React hooks clone and related concepts

Sep 12 2020

Last updated on Feb 18 2025

Getting Closure on Hooks presented by @swyx

// https://www.youtube.com/watch?v=KJP1E-Y-xyo
const React = (function() {
  let hooks = [];
  let idx = 0;

  function useState(initVal) {
    const state = hooks[idx] || initVal;
    const _idx = idx;
    const setState = newVal => {
      hooks[_idx] = newVal;
    };
    idx++;
    return [state, setState];
  }

  function useEffect(cb, depArray) {
    const oldDeps = hooks[idx];
    let hasChanged = true;

    if(oldDeps) {
      hasChanged = depArray.some((dep, i) => !Object.is(dep, oldDeps[i]));
    }
    if(hasChanged) cb();
    hooks[idx] = depArray;
    idx++;
  }

  function render(Component) {
    idx = 0;
    const C = Component();
    C.render();
    return C;
  }

  return { useState, useEffect, render };
})();

function Component() {
  const [count, setCount] = React.useState(1);
  const [text, setText] = React.useState('apple');
  
  React.useEffect(() => {
    console.log('useEffect with count dep')
  }, [count]);
  
  React.useEffect(() => {
    console.log('useEffect empty dep')
  }, []);

  React.useEffect(() => {
    console.log('useEffect no dep')
  });

  return {
    render: () => console.log({count, text}),
    click: () => setCount(count + 1),
    type: word => setText(word)
  }
}

var App = React.render(Component);
App.click();
var App = React.render(Component);
App.type('pear');
var App = React.render(Component);

/*
  useEffect with count dep
  useEffect empty dep
  useEffect no dep
  {count: 1, text: "apple"} 

  useEffect with count dep
  useEffect no dep
  {count: 2, text: "apple"}

  useEffect no dep
  {count: 2, text: "pear"}`
*/

Simple React custom renderer

const React = {
  createElement: (tag, props, ...children) => {
    if (typeof tag === "function") {
      return tag(props);
    }
    let element = { tag, props: { ...props, children } };
    return element;
  }
}

const render = (reactElementOrText, container) => {
  if (['string', 'number'].includes(typeof reactElementOrText)) {
    container.appendChild(document.createTextNode(String(reactElementOrText)));
    return;
  }

  let actualElement = document.createElement(reactElementOrText.tag);
  if (reactElementOrText.props) {
    Object.keys(reactElementOrText.props).filter(p => p !== children).forEach(p => {
      actualElement.setAttribute(p, reactElementOrText.props[p])
    })
  }
  if (reactElementOrText.props.children) {
    reactElementOrText.props.children.forEach(child => {
      render(child, actualElement)
    })
  }
  container.appendChild(actualElement);
}

const App = () => (
  <div className="react-div">
    <h1>Hello</h1>
    <p>Some text here</p>
  </div>
)

render(<App />, document.querySelector("#app"));

You Might Not Need an Effect

Whenever you think of writing useEffect, the only sane thing is to NOT do it. Instead, go to the react docs and re-read the page about why you don’t need an effect. You really don’t. -@TkDodo

When developing an application in React 18+, you may encounter an issue where the useEffect hook is being run twice on mount. This occurs because since React 18, when you are in development, your application is being run in StrictMode by default. In Strict Mode, React will try to simulate the behavior of mounting, unmounting, and remounting a component to help developers uncover bugs during testing. From the user’s perspective, visiting a page shouldn’t be different from visiting it, clicking a link, and then pressing Back. React verifies that your components don’t break this principle by remounting them once in development. In most cases, it should be fine to leave your code as-is, since the useEffect will only run once in production.

Referencing Values with Refs

When you want a component to “remember” some information, but you don’t want that information to trigger new renders, you can use a ref. Typically, you will use a ref when your component needs to “step outside” React and communicate with external APIs. (e.g. storing timeout IDs, DOM elements)

Refs are an escape hatch to hold onto values that aren’t used for rendering. You won’t need them often.
A ref is a plain JavaScript object with a single property called current, which you can read or set.
You can ask React to give you a ref by calling the useRef Hook.
Like state, refs let you retain information between re-renders of a component.
Unlike state, setting the ref’s current value does not trigger a re-render.

function usePrevious(value) {
  const ref = useRef();

  useEffect(() => {
    // this update does not trigger a re-render
    ref.current = value;
  }, [value]);

  return ref.current;
}

`ref` callback function

Instead of a ref object, you may pass a function to the ref attribute. When the <div> DOM node is added to the screen, React will call your ref callback with the DOM node as the argument. When that <div> DOM node is removed, React will call your ref callback with null. React will also call your ref callback whenever you pass a different ref callback.

const scroller = (node: HTMLDivElement | null) => {
  node?.scrollIntoView({ behavior: "smooth" });
};

const ChatWindow = () => {
  return (
    <>
      {Array.from(Array(100).keys()).map((e) => (
        <div key={e}>Chat message: {e}</div>
      ))}
      <div ref={scroller} />
    </>
  );
};

So if you need to interact with DOM nodes directly after they rendered, try not to jump to useRef + useEffect directly, but consider using callback refs instead.

batching and flushSync in rendering

In early versions (React 17 and earlier), React updated the DOM immediately after each state change. Multiple state updates within a single event cycle would cause multiple, unnecessary re-renders, affecting the application’s responsiveness.

const handleUpdate = () => {
  setCount(count + 1); // First update
  setFlag(!flag);      // Second update
  // In pre-batching React, this would cause two separate renders
};

React 18 introduced batching to prevent these issues. Batching means that React groups multiple state updates into a single re-render cycle. This approach ensures that the UI is updated efficiently, reflecting all state changes in one go.

flushSync allows you to opt-out of batching for specific updates, forcing them to be processed immediately. This ensures that critical updates are executed in the correct order, even within a batched state update cycle. (But, use it carefully and not too much, because using it too often can cancel out the performance advantages of batching.)

// https://tigerabrodi.blog/understanding-flushsync-mastering-batching-behavior-in-reactjs
const receiveMessage = () => {
  const newMessage = `Message ${messages.length + 1}`;

  // Update messages and Forces re-render
  flushSync(() => {
    setMessages(prevMessages => [...prevMessages, newMessage]);
  });

  // Scroll to the bottom after messages update
  endOfMessagesRef.current?.scrollIntoView({ behavior: "smooth" });
};

Higher Order Components

HOCs are wrapper components that help provide additional functionality to existing components. While hooks probably replaced most of shared logic concerns, there are still use cases where higher-order components could be useful. For example, you want to fire analytics event on every click of every button, dropdown and link everywhere.

export const withLoggingOnClick = (Component) => {
  return (props) => {
    const log = useLoggingFromSomewhere();

    const onClick = () => {
      // console.info('Log on click something');
      log('Log on click something');
      props.onClick();
    };

    // return original component with all the props
    // and overriding onClick with our own callback
    return <Component {...props} onClick={onClick} />;
  };
};

What’s new in React 17+

React 17:

React 18:

React 19:

Kexi's blog

blog/notes/digital garden

Kexi's blog

Congradulations,
you find the Easter egg

React hooks clone and related concepts

Getting Closure on Hooks presented by @swyx

Simple React custom renderer

You Might Not Need an Effect

Referencing Values with Refs

`ref` callback function

batching and flushSync in rendering

Higher Order Components

What’s new in React 17+

React hooks clone and related concepts

Getting Closure on Hooks presented by @swyx

Simple React custom renderer

You Might Not Need an Effect

Referencing Values with Refs

ref callback function

batching and flushSync in rendering

Higher Order Components

What’s new in React 17+

`ref` callback function