30-Days-Of-React/07_Day_Class_Components/07_class_components.md

<div align="center">
  <h1> 30 Days Of React: Class Components </h1>
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<sub>Author:
<a href="https://www.linkedin.com/in/asabeneh/" target="_blank">Asabeneh Yetayeh</a><br>
<small> October, 2020</small>
</sub>

</div>

[<< Day 6](../06_Day_Map_List_Keys/06_map_list_keys.md) | [Day 8 >>](../07_Day_Class_Components/07_class_components.md)

![30 Days of React banner](../images/30_days_of_react_banner_day_7.jpg)

- [Class Components](#class-components)
  - [Accessing props in Class components](#accessing-props-in-class-components)
- [Exercises](#exercises)
  - [Exercises: Level 1](#exercises-level-1)
  - [Exercises: Level 2](#exercises-level-2)

# Class Components

In the previous sections, we have covered JSX, functional component and props. In this section, we will cover class components or statefull component. Only class based components used to have state and life cycle methods. However, after React version 16.8.0 functional components can have state and life cycle using React Hooks. In 30 Days Of React challenge, we will cover React before 16.8.0 and after, that mean both old and newest version. There are lots of codes written in older version and at some point it may need migration. In addition, to understand React very well someone has to understand class based component too.

All the previous components are functional components. Let us make also class based component. Class based component is made using JavaScript class and it inherits from react Component. Let us learn how to make a class based component by converting all the functional components we made previously. It is not important to convert all but we are converting them for the sake of learning how to change functional components to class components.

```js
// Pure JavaScript class and child
// Imagine this what we import from React package
class Component {
  constructor(props) {}
}

// This how we make class based components by inheriting from the parent
class Child extends Component {
  constructor(props) {
    super(props)
  }
}
```

Functional React component

```js
// index.js

import React from 'react'
import ReactDOM from 'react-dom'
// Header Component
// Functional component
const Header = () => (
  <header>
    <div className='header-wrapper'>
      <h1>Welcome to 30 Days Of React</h1>
      <h2>Getting Started React</h2>
      <h3>JavaScript Library</h3>
      <p>Asabeneh Yetayeh</p>
      <small>Oct 6, 2020</small>
    </div>
  </header>
)
const rootElement = document.getElementById('root')
ReactDOM.render(<Header />, rootElement)
```

Class based React component is a child of React.Component and it has a built-in render method and it may have a constructor.

```js
//index.js

import React from 'react'
import ReactDOM from 'react-dom'

// class based component
class Header extends React.Component {
  render() {
    return (
      <header>
        <div className='header-wrapper'>
          <h1>Welcome to 30 Days Of React</h1>
          <h2>Getting Started React</h2>
          <h3>JavaScript Library</h3>
          <p>Asabeneh Yetayeh</p>
          <small>Oct 7, 2020</small>
        </div>
      </header>
    )
  }
}

const rootElement = document.getElementById('root')
ReactDOM.render(<Header />, rootElement)
```

Let's see the above component with a constructor

```js
//index.js

import React from 'react'
import ReactDOM from 'react-dom'

// class base component
class Header extends React.Component {
  constructor(props) {
    super(props)
    // the code inside the constructor run before any other code
  }
  render() {
    return (
      <header>
        <div className='header-wrapper'>
          <h1>Welcome to 30 Days Of React</h1>
          <h2>Getting Started React</h2>
          <h3>JavaScript Library</h3>
          <p>Asabeneh Yetayeh</p>
          <small>Oct 7, 2020</small>
        </div>
      </header>
    )
  }
}
const rootElement = document.getElementById('root')
ReactDOM.render(<Header />, rootElement)
```

