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Usage

You can use the firestoreUpdate function to update a document from a collection. The first argument is the slash-separated path of the collection. 
// Define a function to update a post
const updatePost = firestoreUpdate('posts')

// Use the function to update a post
const postId = 'post-id-1'
await updatePost(postId, { title: 'My updated post' })

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Subcollections

Collections don’t have to be at the top level of the Firestore, adding slashes will update the document in a subcollection. For example, if you want to update a post in a collection of posts for a specific user: 
// Define a function to update a post
const userId = 'user-id-1'
const updatePostForUser = firestoreUpdate<Post>(`users/${userId}/posts`)

const postId = 'post-id-1'
await updatePostForUser(postId, { title: 'My post' })

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Dynamic Paths

Using subcollections is a common pattern in Firestore since it makes writing security rules much easier. To make our function more reusable, we can use a function that takes an arguments object and returns a string representing the path: 
// Define a function to update a post
const updatePostForUser = firestoreUpdate<Post>(
  ({ userId }) => `users/${userId}/posts`,
)

const postId = 'post-id-1'
const userId = 'user-id-1'

await updatePostForUser(postId, { title: 'My post' }, { userId })
Even more convenient is to pull the userId from the post itself, assuming the post contains it. This can be done by using the data property of the arguments object which contains the data that will be written to the document: 
type Post = {
  title: string
  authorId: string
}

// Define a function to update a post
const updatePostForUser = firestoreUpdate<Post>(
  ({ data }) => `users/${data.authorId}/posts`,
)

const postId = 'post-id-1'
const authorId = 'user-id-1'

await updatePostForUser(postId, { title: 'My post', authorId })

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Return Value

The function returns a Promise that resolves with a DocumentReference to the newly created document. 
const postId = 'post-id-1'
const ref = await updatePost(postId, { title: 'My updated post' })

// The value returned is a DocumentReference
await ref.delete()

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Type Safety

If a type parameter is provided, the returned function will be typesafe. It will allow any Partial of the provided type to be passed as the second argument. It will not allow any other type to be passed as the second argument. 
type Post = {
  title: string
  body: string
}

const updatePost = firestoreUpdate<Post>('posts')

const postId = 'post-id-1'

// This update is allowed because it is a Partial<Post>
await updatePost(postId, { title: 'My updated post' })

// This update is not allowed because it is not a Partial<Post>
await updatePost(postId, { title: 'My updated post', coverImageSrc: '...' })
The provided type definitions will help you to avoid common mistakes, but they are not realtime validation. You can still write invalid data to Firestore if you don’t use the provided functions correctly.

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Options

The second argument to firestoreUpdate is an optional object allowing for additional configuration. This object can have a property addMetadata which, if true, adds metadata to the document. 
// The updatePost function will include metadata to the new document.
const updatePost = firestoreUpdate<Post>('posts', { addMetadata: true })

await updatePost('post-id-1', { title: 'First Post' })
await updatePost('post-id-2', { title: 'Second Post' })
await updatePost('post-id-3', { title: 'Third Post' })

// The posts will be ordered by the time they were updated.
const recentPosts = await firestore
  .collection('posts')
  .orderBy('metadata.timeUpdated', 'asc')
  .limit(10)
  .get()