firestore-posts-likes.md

const db = firebase.firestore();
const FieldValue = firebase.firestore.FieldValue;
const myUserID = '123456';

Creating a post

await db.collection('posts').add({
    title: 'Hello world!',
    postedBy: myUserID,
    likedBy: {},
    likes: 0,
})

Has liked a post

const post = await db.collection('posts').doc('test').get();
const hasLiked = post.likedBy[myUserID] === true

Liking a post

await db.collection('posts').doc('test').update({
    likes: FieldValue.increment(1),
    [`likedBy.${myUserID}`]: true,
})

Unliking a post

await db.collection('posts').doc('test').update({
    likes: FieldValue.increment(-1),
    [`likedBy.${myUserID}`]: FieldValue.delete(),
})

Post.js

function Post({id}) {
    const [post, setPost] = useState(null)
    const [hasLiked, setHasLiked] = useState(false)
    useEffect(() => {
        const unsubscribe = db.collection('posts').doc(id).onSnapshot((doc) => {
            const data = doc.data();
            setPost(data);
            setHasLiked(data.likedBy[myUserID] === true)
        });
        return () => unsubscribe()
    }, [])

    function toggleLike() {
        if (hasLiked) {
            db.collection('posts').doc('test').update({
                likes: FieldValue.increment(-1),
                [`likedBy.${myUserID}`]: FieldValue.delete(),
            })
        } else {
            db.collection('posts').doc('test').update({
                likes: FieldValue.increment(1),
                [`likedBy.${myUserID}`]: true,
            })
        }
    }

    if (!post) {
        return 'Loading...'
    }
    return (
        <div>
            <h1>{{post.title}}</h1>
            <button onClick={toggleLike}>{hasLiked ? 'unlike' : 'like'}</button>
            <p>Likes: {{post.likes}}</p>
        </div>
    )
}