Let's change all the functional component to class based components

```js
// TechList Component
// functional component
const TechList = () => {
  const techs = ['HTML', 'CSS', 'JavaScript']
  const techsFormatted = techs.map((tech) => <li key={tech}>{tech}</li>)
  return techsFormatted
}

// TechList Component
// class base component
class TechList extends React.Component {
  constructor(props) {
    super(props)
  }
  render() {
    const techs = ['HTML', 'CSS', 'JavaScript']
    const techsFormatted = techs.map((tech) => <li key={tech}>{tech}</li>)
    return techsFormatted
  }
}

// Main Component
// Functional Component
const Main = () => (
  <main>
    <div className='main-wrapper'>
      <p>Prerequisite to get started react.js:</p>
      <ul>
        <TechList />
      </ul>
    </div>
  </main>
)

// Main Component
// Class Component
class Main extends React.Component {
  constructor(props) {
    super(props)
  }
  render() {
    return (
      <main>
        <div className='main-wrapper'>
          <p>Prerequisite to get started react.js:</p>
          <ul>
            <TechList />
          </ul>
        </div>
      </main>
    )
  }
}

// Footer Component
// Functional component
const Footer = () => (
  <footer>
    <div className='footer-wrapper'>
      <p>Copyright 2020</p>
    </div>
  </footer>
)

// Footer Component
// Class component
class Footer extends React.Component {
  constructor(props) {
    super(props)
  }
  render() {
    return (
      <footer>
        <div className='footer-wrapper'>
          <p>Copyright 2020</p>
        </div>
      </footer>
    )
  }
}

// The App, or the parent or the container component
// Functional Component
const App = () => (
  <div className='app'>
    <Header />
    <Main />
    <Footer />
  </div>
)

// The App, or the parent or the container component
// Class Component
class App extends React.Component {
  constructor(props) {
    super(props)
  }
  render() {
    return (
      <div className='app'>
        <Header />
        <Main />
        <Footer />
      </div>
    )
  }
}
```

Let's put all the class based components together in one file.

```js
//index.js

import React from 'react'
import ReactDOM from 'react-dom'

// class base component
class Header extends React.Component {
  constructor(props) {
    super(props)
    // the code inside the constructor run before any other code
  }
  render() {
    return (
      <header>
        <div className='header-wrapper'>
          <h1>Welcome to 30 Days Of React</h1>
          <h2>Getting Started React</h2>
          <h3>JavaScript Library</h3>
          <p>Asabeneh Yetayeh</p>
          <small>Oct 7, 2020</small>
        </div>
      </header>
    )
  }
}

// TechList Component
// class base component
class TechList extends React.Component {
  constructor(props) {
    super(props)
  }
  render() {
    const techs = ['HTML', 'CSS', 'JavaScript']
    const techsFormatted = techs.map((tech) => <li key={tech}>{tech}</li>)
    return techsFormatted
  }
}

// Main Component
// Class Component
class Main extends React.Component {
  constructor(props) {
    super(props)
  }
  render() {
    return (
      <main>
        <div className='main-wrapper'>
          <p>Prerequisite to get started react.js:</p>
          <ul>
            <TechList />
          </ul>
        </div>
      </main>
    )
  }
}

// Footer Component
// Class component
class Footer extends React.Component {
  constructor(props) {
    super(props)
  }
  render() {
    return (
      <footer>
        <div className='footer-wrapper'>
          <p>Copyright 2020</p>
        </div>
      </footer>
    )
  }
}

// The App, or the parent or the container component
// Class Component
class App extends React.Component {
  constructor(props) {
    super(props)
  }
  render() {
    return (
      <div className='app'>
        <Header />
        <Main />
        <Footer />
      </div>
    )
  }
}

const rootElement = document.getElementById('root')
ReactDOM.render(<App />, rootElement)
```

## Accessing props in Class components

We stated that props is a means to send data from on component to another or we can call it that props is a data carrier. Therefore, we should handle props in class based component too. We can access props of a class based component using the keyword _this_. See the example below.

```js
// index.js

import React from 'react'
import ReactDOM from 'react-dom'

// class based component
class Header extends React.Component {
  constructor(props) {
    super(props)
    // the code inside the constructor run before any other code
  }
  render() {
    return (
      <header>
        <div className='header-wrapper'>
          <h1>{this.props.data.welcome}</h1>
          <h2>{this.props.data.title}</h2>
          <h3>
            {this.props.data.author.firstName} {this.props.data.author.lastName}
          </h3>
          <small>{this.props.data.date}</small>
        </div>
      </header>
    )
  }
}
const App = () => {
  const data = {
    welcome: 'Welcome to 30 Days Of React',
    title: 'Getting Started React',
    subtitle: 'JavaScript Library',
    author: {
      firstName: 'Asabeneh',
      lastName: 'Yetayeh',
    },
    date: 'Oct 7, 2020',
  }

  return (
    <div className='app'>
      <Header data={data} />
    </div>
  )
}

const rootElement = document.getElementById('root')
ReactDOM.render(<App />, rootElement)
```

