React - Function unaware of state when called from keydown event

Question

I've been working on learning React, and in doing so built a todo app using class components. Recently I have been working to make a copy of the todo app using functions and hooks instead of classes.

Having refactored the code everything appears to be working correctly aside from one use case.

CASE 1: When typing into the input and clicking "Add" button to call addItem() the new todo item adds as expected.

CASE 2: When typing into the input and hitting enter to trigger an event handler which calls addItem() the value of newItem is always the same as its initial value.

I can't for the life of me figure out why addItem() behaves differently when called from the click of the "Add" button versus the press of the "Enter" key.

TodoAppContainer.js

import React, { useState, useEffect } from 'react';
import TodoList from './TodoList';
import TodoForm from './TodoForm';
import { GenerateID } from './generateId';

export default function TodoListContainer(props) {

    const [newItem, setNewItem] = useState('New Todo');
    const [items, setItems] = useState([{
        name: 'Build Todo List App',
        done: true,
        key: GenerateID.next().value
    }]);

    const handleKeyDown = e => {
        if (e.key === 'Enter') addItem();
    };

    const handleChange = ({ target }) => {
        console.log("handleChange");
        // capture text from input field
        const text = target.value;

        // update state value for "newItem"
        setNewItem(text);
    };

    const addItem = () => {
        console.log("addItem");
        // exit early if there is no item
        if (!!!newItem.trim()) return;

        // build new item to add
        const itemToAdd = {
            name: newItem,
            done: false,
            key: GenerateID.next().value
        };

        // update state with new item
        setItems(prevItems => [itemToAdd, ...prevItems]);

        // clear text for new item
        setNewItem('');
    };

    const completeItem = key => {
        console.log('completeItem');
        // create new copy of state items
        const updatedItems = [...items];

        // get the index of the item to update
        const index = updatedItems.findIndex(v => v.key === key);

        // toggle the done state of the item
        updatedItems[index].done = !updatedItems[index].done;

        // update the state
        setItems(updatedItems);
    };

    const removeItem = key => {
        console.log('removeItem');
        // create copy of filtered items
        const filteredItems = items.filter(v => v.key !== key);

        // update the state of items
        setItems(filteredItems);
    }

    // get count of items that are "done"
    const getTodoCount = () => items.filter(v => v.done).length;

    useEffect(() => {
        document.addEventListener('keydown', handleKeyDown);
        return () => document.removeEventListener('keydown', handleKeyDown);
    }, []);

    return (
        <section className='todo-section'>
            <TodoForm
                newItem={newItem}
                handleChange={handleChange}
                addItem={addItem}
            />
            <TodoList
                items={items}
                count={getTodoCount()}
                onClick={completeItem}
                onRemove={removeItem}
            />
        </section>
    );
}

TodoForm.js

import React from 'react';
import PropTypes from 'prop-types';

export default function TodoForm(props) {
    const { newItem, handleChange, addItem } = props;
    return (
        <div className='todo-form'>
            <input type='text' value={newItem} onChange={handleChange} />
            <button onClick={addItem}>Add</button>
        </div>
    )
}

TodoForm.propTypes = {
    newItem: PropTypes.string.isRequired,
    addItem: PropTypes.func.isRequired,
    handleChange: PropTypes.func.isRequired
};

TodoList.js

import React from 'react';
import PropTypes from 'prop-types';

export default function TodoList(props) {
    const { items, count, onClick, onRemove } = props;
    const shrug = '¯\\_(ツ)_/¯';
    const shrugStyles = { fontSize: '2rem', fontWeight: 400, textAlign: 'center' };

    const buildItemHTML = ({ key, name, done }) => {
        const className = done ? 'todo-item done' : 'todo-item';
        return (
            <li className={className} key={key}>
                <span className='item-name' onClick={() => onClick(key)}>{name}</span>
                <span className='remove-icon' onClick={() => onRemove(key)}>&#10006;</span>
            </li>
        );
    };

    return (
        <div>
            <p style={{ margin: 0, padding: '0.75em' }}>{count} of {items.length} Items Complete!</p>
            <ul className='todo-list'>
                {items.length ? items.map(buildItemHTML) : <h1 style={shrugStyles}>{shrug}<br />No items here...</h1>}
            </ul>
        </div>
    );
};

TodoList.propTypes = {
    count: PropTypes.number.isRequired,
    items: PropTypes.array.isRequired,
    onClick: PropTypes.func.isRequired,
    onRemove: PropTypes.func.isRequired
};

Answer 1

This is happening because the you are adding the event listener inside useEffect and at that time the value of newItem is your initial newItem.

To make it work , you can add newItem to dependecy array to update the event listeners, every time newItem updates.

useEffect(() => {
        document.addEventListener('keydown', handleKeyDown);
        return () => document.removeEventListener('keydown', handleKeyDown);
    }, [newItem]);

This however is just a solution but not the recommended solution. Adding event listeners this way is not very React-ish.

Instead of writing event listeners in useEffect.You should instead bind an event like this

export default function TodoForm(props) {
    const { newItem, handleChange, addItem ,handleKeyDown} = props;
    return (
        <div className='todo-form'>
            <input type='text' 
                value={newItem} 
                onChange={handleChange} 
                onKeyDown={handleKeyDown}// this is new
             />
            <button onClick={addItem}>Add</button>
        </div>
    )
}

And don't forget to add it in the parent component

<TodoForm
                newItem={newItem}
                handleChange={handleChange}
                handleKeydown={handleKeydown}//this is new
                addItem={addItem}
            />