-dom";

type FuncComponent = (props: VDomAttributes, children: VDomNode[]) => JSX.Element

const App: FuncComponent = (props: VDomAttributes, __: VDomNode[]) => {
    return <div>
        <>
            <h1>H1</h1>
            <h2>{props["example"]?.toString()}</h2>
            {
                props["show"] ? <p>show</p> : <></>
            }
            <h1>H1</h1>
        </>
    </div>
}
const app = document.getElementById('app')

const renderButton = document.createElement('button')
renderButton.textContent = 'Render'
let cnt = 0
renderButton.addEventListener('click', () => {
    const vDom: VDomNode = App({
        "example": (new Date()).toString(),
        "show": cnt % 2 === 0
    }, []) as unknown as VDomNode
    cnt++
    render(vDom, app!)
})
document.body.appendChild(renderButton)

Now if you click the renderButton, the rendered result will repeat once, since, well, all our current logic is simply putting the rendered vDOM into the document.

If you add a console.log in the commit function, you can see the alternate fiber being printed out.

Now we need to define how we handle the old fiber and the new fiber, and mutate the DOM based on the information. The rules is as follows.

For each new fiber,

• If there was an old fiber, we compare the content of the old fiber with the new fiber, if they are different, we replace the old DOM node with the new DOM node, or else we copy the old DOM node to the new DOM node. Please note that, by two vDOM being equal, we mean their tags and all properties are equal. Their children can be different.
• If there has no old fiber, we create a new DOM node and append it to the parent.
• If, for the new fiber, it doesn't have a child or a sibling, but its old fiber has a child or a sibling, we recursively remove the old child or sibling.

Kind of confused? Well, I'll just show the code. We first delete the old DOM creation. Then apply the rules above.

The first rule, if there is an old fiber, we compare the content of the old fiber with the new fiber. If they are different, we replace the old DOM node with the new DOM node, or else we copy the old DOM node to the new DOM node.

export function vDOMEquals(a: VDomNode, b: VDomNode): boolean {
    if (isString(a) && isString(b)) {
        return a === b
    } else if (isElement(a) && isElement(b)) {
        let ret = a.tag === b.tag && a.key === b.key
        if (!ret) return false
        if (a.props && b.props) {
            const aProps = a.props
            const bProps = b.props
            const aKeys = Object.keys(aProps)
            const bKeys = Object.keys(bProps)
            if (aKeys.length !== bKeys.length) return false
            for (let i = 0; i < aKeys.length; i++) {
                const key = aKeys[i]
                if (key === 'key') continue
                if (aProps[key] !== bProps[key]) return false
            }
            for (let i = 0; i < bKeys.length; i++) {
                const key = bKeys[i]
                if (key === 'key') continue
                if (aProps[key] !== bProps[key]) return false
            }
            return true
        } else {
            return a.props === b.props
        }
    } else {
        return false
    }
}

Then I made some small refactor,

export interface Fiber {
  type: string
  props: VDomAttributes
  parent: Fiber | null
  child: Fiber | null
  sibling: Fiber | null
  dom: HTMLElement | Text | null
  alternate: Fiber | null
  committed: boolean
}

Now, when it comes to commit, we have an extra alternative field to compare the old fiber with the new fiber.

This is the original commit function,

function commit() {
    function commitChildren(fiber: Fiber | null) {
        if(!fiber) {
            return
        }
        if(fiber.dom && fiber.parent?.dom) {
            fiber.parent.dom.appendChild(fiber.dom)
            fiber.committed = true
        }
        if(fiber.dom && fiber.parent && isFragment(fiber.parent.vDom) && !fiber.committed) {
            let parent = fiber.parent
            // find the first parent that is not a fragment
            while(parent && isFragment(parent.vDom)) {
                // the root element is guaranteed to not be a fragment has has a non-fragment parent
                parent = parent.parent!
            }
            parent.dom?.appendChild(fiber.dom!)
            fiber.committed = true
        }
        commitChildren(fiber.child)
        commitChildren(fiber.sibling)
    }
    commitChildren(wip)
    wipParent?.appendChild(wip!.dom!)
    wip!.committed = true
    wip = null
}

We will change the name a bit. The old name is just wrong (I'm sorry for that).

let oldFiber: Fiber | null = null

function commit() {
    function commitChildren(fiber: Fiber | null) {
        if(!fiber) {
            return
        }
        if(fiber.dom && fiber.parent?.dom) {
            fiber.parent.dom.appendChild(fiber.dom)
            fiber.committed = true
        }
        commitChildren(fiber.child)
        commitChildren(fiber.sibling)
    }
    commitChildren(wip)
    wipParent?.appendChild(wip!.dom!)
    wip!.committed = true
    oldFiber = wip
    wip = null
}

