layout: "../layouts/BlogPost.astro" title: "React hooks clone and related concepts" slug: react-hooks-clone-and-related-concepts description: "" added: "Sep 12 2020" tags: [react, code]
// 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"}`
*/
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 }
}
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.
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)
current
, which you can read or set.useRef
Hook.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} />;
};
};
React 17:
React 18:
React 19: