An Introduction to React.js Hooks

Written by Adam Burdette

Hooks replace classes by providing similar functionality to class methods. They’re a fantastic tool for improving the modularity and readability of code across an application. With the release of version 0.59, React Native developers can now take advantage of hooks and all of their benefits.


Below is an introduction to the built-in hooks supplied by React, some ideas and rules for writing custom hooks, some useful hook libraries, and a real world authentication example.


Hook Definitions

The

useState

and

useEffect

hooks are the heavy lifters and will replace most of the functionality required by classes. Other built-in hooks provide some of the less frequently used React features that previously required classes.

useState

This hook is a replacement for

state

and

setState

. Calling

useState

creates a single state variable of any type with an optional initial value. Multiple and single calls can be used to contain an object called

state

.

    
import React, { useState } from 'react';

const WithState = (props) => {
  // Note: Unlike a class's setState, the merge for objects is not handled by setState.
  const [state, setState] = useState({foo: 'bar'});
  const [input, setInput] = useState('');

  ...
}
    
  

useState

returns an array whose first input will always be the current value of the state, and whose second value is a function used to update that value. The standard naming convention is

name/set{name}

.


useEffect

This hook allows side effects to be used in functional components. This is somewhat of a drop-in for class lifecycle functions such as

componentDidMount

and

componentDidUpdate

.

The function takes two arguments: a function with an effect to call, and an optional array with values on which to base the refreshing of that effect. The return is a cleanup function for the effect. A popular use case this could replace is subscribing to an event in

componentDidMount

and unsubscribing in

componentWillUnmount

.

    
import React, {useEffect, useState} from 'react';
import { AppState } from "react-native";

const WithSubscription = (props) => {
  const [state, setState] = useState(AppState.currentState);
  const handleStateChange = state => setState(state);

  useEffect(() => {
    AppState.addEventListener("change", handleStateChange);

    // This is called when the component unmounts or the effect is called again.
    return () => AppState.removeEventListener("change", handleStateChange);
  });

  ...
}
    
  

The second argument is an optional but important array designating properties that, when updated, will cause the effect to be executed again. Ideally, all properties outside of the

useEffect

hook should be provided to this argument. This is equivalent to:

    
componentDidUpdate = (prevProps) => {
  const {foo} = this.props
  if(prevProps.foo !== foo)
    {
      // Effect
    }
}

// -----------------------------------------

useEffect(() => {
  //Effect
}, [foo])
    
  

If the array is not provided, the effect will be run on every render. Conversely, if an empty array is passed, it is equivalent to

componentDidMount

and will only run once.


useContext

This hook provides the component with access to a context.


useMemo

This hook allows for the execution of expensive operations, keeping the value and only recomputing when specific properties change, like

useEffect

. This is useful when expensive non-primitive data is causing a

useEffect

to run on every render. Remember

[] !== []

and

{} !== {}

!


useCallback

This hook is similar to

useMemo

, but will keep a function reference. Function instances are often redefined, so if this is causing rerenders, the hook can memorize the function. Again, remember

() => {} !== () => {}

.

Callbacks were created for both the navigate function and the dispatch wrapped login action using

useCallback
. This is to prevent rerenders as the function instances may change.


useReducer

Like a light form of Redux, this hook provides a dispatch function and a state, while taking a reducer and an initial state. Note that this is not a full replacement for Redux, though it could be used in certain situations.


useRef

As the name suggests, this hook allows components to use refs.


Check out the documentation for more information on these and other available hooks.


Custom Hooks

Among their many benefits, hooks can be customized to centralize and reuse logic. For example, consider the need to have a selected item in a list.

    
class ListWithSelection extends Component {
  constructor(props){
    super(props);
    const {initialValue = ''} = props // Initial value
    this.state = {selectedId: initialValue};
  }

  setSelectedId(id) {
    this.setState({selectedId: id});
  }

  render() {
    const { selectedId } = this.state;

    return (
      <ScrollView>
        {
          this.props.items.map(({id}) => (
            <ListItem
              onPress={() => this.setSelectedId(id)}
              isSelected={selectedId === id}
            />
        ))
        }
      </ScrollView>
    )
  }
}
    
  

If this functionality appears in multiple parts of the app, it is likely that this state and logic is repeated. This is a great opportunity to implement a reusable hook.

    
const useSelectedId = (initialValue = '') => {
  const [selectedId, setSelectedId] = useState(initialValue);
  const isIdSelected = id => selectedId === id;

  return {
    selectedId,
    setSelectedId,
    isIdSelected,
  }
}
    
  

Then, refactor the class into a functional component using the hook above.

    
const ListWithSelection = ({ initialValue, items }) => {
  const {
    selectedId,
    setSelectedId,
    isIdSelected,
    items,
  } = useSelectedId(initialValue);

  return (
    <ScrollView>
      {
        items.map(({ id }) => (
          <ListItem
            key={id}
            onPress={() => setSelectedId(id)}
            isSelected={isIdSelected(id)}
          />
        ))}
    </ScrollView>
  )
}
    
  

The component is no longer a class, the line count has gone down significantly, and the

this
keyword is gone. The code has been written and is now reusable any time this logic is needed.


