仓库源文，站点原文

title: "React hooks clone and related concepts" description: "" added: "Sep 12 2020" tags: [react, code]

updatedDate: "Dev 4 2024"

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"}`
*/

Vanilla React data fetching

If you're going to fetch in useEffect(), you should at least make sure that you're handling:

import * as React from "react"

export default function useQuery(url) {
  const [data, setData] = React.useState(null)
  const [isLoading, setIsLoading] = React.useState(true)
  const [error, setError] = React.useState(null)

  React.useEffect(() => {
    let ignore = false  // isCancelled

    const handleFetch = async () => {
      setData(null)
      setIsLoading(true)
      setError(null)

      try {
        const res = await fetch(url)

        if (ignore) {
          return 
        }

        if (res.ok === false) {
          throw new Error(`A network error occurred.`)
        }

        const json = await res.json()

        setData(json)
        setIsLoading(false)
      } catch (e) {
        setError(e.message)
        setIsLoading(false)
      }
    }

    handleFetch()

    return () => {
      ignore = true
    }
  }, [url])

  return { data, isLoading, error }
}

In reality, we still need to think about:

  1. For every component that needs the same data, we have to refetch it.
  2. It's possible that while fetching to the same endpoint, one request could fail while the other succeeds.
  3. If our state is moved to "global", we've just introduced a small, in-memory cache. Since we've introduced a cache, we also need to introduce a way to invalidate it.
  4. Context often becomes confusing over time. A component subscribed to QueryContext will re-render whenever anything changes – even if the change isn't related to the url it cares about.
  5. We're treating asynchronous state as if it were synchronous state.

That's why React Query was created.

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)

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.

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: