componentDidMount() Equivalent in React Hook

Getting Started with a Class-based Component

• $ npx create-react-app todo-fecther

• $ cd todo-fetcher

• $ mkdir src/components && cd src/components

• $ touch TodoBoard.js Todo.js

• Edit the 2 files

  // src/components/TodoBoard.js
  import React from 'react';
  import Todo from './Todo';
   
  class TodoBoard extends React.Component {
    constructor() {
      super();
      this.state = {
        todos: [],
      };
    }
   
    componentDidMount() {
      const url = 'https://jsonplaceholder.typicode.com/todos';
      fetch(url)
        .then((response) => {
          if (response.status === 200) {
            return response.json();
          } else {
            throw 'erred';
          }
        })
        .then((data) => {
          this.setState = { todos: data };
        })
        .catch((error) => window.alert(error));
    }
   
    render() {
      return (
        <div>
          {this.state.todos.map((todo) => (
            <Todo key={todo.id} {...todo} />
          ))}
        </div>
      );
    }
  }
   
  export default TodoBoard;

  // src/components/Todo.js
  // This is a function component already
  import React from 'react';
   
  const Todo = (props) => {
    return (
      <div>
        <p>User ID: {props.userId}</p>
        <p>ID: {props.id}</p>
        <p>Title: {props.title}</p>
        <p>Status: {props.completed}</p>
      </div>
    );
  };
   
  export default Todo;

• Modify src/index.js to the following

  // src/index.js
  import React from 'react';
  import ReactDOM from 'react-dom';
  import TodoBoard from './components/TodoBoard';
   
  ReactDOM.render(<TodoBoard />, document.getElementById('root'));

• $ npm start

(Refer to facebook/create-react-app for more info about setting up a CRA project)

Re-write TodoBoard.js to a Function Component with Hooks

// src/components/TodoBoard.js
import React, { useState, useEffect } from 'react';
 
const TodoBoard = () => {
  const [todos, setTodos] = useState([]);
  useEffect(() => {
    const url = 'https://jsonplaceholder.typicode.com/todos';
    fetch(url)
      .then((response) => {
        if (reponse.status === 200) {
          return response.json();
        } else {
          throw 'erred';
        }
      })
      .then((data) => setTodos(data))
      .catch((error) => window.alert(error));
    // Refer to Takeaway
  }, []);
 
  return (
    <div>
      {this.state.todos.map((todo) => (
        <Todo key={todo.id} {...todo} />
      ))}
    </div>
  );
};
 
export default TodoBoard;

Takeaway

• The second argument for useEffect() hook is the dependency part of the effect. The certain useEffect() is triggered when any of the state in the dependency array, e.g., foo or bar in [foo, bar], changes
• If useEffect() has an empty dependency array, it is only triggered once when the component is loaded, just like componentDidMount() in class-based components
• Don't forget to clean up the effect in some cases, or the effect would cause memory-leak!