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Equivalent Class Component

Functional Component with State Hook

Effect Hook

Example Class Component

Example Functional Component using useEffect

Custom Hooks
Resources

Introduction

Hooks are a new feature introduced in React16.8. I’ll try to explain what they are in the following order.

What are Hooks?
Rules of Hooks
State Hook
Effect Hook
Custom Hook

What are Hooks?

According to the official React documentation,

Hooks let you use state and other React features without writing a class.

Yup. That’s exactly it!.
Now you can use some React features such as state in functional components thanks to hooks!!

I’ll introduce 3 following hooks in this post.

(1)State Hook: It lets you use state and setState in functional components.
(2)Effect Hook: It lets you perform side effects such as data fetching in functional components.
(3)Custom Hooks: Building your custom hooks lets you extract component logic into reusable functions.

Rules of Hooks

There are 2 ground rules you have to follow to use hooks safely.

(1) Only Call Hooks at the Top Level!! Don’t call Hooks inside loops, conditions, or nested functions.

(2) Only Call Hooks from React Functions!! Don’t call Hooks from regular JavaScript functions.

State Hook

You can use state and setState in functional components by using a hook called useState.

Let’s see how to use useState through comparing a functional componentusing useState to the equivalent class component.

Equivalent Class Component

It does the following 2 things.

(1) Define the count state and its initialState 0.
(2) Add 1 to count each time setState is called.

class Example extends React.Component {
  constructor(props) {
    super(props)
    this.state = {
      count: 0,
    }
  }

  render() {
    return (
      <div>
        <p>You clicked{this.state.count} times</p>
        <button onClick={() => this.setState({ count: this.state.count + 1 })}>
          Click Me
        </button>
      </div>
    )
  }
}

Functional Component with State Hook

Now it’s time to create a functional component using useState which does the same thing as the class component above.

The basic syntax of useState looks like this!

const [state, setState] = useState(initialState)

It’s like you define the state, setState and initialState all together.

If you want to define the same state as the one in the class component above, it would look like this.

import React, { useState } from 'react'

function Counter() {
  const [count, setCount] = useState(0)
}

Notice 3 things in the code above!

(1) count is the equivalent ofthis.state={count:0} in the class component.
(2) setCount is the equivalent of setState part in the class component.
(3) 0 is the initial state of count.

While you are writing up the rest of the code, keep these 2 things in mind.

(1) You can use count directory!(no need to do this.count.)
(2) You can update the state by using setCount.

import React, { useState } from 'react'

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

  return (
    <div>
      // (1) You can use count directory!
      <p>You clicked {count} times</p>
      // (2) You can update the state by using setCount.
      <button onClick={() => setCount(count + 1)}> Click me</button>
    </div>
  )
}

export default Counter

As a side note, you can define multiple states like the code below.

import React, { useState } from 'react'

function Counter() {
  const [count, setCount] = useState(0)
  const [name, setName] = useState('')

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}> Click me</button>
      <p>My name is {name}.</p>
      <button onClick={() => setName('テスト太郎')}>Show my name</button>
    </div>
  )
}

export default Counter

Effect Hook

You can perform side effects in functional components by using a hook called useEffect!

Let’s see how to use useEffec through comparing a functional componentusing useEffect to the equivalent class component.

Example Class Component

In class components, we perform side effects such as fetching data and changing the DOM in componentDidMount componentDidUpdate.

Here, it outputs It did mount in the console after a component is mounted and outputs It did get updated after updating occurs.

import React from 'react'

class Effect extends React.Component {
  constructor(props) {
    super(props)
    this.state = {
      count: 0,
    }
  }

  componentDidMount() {
    console.log('It did mount.')
  }

  componentDidUpdate() {
    console.log('It did get updated.')
  }

  render() {
    return (
      <div>
        <h1>You clicked {this.state.count} times</h1>
        <button onClick={() => this.setState({ count: this.state.count + 1 })}>
          Click me
        </button>
      </div>
    )
  }
}

Example Functional Component using useEffect

The useEffect hook is like a combination of componentDidMount, componentDidUpdate and componentWillUnmount.
It runs after every render including the first render.
When you are building react applications with hooks this is where you perform side effects.

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

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

  useEffect(() => {
    console.log('It got rendered')
  })

  return (
    <div>
      <h1>You clicked {count} times</h1>
      <button onClick={() => setCount(count + 1)}> Click me</button>
    </div>
  )
}

Custom Hooks

As I mentioned above, building your custom hooks lets you extract component logic into reusable functions.

Let’s suppose there are two components like below.

(1) A component called Status which returns Logged in if it receives id = 1.
(2) A component called Message which returns Welocme Back if it receives id = 1.

export default function Status(props) {
  const [isLoggedIn, setIsLoggedIn] = useState(false)

  const handleStateChange = id => {
    if (id === 1) {
      setIsLoggedIn(true)
    } else {
      setIsLoggedIn(false)
    }
  }

  useEffect(() => {
    handleStateChange(props.user.id)
  })

  const status = isLoggedIn ? 'Logged in' : 'Sign up'

  return (
    <>
      <h1>Status: {status}</h1>
    </>
  )
}

export default function Message(props) {
  const [isLoggedIn, setIsLoggedIn] = useState(false)

  const handleStateChange = id => {
    if (id === 1) {
      setIsLoggedIn(true)
    } else {
      setIsLoggedIn(false)
    }
  }

  useEffect(() => {
    handleStateChange(props.user.id)
  })

  const message = isLoggedIn ? 'Welcome Back' : 'Who are you??'

  return (
    <>
      <h1>Message: {message}</h1>
    </>
  )
}

As you probably noticed, it’s very redundant. You can build a custom hook to extract the same logic exists in both components into one reusable function.

※It is very important that you name your custom hook starting with use.
In this case, I named my custom hook useLogIn.

import { useState, useEffect } from 'react'

export default function useLogIn(userId) {
  const [isLoggedIn, setIsLoggedIn] = useState(false)

  // The login you want to reuse.
  const handleStateChange = id => {
    if (id === 1) {
      setIsLoggedIn(true)
    } else {
      setIsLoggedIn(false)
    }
  }

  // Perform side effects in useEffect.
  useEffect(() => {
    handleStateChange(userId)
  })

  return isLoggedIn
}

Using useLogIn, we can simplify Status and Message components.

import React from 'react'
import useLogIn from './useLogIn'

export default function Status(props) {
  const status = useLogIn(props.user.id) ? 'Logged in' : 'Sign up'
  return (
    <>
      <h1>Status: {status}</h1>
    </>
  )
}

import React from 'react'
import useLogIn from './useLogIn'

export default function Message(props) {
  const message = useLogIn(props.user.id) ? 'Welcome Back' : 'Who are you??'
  return (
    <>
      <h1>Message: {message}</h1>
    </>
  )
}

The custom hooks can be used for other cool stuff, check out the official document about Building Your Own Hooks.

React hooks for noobs

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