The Rules

The React team has provided a few rules to keep in mind when using hooks. Luckily, React has created an ESLint plugin to enforce these rules.


Only Call Hooks At the Top Level of Your Functional Component This means no conditional calls to or looping with hooks. Not following this rule can lead to inconsistencies between renderings.


Bad

    
if(condition){
  useEffect(...)
}
    
  

Good

    
useEffect(() => { if(condition){...}, [condition] })
    
  

Only Call Hooks from React Functions This includes functions called from the render of a functional component. Calling a hook from a non-React function will trigger an error screen, and jest tests will fail.


Hook Libraries


react-redux

Instead of using the

connect
HOC, hooks can be used to get data and dispatch actions.


react-native-hooks

A collection of hooks to encapsulate parts of the React Native API.



react-navigation-hooks

Hooks for the

react-navigation
API. This library is fairly new and a great place to contribute and get involved.


react-spring

This library supports all react-based platforms and has hooks to handle complex animation logic.



react-apollo-hooks

In 3.0 Apollo will be adding hooks to

react-apollo
, but for now this library has a great set of hooks for your Apollo GraphQL needs.


A Real World Example

Let’s review a common use case: authentication. The user provides some input, an action is called to cause some reaction in the state, and the UI responds to any state change. Thanks to some of the hook libraries listed in the previous section, a class is no longer needed to implement this functionality.

    
import React, { useState, useEffect, useCallback } from 'react';
import { useDispatch, useSelector } from 'react-redux';
import { useNavigation } from 'react-navigation-hooks';
import { login } from '../../actions/session';

const Auth = () => {
  // State
  const [email, setEmail] = useState('');
  const [password, setPassword] = useState('');

  // Navigation
  const {navigate} = useNavigation();

  // Redux - mapStateToProps/mapDispatchToProps
  const dispatch = useDispatch();
  const loginCallback = useCallback(
    () => dispatch(login({email, password})),
    [dispatch]
  )
  const {loaded} = useSelector(({session: {loginState}}) => ({
    loaded: loginState.loaded,
  }));

  // componentDidUpdate
  const navCallback = useCallback(() => navigate(routes.app));
  useEffect(() => {
    if(loaded){
      navCallback();
    }
  }, [navCallback, loaded]);

  <View
    style={style.container}
    testID="authContainer"
  >
    <TextInput
      value={email}
      placeholder="Email"
      onChangeText={setEmail}
    />
    <TextInput
      value={password}
      label="Password"
      secureTextEntry
      onChangeText={setPassword}
    />
    <Button
      title="Log In"
      onPress={() => loginCallback()}
    >
    </Button>
  </View>
}
    
  

After implementing all of the necessary hooks in the example above, the function looks a bit bulky and cluttered. Luckily, hooks are functions and can be refactored. Let’s create a custom hook and call it

useAuthHooks
to clean up the function.

    
import { useState, useEffect, useCallback } from 'react';
import { useDispatch, useSelector } from 'react-redux';
import { useNavigation } from 'react-navigation-hooks';
import { login } from '../../actions/session';

const useAuthHooks = () => {
  const [email, setEmail] = useState('');
  const [password, setPassword] = useState('');

  const navigation = useNavigation();
  const { navigate } = navigation;

  const dispatch = useDispatch();
  const loginCallback = useCallback(
    () => dispatch(login({email, password})),
    [dispatch]
  )
  const {loaded} = useSelector(({session: {loginState}}) => ({
    loaded: loginState.loaded,
  }));

  const navCallback = useCallback(() => navigate(routes.app));
  useEffect(() => {
    if(loaded){
      navCallback();
    }
  }, [navCallback, loaded]);

  return {
    state: {
      email,
      setEmail,
      password,
      setPassword
    },
    navigation,
    dispatch,
    loaded,
    login: loginCallback,
  }
}
    
  

This file now contains all the logic needed for the

Auth
component and the size of the component has been greatly decreased.

    
import React from 'react';
import useAuthHooks from './useAuthHooks';

const Auth = () => {
  const {
    email,
    setEmail,
    password,
    setPassword,
    loginCallback
  }= useAuthHooks()

  return (
    <View
      style={style.container}
      testID="authContainer"
    >
      <TextInput
        value={email}
        placeholder="Email"
        onChangeText={setEmail}
      />
      <TextInput
        value={password}
        label="Password"
        secureTextEntry
        onChangeText={setPassword}
      />
      <Button
        title="Log In"
        onPress={() => loginCallback()}
      />
    </View>
  )
}
    
  

As you can see, hooks can greatly improve the readability and reusability of your React components, speeding up the development process and easing the chore of refactoring.


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