script.js

/** @jsxRuntime classic */

function createElement(type, props, ...children) {
  return {
    type,
    props: {
      ...props,
      children: children
        .flat()
        .map((c) => (typeof c === "object" ? c : createTextElement(c))),
    },
  };
}

function createTextElement(text) {
  return {
    type: "TEXT_ELEMENT",
    props: {
      nodeValue: text,
      children: [],
    },
  };
}

let nextUnitOfWork = null;
let currentRoot = null;
let wipRoot = null;
let deletions = null;

function commitWork(fiber) {
  if (!fiber) {
    return;
  }

  // crawl up the fiber tree until we find a fiber with  a dom node
  let domParentFiber = fiber.parent;
  while (!domParentFiber.dom) {
    domParentFiber = domParentFiber.parent;
  }
  const domParent = domParentFiber.dom;

  if (fiber.effectTag === "PLACEMENT" && fiber.dom != null) {
    domParent.appendChild(fiber.dom);
  } else if (fiber.effectTag === "UPDATE" && fiber.dom != null) {
    updateDom(fiber.dom, fiber.alternate.props, fiber.props);
  } else if (fiber.effectTag === "DELETION") {
    commitDeletion(fiber, domParent);
  }

  commitWork(fiber.child);
  commitWork(fiber.sibling);
}

function commitDeletion() {
  if (fiber.dom) {
    domParent.removeChild(fiber.dom);
  } else {
    commitDeletion(fiber.child, domParent);
  }
}

function commitRoot() {
  deletions.forEach(commitWork);
  commitWork(wipRoot.child);
  currentRoot = wipRoot;
  wipRoot = null;
}

function render(element, container) {
  wipRoot = {
    dom: container,
    props: {
      children: [element],
    },
    alternate: currentRoot,
  };
  deletions = [];
  nextUnitOfWork = wipRoot;
}

const isEvent = (key) => key.startsWith("on");
const isProp = (key) => key !== "children" && !isEvent(key);
const isNew = (prev, next) => (key) => prev[key] !== next[key];
const isGone = (prev, next) => (key) => !(key in next);

function updateDom(dom, prevProps, nextProps) {
  // remove old or changed event listeners
  Object.keys(prevProps)
    .filter(isEvent)
    .filter((key) => !(key in nextProps) || isNew(prevProps, nextProps)(key))
    .forEach((name) => {
      const eventType = name.toLowerCase().substring(2);
      dom.removeEventListener(eventType, prevProps[name]);
    });

  // remove old properties
  Object.keys(prevProps)
    .filter(isProp)
    .filter(isGone(prevProps, nextProps))
    .forEach((name) => {
      dom[name] = "";
    });

  // set new or changed properties
  Object.keys(nextProps)
    .filter(isProp)
    .filter(isNew(prevProps, nextProps))
    .forEach((name) => (dom[name] = nextProps[name]));

  // add back event listeners
  Object.keys(nextProps)
    .filter(isEvent)
    .filter(isNew(prevProps, nextProps))
    .forEach((name) => {
      const eventType = name.toLowerCase().substring(2);
      dom.addEventListener(eventType, nextProps[name]);
    });
}

function createDom(fiber) {
  // recursively render the fiber and its children
  // a fiber is a unit of work; one fiber = one element
  const dom =
    fiber.type === "TEXT_ELEMENT"
      ? document.createTextNode("")
      : document.createElement(fiber.type);

  // assign the element props to our node
  updateDom(dom, {}, fiber.props);
  return dom;
}

function workLoop(deadline) {
  let shouldYield = false;
  while (nextUnitOfWork && !shouldYield) {
    nextUnitOfWork = performUnitOfWork(nextUnitOfWork);
    shouldYield = deadline.timeRemaining() < 1;
  }

