All the fundamental React.js Core Concepts that you should know as a React Developer | by Nadim Mahmud | May, 2021 | Medium

const [ user, setUser ] = useState(JSON.parse(localStorage.getItem('profile'))); //convert to object

const logout = () =>{
        dispatch({type: 'LOGOUT'});
        history.push("/");
        setUser(null);
    }

    useEffect(()=>{
        const token = user?.token;

        //JWT check if token expired
        if(token){
            const decodedToken = decode(token)
            if(decodedToken.exp*1000 < newDate().getTime()) logout();
        }
        setUser(JSON.parse(localStorage.getItem('profile')))
    },[location])

const [animateHeader, setAnimateHeader] = useState(false)

useEffect(() => {
  const listener = () =>
  window.scrollY > 140 ? setAnimateHeader(true) : setAnimateHeader(false)
  window.addEventListener('scroll', listener)
  return () => {
    window.removeEventListener('scroll', listener)
  }
}, [])

// Example usage with TailwindCSS:
<div className={`bg-header/75 transition duration-500 ease-in-out ${animateHeader && 'bg-header/[0.95]'}`}>...</div>

function App() {
  const [checked, setChecked] = useState(false);
  const toggleChecked = () => setChecked(value => !value);
  return (
    <input
      type="checkbox"
      checked={checked}
      onChange={toggleChecked}
    />
  );
}

// making all uppercase
city.toUpperCase();

//removing spaces and making all lowercase
city = city.trim().toLowerCase();

//capitalizing the first letter
cityCapitalized = city.charAt(0).toUpperCase() + city.slice(1);

onClick={() => window.open(url, "_blank")}

import { useEffect } from 'react';

export const useClickOutside = (ref, setIsModalOpen) => {
  useEffect(() => {
    function handleClickOutside(event) {
      if (ref.current && !ref.current.contains(event.target)) {
        setIsModalOpen(false)
      }
    }
    document.addEventListener("mousedown", handleClickOutside);
    return () => {
      document.removeEventListener("mousedown", handleClickOutside);
    };
  }, [ref]);
};

this.state = {
	zombies:[
    {
    	name: 'Zombie'
    },{
    	name: 'Zombie'
    } {
    	name: 'Zombie'
    } {
    	name: 'Zombie'
    } {
    	name: 'Zombie'
    }
    ]
}

render() {
	return (
    	<div>
        {
        	this.state.zombies.map(zombie => <h1>{zombie.name}</h1>)
        }
        
        </div>
    )
}

import React, { useState, useEffect } from 'react';

function Example() {
  const [count, setCount] = useState(0);


  // Similar to componentDidMount and componentDidUpdate:
  useEffect(() => {
    // Update the document title using the browser API
    document.title = `You clicked ${count} times`;
  });

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

render() {
  const elements = ['one', 'two', 'three'];

  const items = []

  for (const [index, value] of elements.entries()) {
    items.push(<li key={index}>{value}</li>)
  }

  return (
    <div>
      {items}
    </div>
  )
}

////////// or:

render: function() {
  const elements = ['one', 'two', 'three'];
  return (
    <ul>
      {elements.map((value, index) => {
        return <li key={index}>{value}</li>
      })}
    </ul>
  )
}

const useStyles = makeStyles((theme) => ({}));

<Grid container><Grid item></Grid></Grid>

import { createMuiTheme } from '@material-ui/core/styles';

// A custom theme for this app
const theme = createMuiTheme({});

export default theme;

this.setState({ name: 'Matt' }, () => console.log(this.state.name))