Appending, Copying and Replacing

So what should we do? Our old logic is only appending, so we extract that,

export function render(vDom: VDomNode, parent: HTMLElement) {
    wip = {
        parent: null,
        sibling: null,
        child: null,
        vDom: vDom,
        dom: null,
        committed: false,
        alternate: oldFiber,
    }
    wipParent = parent
    nextUnitOfWork = wip
}

We need to delay the construction of the DOM until the commit phase, to provide more flexibility.

function reconcile(fiber: Fiber, isFragment: boolean) {
    if (isElement(fiber.vDom)) {
        const elements = fiber.vDom.children ?? []
        let index = 0
        let prevSibling = null

        while (index < elements.length) {
            const element = elements[index]
            const newFiber: Fiber = {
                parent: isFragment ? fiber.parent : fiber,
                dom: null,
                sibling: null,
                child: null,
                vDom: element,
                committed: false,
                alternate: null,
            }

            if (index === 0) {
                fiber.child = newFiber
            } else {
                prevSibling!.sibling = newFiber
            }
            prevSibling = newFiber
            index++
        }
    }
}

function performUnitOfWork(nextUnitOfWork: Fiber | null): Fiber | null {
    if(!nextUnitOfWork) {
        return null
    }
    const fiber = nextUnitOfWork
    const isFragment = isElement(fiber.vDom) && fiber.vDom.tag === '' && fiber.vDom.kind === 'fragment'

    if(!fiber.dom && !isFragment) {
        fiber.dom = createDom(fiber.vDom)
    }

    reconcile(fiber, isFragment)

    if (fiber.child) {
        return fiber.child
    }
    let nextFiber: Fiber | null = fiber
    while (nextFiber) {
        if (nextFiber.sibling) {
          return nextFiber.sibling
        }
        nextFiber = nextFiber.parent
    }
    return null
}

Following the first and second rule, we refactor them into the following code,

function reconcile(fiber: Fiber, isFragment: boolean) {
    if (isElement(fiber.vDom)) {
        const elements = fiber.vDom.children ?? []
        let index = 0
        let prevSibling = null

        let currentOldFiber = fiber.alternate?.child ?? null
        while (index < elements.length) {
            const element = elements[index]
            const newFiber: Fiber = {
                parent: isFragment ? fiber.parent : fiber,
                dom: null,
                sibling: null,
                child: null,
                vDom: element,
                committed: false,
                alternate: currentOldFiber,
            }

            if (index === 0) {
                fiber.child = newFiber
            } else {
                prevSibling!.sibling = newFiber
            }
            prevSibling = newFiber
            currentOldFiber = currentOldFiber?.sibling ?? null
            index++
        }
    }
}

Please always keep in mind that in javascript, all values are references. If we have fiber.dom = fiber.alternate.dom, then fiber.dom and fiber.alternate.dom will point to the same object. If we change fiber.dom, fiber.alternate.dom will also change, and vice versa. That's why when replacing, we simply used fiber.alternate.dom?.replaceWith(fiber.dom). This will replace the old DOM with the new DOM. While previous parents, if copied, have the fiber.alternate.dom for their DOM, their DOM will also be replaced.

However, we hadn't handled deletion yet.

Some Mishaps

Okay, previous code contains some bugs that I spotted as I am writing more complex jsx, so, before implementing the deletion, let's fix them.

Previously there was a bug- we can not pass list to props, let's use this chance to fix it.

import { render } from "./runtime";
import { createElement, fragment, VDomAttributes, VDomNode } from "./v-dom";

type FuncComponent = (props: VDomAttributes, children: VDomNode[]) => JSX.Element

const App: FuncComponent = (props: VDomAttributes, __: VDomNode[]) => {
    return <div>
        <>
            <h1>H1</h1>
            <h2>{props["example"]?.toString()}</h2>
            {
                props["show"] ? <p>show</p> : <></>
            }
            <h1>H1</h1>
        </>
    </div>
}
const app = document.getElementById('app')

const renderButton = document.createElement('button')
renderButton.textContent = 'Render'
let cnt = 0
renderButton.addEventListener('click', () => {
    const vDom: VDomNode = App({
        "example": (new Date()).toString(),
        "show": cnt % 2 === 0
    }, []) as unknown as VDomNode
    cnt++
    render(vDom, app!)
})
document.body.appendChild(renderButton)

Then you just fix the type things- only one error for me, so, do it yourself please.