  // if we've completed all the work, let's commit the whole fiber tree to the dom
  if (!nextUnitOfWork && wipRoot) {
    commitRoot();
  }

  requestIdleCallback(workLoop); // setTimeout-like, runs when main is idle
}

requestIdleCallback(workLoop);

function performUnitOfWork(fiber) {
  if (fiber.type instanceof Function) {
    updateFunctionComponent(fiber);
  } else {
    updateHostComponent(fiber);
  }

  // search for the next unit of work
  if (fiber.child) return fiber.child;
  let nextFiber = fiber;
  while (nextFiber) {
    if (nextFiber.sibling) return nextFiber.sibling;
    nextFiber = nextFiber.parent;
  }
}

let wipFiber = null;
let hookIndex = null;
function updateFunctionComponent(fiber) {
  wipFiber = fiber;
  hookIndex = 0;
  wipFiber.hooks = [];
  // for FC, we actually run the function to get the children
  const children = [fiber.type(fiber.props)];
  reconcileChildren(fiber, children);
}

function updateHostComponent(fiber) {
  if (!fiber.dom) {
    fiber.dom = createDom(fiber);
  }
  reconcileChildren(fiber, fiber.props.children);
}

function reconcileChildren(wipFiber, elements) {
  let index = 0;
  let oldFiber = wipFiber.alternate && wipFiber.alternate.child;
  let prevSibling = null;

  // for each child elem, create a fiber
  while (index < elements.length || oldFiber != null) {
    const element = elements[index];
    let newFiber = null;

    const sameType = oldFiber && element && element.type == oldFiber.type;

    if (sameType) {
      // update the node, this fiber is the same
      newFiber = {
        type: oldFiber.type,
        props: element.props,
        dom: oldFiber.dom,
        parent: wipFiber,
        alternate: oldFiber,
        effectTag: "UPDATE",
      };
    }

    if (element && !sameType) {
      // add this node
      newFiber = {
        type: element.type,
        props: element.props,
        dom: null,
        parent: wipFiber,
        alternate: null,
        effectTag: "PLACEMENT",
      };
    }

    if (oldFiber && !sameType) {
      // delete fiber's node
      oldFiber.effectTag = "DELETION";
      deletions.push(oldFiber);
    }

    if (oldFiber) {
      oldFiber = oldFiber.sibling;
    }

    // add to the fiber tree, setting it as either a child or sibling
    if (index === 0) {
      wipFiber.child = newFiber;
    } else if (element) {
      prevSibling.sibling = newFiber;
    }
    prevSibling = newFiber;
    index++;
  }
}

function useState(initial) {
  const oldHook =
    wipFiber.alternate &&
    wipFiber.alternate.hooks &&
    wipFiber.alternate.hooks[hookIndex];
  const hook = {
    state: oldHook ? oldHook.state : initial,
    queue: [],
  };

  const actions = oldHook ? oldHook.queue : [];
  actions.forEach((action) => {
    hook.state = action(hook.state);
  });

  const setState = (action) => {
    hook.queue.push(action);
    wipRoot = {
      dom: currentRoot.dom,
      props: currentRoot.props,
      alternate: currentRoot,
    };
    nextUnitOfWork = wipRoot;
    deletions = [];
  };
  wipFiber.hooks.push(hook);
  hookIndex++;
  return [hook.state, setState];
}

const CoolReact = {
  createElement,
  render,
  useState,
};

/** Example function component */
function NameTag(props) {
  return <h1>Hi {props.name}</h1>;
}

function Counter(props) {
  const [state, setState] = CoolReact.useState(props.initial);
  const emoji = new Array(state).fill("🔥");
  return (
    <div>
      <button onClick={() => setState((c) => c + 1)}>{state} Clicks</button>
      <p>{emoji}</p>
    </div>
  );
}

/* @jsx CoolReact.createElement */
const element = (
  <div id="foo">
    <h1>very important website</h1>
    <p id="p">I get to see my daddy on july 14th</p>
    <img src="public/cameron-poe.gif" />
    <NameTag name="cameron" />
    <Counter initial={5} />
  </div>
);

const container = document.getElementById("root");
CoolReact.render(element, container);