from-classes-to-react-hooks

A guide/cheatsheet to help you migrate from ES6 class components to the React Hooks API.

State

Classes

import React from 'react';

class Counter extends React.Component {
  constructor(props) {
    super(props);

    this.state = {
      count: 0
    };

    this.increment = this.increment.bind(this);
    this.decrement = this.decrement.bind(this);
  }

  increment() {
    this.setState(prevState => ({ count: (prevState.count += 1) }));
  }

  decrement() {
    this.setState(prevState => ({ count: (prevState.count -= 1) }));
  }

  render() {
    return (
      <div>
        <div>{this.state.count}</div>
        <button onClick={this.increment}>Increment</button>
        <button onClick={this.decrement}>Decrement</button>
      </div>
    );
  }
}

React Hooks

import React, { useState } from "react";

function Counter () {
  const [count, setCount] = useState(0);
  return (
    <div>
      <div>{count}</div>
      <button onClick={() => setCount(count => count + 1)}>Increment</button>
      <button onClick={() => setCount(count => count - 1)}>Decrement</button>
    </div>
  );
}

Event Handlers

Classes

import React from "react";

class Clicker extends React.Component {
  constructor(props) {
    super(props);

    this.handleClick = this.handleClick.bind(this);
  }

  handleClick(event) {
    alert(`You just clicked ${this.props.id}`);
    event.stopPropagation();
  }

  render() {
    return <div onClick={this.handleClick}>Click me!</div>;
  }
}

React Hooks

import React, { useCallback } from 'react';

function Clicker({ id }) {
  const handleClick = useCallback(
    event => {
      alert(`You just clicked ${id}`);
      event.stopPropagation();
    },
    [id]
  );

  return <div onClick={handleClick}>Click me!</div>;
}

Component Lifecycle / Effects

componentDidMount / useEffect

Making Initial API Call

Classes

import React from "react";

function fetchFakeData() {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve([{ id: 1, name: "Foo" }, { id: 2, name: "Bar" }]);
    }, 1000);
  });
}

class App extends React.Component {
  constructor(props) {
    super(props);

    this.state = {
      data: []
    };
  }

  componentDidMount() {
    fetchFakeData().then(fakeData => this.setState({ data: fakeData }));
  }

  render() {
    return (
      <div>
        {this.state.data.length === 0
          ? "Loading..."
          : this.state.data.map(({ id, name }) => <span key={id}>{name}</span>)}
      </div>
    );
  }
}

React Hooks

function fetchFakeData() {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve([{ id: 1, name: "Foo" }, { id: 2, name: "Bar" }]);
    }, 1000);
  });
}

function App() {
  const [data, setData] = useState([]);

  useEffect(() => {
    fetchFakeData().then(fakeData => setData(fakeData));
  }, []);

  return (
    <div>
      {data.length === 0
        ? "Loading..."
        : data.map(({ id, name }) => <span key={id}>{name}</span>)}
    </div>
  );
}

componentDidUpdate / useEffect

Synchronizing State When Props Change

Classes

function makeRandNumWith(a, b) {
  return Math.floor(Math.random() * b) + a;
}

class Parent extends React.Component {
  constructor(props) {
    super(props);

    this.state = {
      parentCount: 0
    };
  }

  render() {
    return (
      <div>
        <span>Parent count: {this.state.parentCount}</span>
        <button
          onClick={() => this.setState({ parentCount: makeRandNumWith(0, 10) })}
        >
          Randomize & Sync
        </button>
        <Child parentCount={this.state.parentCount} />
      </div>
    );
  }
}

class Child extends React.Component {
  constructor(props) {
    super(props);

    this.state = {
      childCount: props.parentCount
    };
  }

  componentDidUpdate(prevProps) {
    if (prevProps.parentCount !== this.props.parentCount) {
      this.setState({ childCount: this.props.parentCount });
    }
  }

  render() {
    return (
      <div>
        <span>Child count: {this.state.childCount}</span>
        <button
          onClick={() =>
            this.setState(prevState => ({
              childCount: prevState.childCount + 1
            }))
          }
        >
          Increment Child Count
        </button>
      </div>
    );
  }
}

React Hooks

import { useEffect, useState } from "react";

function makeRandNumWith(a, b) {
  return Math.floor(Math.random() * b) + a;
}

function Parent() {
  const [parentCount, setParentCount] = useState(0);

  return (
    <div>
      <span>Parent count: {parentCount}</span>
      <button onClick={() => setParentCount(makeRandNumWith(0, 10))}>
        Randomize & Sync
      </button>
      <Child parentCount={parentCount} />
    </div>
  );
}

function Child({ parentCount }) {
  const [childCount, setChildCount] = useState(parentCount);

  useEffect(() => {
    setChildCount(parentCount);
  }, [parentCount]);
  return (
    <div>
      <span>Child count: {childCount}</span>
      <button onClick={() => setChildCount(x => x + 1)}>
        Increment Child Count
      </button>
    </div>
  );
}