However, if we have the following code,

export function vDOMEquals(a: VDomNode, b: VDomNode): boolean {
    if (isString(a) && isString(b)) {
        return a === b
    } else if (isElement(a) && isElement(b)) {
        let ret = a.tag === b.tag && a.key === b.key
        if (!ret) return false
        if (a.props && b.props) {
            const aProps = a.props
            const bProps = b.props
            const aKeys = Object.keys(aProps)
            const bKeys = Object.keys(bProps)
            if (aKeys.length !== bKeys.length) return false
            for (let i = 0; i < aKeys.length; i++) {
                const key = aKeys[i]
                if (key === 'key') continue
                if (aProps[key] !== bProps[key]) return false
            }
            for (let i = 0; i < bKeys.length; i++) {
                const key = bKeys[i]
                if (key === 'key') continue
                if (aProps[key] !== bProps[key]) return false
            }
            return true
        } else {
            return a.props === b.props
        }
    } else {
        return false
    }
}

Our thing broke again...

Okay, this is because children can be nested arrays in the above case, we need to flat them.

But that's not enough, ugh, our createDom only recognize either string or element, not integer, so, we need toString the numbers.

function buildDom(fiber: Fiber, fiberIsFragment: boolean) {
    if(fiber.dom) return
    if(fiberIsFragment) return
    fiber.dom = createDom(fiber.vDom)
}

function performUnitOfWork(nextUnitOfWork: Fiber | null): Fiber | null {
    if(!nextUnitOfWork) {
        return null
    }
    const fiber = nextUnitOfWork
    const fiberIsFragment = isFragment(fiber.vDom)

    reconcile(fiber)

    buildDom(fiber, fiberIsFragment);

    if (fiber.child) {
        return fiber.child
    }
    let nextFiber: Fiber | null = fiber
    while (nextFiber) {
        if (nextFiber.sibling) {
          return nextFiber.sibling
        }
        nextFiber = nextFiber.parent
    }
    return null
}

Okay, things work now- kind of.

If you hit the render button, the list is updated- but the old element still remains. We need to delete the old element.

Remove

We restate the rule here- for any new fiber, if it does not have a child or a sibling, but its old fiber has a child or a sibling, we recursively remove the old child or sibling.

function commit() {
    function commitChildren(fiber: Fiber | null) {
        if(!fiber) {
            return
        }
        if(fiber.dom && fiber.parent?.dom) {
            fiber.parent?.dom?.appendChild(fiber.dom)
            fiber.committed = true
        }

        if(fiber.dom && fiber.parent && isFragment(fiber.parent.vDom) && !fiber.committed) {
            let parent = fiber.parent
            // find the first parent that is not a fragment
            while(parent && isFragment(parent.vDom)) {
                // the root element is guaranteed to not be a fragment has has a non-fragment parent
                parent = parent.parent!
            }
            parent.dom?.appendChild(fiber.dom!)
            fiber.committed = true
        }

        commitChildren(fiber.child)
        commitChildren(fiber.sibling)
    }
    commitChildren(wip)
    wipParent?.appendChild(wip!.dom!)
    wip!.committed = true
    oldFiber = wip
    wip = null
}

If you don't do recursive remove, some old elements will dangle when you have multiple things requires deletion. You can change to,

function commit() {
    function commitToParent(fiber: Fiber | null) {
        if(!fiber) {
            return
        }
        if(fiber.dom && fiber.parent?.dom) {
            fiber.parent?.dom?.appendChild(fiber.dom)
            fiber.committed = true
        }

        if(fiber.dom && fiber.parent && isFragment(fiber.parent.vDom) && !fiber.committed) {
            let parent = fiber.parent
            // find the first parent that is not a fragment
            while(parent && isFragment(parent.vDom)) {
                // the root element is guaranteed to not be a fragment has has a non-fragment parent
                parent = parent.parent!
            }
            parent.dom?.appendChild(fiber.dom!)
            fiber.committed = true
        }

        commitToParent(fiber.child)
        commitToParent(fiber.sibling)
    }
    commitToParent(wip)
    wipParent?.appendChild(wip!.dom!)
    wip!.committed = true
    oldFiber = wip
    wip = null
}

For reference.

Summary

This is a hard chapter- but pretty traditional coding, to be honest. However, up to now, you have understand how React works from bottom to top.

Actually, things can already work now- we can manually trigger a re-render whenever we change the props. However, such frustrating manual work is not what we want. We want the reactivity to be automatic. So, we will talk about hooks in the next chapter.

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