React Hooks, What is it? How to use it? Pt.2 - Using "useState" and "useEffect"

#react #100daysofcode #reacthooks #javascript

On this second part, Following our project sample, I'm going to show you how to use useState and useEffect.

Third Step: Continuing to use "useState"

So now we'll be able to type a new book to our list and save it:

// We need to import useState from react
import React, { useState } from 'react';

export default function App() {
  const [books, setBooks] = useState(['Javascript', 'React']);
  /* Here we're going to create a new state to save the value typed for newBook 
  */ 
  const [newBook, setNewBook] = useState('');

  /* Here we're going to create a function to bind this value inputted
  */
  const handleBookInput = e => setNewBook(e.target.value);

  /* Here we're going to pass as argument 'newBook' xD */
  const addBook = () => setBooks(newBook);

  return (
    <>
      <input 
        value={newBook}
        onChange={handleBookInput}
        placeholder="add here you new book"
        onKeyPress={e => e.key === 'Enter' && addBook()}
      />
      <button onClick={addBook}>Add Book!</button>
      <ul>
        {books.map((book, index) => <li key={index}>{book}</li>)}
      </ul>
    </>
  );
};

I'm going to show the diffs between first and second code, these codes do the same thing 😄

--- import React from 'react';
+++ import React, { useState } from 'react';

--- class App extends React.Component {
+++ export default function App() {
---   state = {
---     books: ['Javascript', 'React'],
---     newBook: '',
---   }
+++  const [books, setBooks] = useState(['Javascript', 'React']);
+++  const [newBook, setNewBook] = useState('');

---  handleBookInput = e => this.setState({ newBook: e.target.value });
+++  const handleBookInput = e => setNewBook(e.target.value);

---  addBook = () => {
---    this.setState({
---      books: [...this.state.books, this.state.newBook],
---      newBook: '',
---    });
---  }
+++  const addBook = () => setBooks(newBook);

---  render() {
    return (
      <>
        <input
---       value={this.state.newBook}
+++       value={newBook}
---       onChange={this.handleBookInput}
+++       onChange={handleBookInput}
          placeholder="add here you new book"
---       onKeyPress={e => e.key === 'Enter' && this.addBook()}
+++       onKeyPress={e => e.key === 'Enter' && addBook()}          
        />
---     <button onClick={this.addBook}>Add Book!</button>
+++     <button onClick={addBook}>Add Book!</button>
        <ul>
---       {this.state.books.map(
---         (book, index) => <li key={index}>{book}</li>
---       )}
+++       {books.map((book, index) => <li key={index}>{book}</li>)}
        </ul>
      </>
    );
---}
+++};

---export default App;
+++

Fourth Step: Using useEffect

Previously i show to you how to use state in a functional component, now i'm going to show how to use lifecycle methods;
First, These is the most used lifecycle methods from React.Component:
- componentDidMount()
- componentDidUpdate()
- componentWillUnmount()
Seat down, the way to use it in hooks is so hard and expensive 😄 :

  useEffect(() => {}, []); // xD easier, right? hehehe

let's put these methods to our case, we'll use componentDidUpdate to change document.title to show how many books are added, first in Class Component without hooks:

import React from 'react';

class App extends React.Component {
  state = {
    books: ['Javascript', 'React'],
    newBook: '',
  }

  componentDidUpdate() { 
    document.title = `Current books added: ${this.state.books.length}`
  }

  handleBookInput = e => this.setState({ newBook: e.target.value });

  addBook = () => {
    this.setState({
      books: [...this.state.books, this.state.newBook],
      newBook: '',
    });
  }

  render() {
    return (
      <>
        <input
          value={this.state.newBook}
          onChange={this.handleBookInput}
          placeholder="add here you new book"
          onKeyPress={e => e.key === 'Enter' && this.addBook()}
        />
        <button onClick={this.addBook}>Add Book!</button>
        <ul>
          {this.state.books.map(
            (book, index) => <li key={index}>{book}</li>
          )}
        </ul>
      </>
    )
  }
};

export default App;

the same using hooks:

// We need to import useEffect from react
import React, { useState, useEffect } from 'react';

export default function App() {
  const [books, setBooks] = useState(['Javascript', 'React']);
  const [newBook, setNewBook] = useState('');

  const handleBookInput = e => setNewBook(e.target.value);

  const addBook = () => setBooks(newBook);

  /* using useEffect you can use all lifecycle methods 
  to use componentDidMount() {} in hooks you should code something like that:
  useEffect(() => {
    ....
  }, []);
  here useEffect get 2 arguments a function and a array, inside 
  the function will contain your code, on componentDidMount the array value
  need to be empty.

  to use componentDidUpdate() {} in hooks the code will look like changing only one thing:
  useEffect(() => {
    ....
  }, [state_to_observe]);
  here use effect get 2 arguments like the code before, but the only change is on the
  second argument receives the state observer (Note: if do you want to observer many states 
  you can put that states inside the array), in our sample we're going to use useEffect with state observer.
  */

  /* When that state has any changes, the function inside useEffect is called xD  easy peasy, right?
  */

  useEffect(() => {
    document.title = `Current books added: ${books.length}`
  }, [books]);

  return (
    <>
      <input 
        value={newBook}
        onChange={handleBookInput}
        placeholder="add here you new book"
        onKeyPress={e => e.key === 'Enter' && addBook()}
      />
      <button onClick={addBook}>Add Book!</button>
      <ul>
        {books.map((book, index) => <li key={index}>{book}</li>)}
      </ul>
    </>
  );
};