30-Days-Of-React/09_Day_Conditional_Rendering/09_conditional_rendering.md

<div align="center">
  <h1> 30 Days Of React: Conditional Rendering</h1>
  <a class="header-badge" target="_blank" href="https://www.linkedin.com/in/asabeneh/">
  <img src="https://img.shields.io/badge/style--5eba00.svg?label=LinkedIn&logo=linkedin&style=social">
  </a>
  <a class="header-badge" target="_blank" href="https://twitter.com/Asabeneh">
  <img alt="Twitter Follow" src="https://img.shields.io/twitter/follow/asabeneh?style=social">
  </a>

<sub>Author:
<a href="https://www.linkedin.com/in/asabeneh/" target="_blank">Asabeneh Yetayeh</a><br>
<small> October, 2020</small>
</sub>

</div>

[<< Day 8](../08_Day_States/08_states.md) | [Day 10 >>](../10_React_Project_Folder_Structure/10_react_project_folder_structure.md)

![30 Days of React banner](../images/30_days_of_react_banner_day_9.jpg)

# Conditional Rendering

As we can understand from the term, conditional rendering is a way to render different JSX or component at different condition. We can implement conditional rendering using regular if and else statement, ternary operator and &&. Let's implement a different conditional rendering.

## Conditional Rendering using If and Else statement

In the code below, we have an initial state of loggedIn which is false. If the state is false we inform user to log in otherwise we welcome the user.

```js
// index.js
import React from 'react'
import ReactDOM from 'react-dom'

// class based component
class Header extends React.Component {
  render() {
    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>
          <p>Select a country for your next holiday</p>
        </div>
      </header>
    )
  }
}

class App extends React.Component {
  state = {
    loggedIn: false,
  }

  render() {
    const data = {
      welcome: '30 Days Of React',
      title: 'Getting Started React',
      subtitle: 'JavaScript Library',
      author: {
        firstName: 'Asabeneh',
        lastName: 'Yetayeh',
      },
      date: 'Oct 9, 2020',
    }

    // conditional rendering using if and else statement

    let status

    if (this.state.loggedIn) {
      status = <h3>Welcome to 30 Days Of React</h3>
    } else {
      status = <h3>Please Login</h3>
    }

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

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

Let's add a method which allow as to toggle the status of the user. We should have a button to handle event for logging in and logging out.

```js
// index.js
import React from 'react'
import ReactDOM from 'react-dom'

// A button component
const Button = ({ text, onClick, style }) => (
  <button style={style} onClick={onClick}>
    {text}
  </button>
)

// CSS styles in JavaScript Object
const buttonStyles = {
  backgroundColor: '#61dbfb',
  padding: 10,
  border: 'none',
  borderRadius: 5,
  margin: '3px auto',
  cursor: 'pointer',
  fontSize: 22,
  color: 'white',
}

// class based component
class Header extends React.Component {
  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>
    )
  }
}

class App extends React.Component {
  state = {
    loggedIn: false,
  }
  handleLogin = () => {
    this.setState({
      loggedIn: !this.state.loggedIn,
    })
  }

  render() {
    const data = {
      welcome: '30 Days Of React',
      title: 'Getting Started React',
      subtitle: 'JavaScript Library',
      author: {
        firstName: 'Asabeneh',
        lastName: 'Yetayeh',
      },
      date: 'Oct 9, 2020',
    }

    let status
    let text

    if (this.state.loggedIn) {
      status = <h1>Welcome to 30 Days Of React</h1>
      text = 'Logout'
    } else {
      status = <h3>Please Login</h3>
      text = 'Login'
    }

    return (
      <div className='app'>
        <Header data={data} />
        {status}
        <Button text={text} style={buttonStyles} onClick={this.handleLogin} />
      </div>
    )
  }
}

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

How about if our condition is more than two? Like pure JavaScript we can use if else if statement. In general, conditional rendering is not different from pure JavaScript conditional statement.