As you can see in the above example, to get the data out from props we have write _props.data_ every time. We can avoid this repetition using destructuring.

```js
// index.js

import React from 'react'
import ReactDOM from 'react-dom'

// class based component
class Header extends React.Component {
  constructor(props) {
    super(props)
    // the code inside the constructor run before any other code
  }
  render() {
    console.log(this.props.data)
    const {
      welcome,
      title,
      subtitle,
      author: { firstName, lastName },
      date,
    } = this.props.data

    return (
      <header>
        <div className='header-wrapper'>
          <h1>{welcome}</h1>
          <h2>{title}</h2>
          <h3>{subtitle}</h3>
          <p>
            {firstName} {lastName}
          </p>
          <small>{date}</small>
        </div>
      </header>
    )
  }
}
const App = () => {
  const data = {
    welcome: 'Welcome to 30 Days Of React',
    title: 'Getting Started React',
    subtitle: 'JavaScript Library',
    author: {
      firstName: 'Asabeneh',
      lastName: 'Yetayeh',
    },
    date: 'Oct 6, 2020',
  }

  return (
    <div className='app'>
      <Header data={data} />
    </div>
  )
}

const rootElement = document.getElementById('root')
ReactDOM.render(<App />, rootElement)
```

As you can see, the above code cleaner than the previous. Now, let's clean all the components we have and put all together.

```js
// index.js

import React from 'react'
import ReactDOM from 'react-dom'

// class based component
class Header extends React.Component {
  constructor(props) {
    super(props)
    // the code inside the constructor run before any other code
  }
  render() {
    console.log(this.props.data)
    const {
      welcome,
      title,
      subtitle,
      author: { firstName, lastName },
      date,
    } = this.props.data

    return (
      <header>
        <div className='header-wrapper'>
          <h1>{welcome}</h1>
          <h2>{title}</h2>
          <h3>{subtitle}</h3>
          <p>
            {firstName} {lastName}
          </p>
          <small>{date}</small>
        </div>
      </header>
    )
  }
}

// TechList Component
// class base component
class TechList extends React.Component {
  constructor(props) {
    super(props)
  }
  render() {
    const { techs } = this.props
    const techsFormatted = techs.map((tech) => <li key={tech}>{tech}</li>)
    return techsFormatted
  }
}

// Main Component
// Class Component
class Main extends React.Component {
  constructor(props) {
    super(props)
  }
  render() {
    return (
      <main>
        <div className='main-wrapper'>
          <p>Prerequisite to get started react.js:</p>
          <ul>
            <TechList techs={this.props.techs} />
          </ul>
        </div>
      </main>
    )
  }
}

// Footer Component
// Class component
class Footer extends React.Component {
  constructor(props) {
    super(props)
  }
  render() {
    return (
      <footer>
        <div className='footer-wrapper'>
          <p>Copyright {this.props.date.getFullYear()}</p>
        </div>
      </footer>
    )
  }
}

class App extends React.Component {
  render() {
    const data = {
      welcome: 'Welcome to 30 Days Of React',
      title: 'Getting Started React',
      subtitle: 'JavaScript Library',
      author: {
        firstName: 'Asabeneh',
        lastName: 'Yetayeh',
      },
      date: 'Oct 7, 2020',
    }
    const techs = ['HTML', 'CSS', 'JavaScript']

    return (
      <div className='app'>
        <Header data={data} />
        <Main techs={techs} />
        <Footer date={new Date()} />
      </div>
    )
  }
}

const rootElement = document.getElementById('root')
ReactDOM.render(<App />, rootElement)
```

Most of the time the container or the parent component can be written as class component and others as functional or presentational components. Data usually flows from parent components to child component and it is unidirectional. However, the latest version of react can allow us to write every component in our application only with functional components. This was impossible in previous versions.
In next section, we will cover state which is the heart of React. State allows React component to rerender when whenever there is a change in state.

# Exercises

## Exercises: Level 1

1. How do you write a pure JavaScript function
2. What is inheritance and how do you make a child from a parent class?
3. What is class based React component ?
4. What is the difference between functional React component and class based React component ?
5. What is the use cases of class based component ?
6. What is React life cycle ? (not covered yet) ?
7. What is state in React ?

## Exercises: Level 2

Learn more about class based component by changing previous days exercises to class based components

🎉 CONGRATULATIONS ! 🎉

[<< Day 6](../06_Day_Map_List_Keys/06_map_list_keys.md) | [Day 8 >>](../07_Day_Class_Components/07_class_components.md)