## Conditional Rendering using Ternary Operator

Ternary operator is an an alternative for if else statement. However, there is more use cases for ternary operator than if else statement. For example, use can use ternary operator inside styles, className or many places in a component than regular if else statement.

```js
// index.js
import React from 'react'
import ReactDOM from 'react-dom'

// A button component
const Button = ({ text, onClick, style }) => (
  <button style={style} onClick={onClick}>
    {text}
  </button>
)

// CSS styles in JavaScript Object
const buttonStyles = {
  backgroundColor: '#61dbfb',
  padding: 10,
  border: 'none',
  borderRadius: 5,
  margin: '3px auto',
  cursor: 'pointer',
  fontSize: 22,
  color: 'white',
}

// class based component
class Header extends React.Component {
  render() {
    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>
    )
  }
}

class App extends React.Component {
  state = {
    loggedIn: false,
  }
  handleLogin = () => {
    this.setState({
      loggedIn: !this.state.loggedIn,
    })
  }

  render() {
    const data = {
      welcome: '30 Days Of React',
      title: 'Getting Started React',
      subtitle: 'JavaScript Library',
      author: {
        firstName: 'Asabeneh',
        lastName: 'Yetayeh',
      },
      date: 'Oct 9, 2020',
    }

    let status = this.state.loggedIn ? (
      <h1>Welcome to 30 Days Of React</h1>
    ) : (
      <h3>Please Login</h3>
    )

    return (
      <div className='app'>
        <Header data={data} />
        {status}
        <Button
          text={this.state.loggedIn ? 'Logout' : 'Login'}
          style={buttonStyles}
          onClick={this.handleLogin}
        />
      </div>
    )
  }
}

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

In addition to JSX, we can also conditionally render a component. Let's change the above conditional JSX to a component.

```js
// index.js
import React from 'react'
import ReactDOM from 'react-dom'

// A button component
const Button = ({ text, onClick, style }) => (
  <button style={style} onClick={onClick}>
    {text}
  </button>
)

// CSS styles in JavaScript Object
const buttonStyles = {
  backgroundColor: '#61dbfb',
  padding: 10,
  border: 'none',
  borderRadius: 5,
  margin: '3px auto',
  cursor: 'pointer',
  fontSize: 22,
  color: 'white',
}

// class based component
class Header extends React.Component {
  render() {
    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 Login = () => (
  <div>
    <h3>Please Login</h3>
  </div>
)
const Welcome = (props) => (
  <div>
    <h1>Welcome to 30 Days Of React</h1>
  </div>
)

class App extends React.Component {
  state = {
    loggedIn: false,
  }
  handleLogin = () => {
    this.setState({
      loggedIn: !this.state.loggedIn,
    })
  }

  render() {
    const data = {
      welcome: '30 Days Of React',
      title: 'Getting Started React',
      subtitle: 'JavaScript Library',
      author: {
        firstName: 'Asabeneh',
        lastName: 'Yetayeh',
      },
      date: 'Oct 9, 2020',
    }

    const status = this.state.loggedIn ? <Welcome /> : <Login />

    return (
      <div className='app'>
        <Header data={data} />
        {status}
        <Button
          text={this.state.loggedIn ? 'Logout' : 'Login'}
          style={buttonStyles}
          onClick={this.handleLogin}
        />
      </div>
    )
  }
}

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

## Conditional Rendering using && Operator

The && operator render the right JSX operand if the left operand(expression) is true.

```js
// index.js
import React from 'react'
import ReactDOM from 'react-dom'

// A button component
const Button = ({ text, onClick, style }) => (
  <button style={style} onClick={onClick}>
    {text}
  </button>
)

// CSS styles in JavaScript Object
const buttonStyles = {
  backgroundColor: '#61dbfb',
  padding: 10,
  border: 'none',
  borderRadius: 5,
  margin: '3px auto',
  cursor: 'pointer',
  fontSize: 22,
  color: 'white',
}

// class based component
class Header extends React.Component {
  render() {
    console.log(this.props.data)
    const {
      welcome,
      title,
      subtitle,
      author: { firstName, lastName },
      date,
    } = this.props.data

    return (
      <header style={this.props.styles}>
        <div className='header-wrapper'>
          <h1>{welcome}</h1>
          <h2>{title}</h2>
          <h3>{subtitle}</h3>
          <p>
            {firstName} {lastName}
          </p>
          <small>{date}</small>
        </div>
      </header>
    )
  }
}
const Login = () => (
  <div>
    <h3>Please Login</h3>
  </div>
)
const Welcome = (props) => (
  <div>
    <h1>Welcome to 30 Days Of React</h1>
  </div>
)

class App extends React.Component {
  state = {
    loggedIn: false,
    techs: ['HTML', 'CSS', 'JS'],
  }
  handleLogin = () => {
    this.setState({
      loggedIn: !this.state.loggedIn,
    })
  }

  render() {
    const data = {
      welcome: '30 Days Of React',
      title: 'Getting Started React',
      subtitle: 'JavaScript Library',
      author: {
        firstName: 'Asabeneh',
        lastName: 'Yetayeh',
      },
      date: 'Oct 9, 2020',
    }

    // We can destructure state

    const { loggedIn, techs } = this.state

    const status = loggedIn ? <Welcome /> : <Login />

    return (
      <div className='app'>
        <Header data={data} />
        {status}
        <Button
          text={loggedIn ? 'Logout' : 'Login'}
          style={buttonStyles}
          onClick={this.handleLogin}
        />
        {techs.length === 3 && (
          <p>You have all the prerequisite courses to get started React</p>
        )}
        {!loggedIn && (
          <p>
            Please login to access more information about 30 Days Of React
            challenge
          </p>
        )}
      </div>
    )
  }
}

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

In the previous section, we used alert box to greet people and also to display time. Let's render the greeting and time on browser DOM instead of displaying on alert box.

```js
// index.js
import React from 'react'
import ReactDOM from 'react-dom'

// class based component
class Header extends React.Component {
  render() {
    console.log(this.props.data)
    const {
      welcome,
      title,
      subtitle,
      author: { firstName, lastName },
      date,
    } = this.props.data

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

const Message = ({ message }) => (
  <div>
    <h1>{message}</h1>
  </div>
)
const Login = () => (
  <div>
    <h3>Please Login</h3>
  </div>
)
const Welcome = (props) => (
  <div>
    <h1>Welcome to 30 Days Of React</h1>
  </div>
)

// A button component
const Button = ({ text, onClick, style }) => (
  <button style={style} onClick={onClick}>
    {text}
  </button>
)

// TechList Component
// class base component
class TechList extends React.Component {
  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 {
  render() {
    const {
      techs,
      greetPeople,
      handleTime,
      loggedIn,
      handleLogin,
      message,
    } = this.props
    console.log(message)

    const status = loggedIn ? <Welcome /> : <Login />
    return (
      <main>
        <div className='main-wrapper'>
          <p>Prerequisite to get started react.js:</p>
          <ul>
            <TechList techs={this.props.techs} />
          </ul>
          {techs.length === 3 && (
            <p>You have all the prerequisite courses to get started React</p>
          )}
          <div>
            <Button
              text='Show Time'
              onClick={handleTime}
              style={buttonStyles}
            />{' '}
            <Button
              text='Greet People'
              onClick={greetPeople}
              style={buttonStyles}
            />
            {!loggedIn && <p>Please login to access more information about 30 Days Of React challenge</p>}
          </div>
          <div style={{ margin: 30 }}>
            <Button
              text={loggedIn ? 'Logout' : 'Login'}
              style={buttonStyles}
              onClick={handleLogin}
            />
            <br />
            {status}
          </div>
          <Message message={message} />
        </div>
      </main>
    )
  }
}

// CSS styles in JavaScript Object
const buttonStyles = {
  backgroundColor: '#61dbfb',
  padding: 10,
  border: 'none',
  borderRadius: 5,
  margin: '3px auto',
  cursor: 'pointer',
  fontSize: 22,
  color: 'white',
}

// 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 {
  state = {
    loggedIn: false,
    techs: ['HTML', 'CSS', 'JS'],
    message: 'Click show time or Greet people to change me',
  }
  handleLogin = () => {
    this.setState({
      loggedIn: !this.state.loggedIn,
    })
  }
  showDate = (time) => {
    const months = [
      'January',
      'February',
      'March',
      'April',
      'May',
      'June',
      'July',
      'August',
      'September',
      'October',
      'November',
      'December',
    ]

    const month = months[time.getMonth()].slice(0, 3)
    const year = time.getFullYear()
    const date = time.getDate()
    return `${month} ${date}, ${year}`
  }
  handleTime = () => {
    let message = this.showDate(new Date())
    this.setState({ message })
  }
  greetPeople = () => {
    let message = 'Welcome to 30 Days Of React Challenge, 2020'
    this.setState({ message })
  }

  render() {
    const data = {
      welcome: '30 Days Of React',
      title: 'Getting Started React',
      subtitle: 'JavaScript Library',
      author: {
        firstName: 'Asabeneh',
        lastName: 'Yetayeh',
      },
      date: 'Oct 9, 2020',
    }

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

        <Main
          techs={techs}
          handleTime={this.handleTime}
          greetPeople={this.greetPeople}
          loggedIn={this.state.loggedIn}
          handleLogin={this.handleLogin}
          message={this.state.message}
        />

        <Footer date={new Date()} />
      </div>
    )
  }
}

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

## Exercises

### Exercises: Level 1

1. What is conditional rendering?
2. How do you implement conditional rendering?
3. Which method of conditional rendering do you prefer?

### Exercises: Level 2

1. Make a single page application which changes the body of the background based on the time of the day(Autumn, Winter, Spring, Summer)
2. Make a single page application which change the body of the background based on the time of the day(Morning, Noon, Evening, Night)

### Exercises: Level 3

Coming

🎉 CONGRATULATIONS ! 🎉

[<< Day 8](../08_Day_States/08_states.md) | [Day 10 >>](../10_React_Project_Folder_Structure/10_react_project_folder_structure.md)