​
Usage

You can use the firestoreGet function to get a document from a specified path. The first argument is the slash-separated path of the collection containing the document. 
// Define a function to add a post
const getPost = firestoreGet('posts')

// Use the function to get a post
const postId = 'post-id-1'
const myPost = await getPost(postId)

console.log(myPost.title) // 'My First Post'

​
Subcollections

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

const commentId = 'comment-id-1'
const myComment = await getComment(commentId)

console.log(myComment.body) // 'What a great 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 add a post
const getComment = firestoreGet<Comment>(
  ({ postId }) => `posts/${postId}/comments`,
)

const postId = 'post-id-1'
const commentId = 'comment-id-1'

await getComment(commentId, { postId })

console.log(myComment.body) // 'What a great post!'

​
Return Value

The function returns a Promise that resolves with either the expanded document data or undefined if the document does not exist. The document data will be expanded to include the document ID as a property id. 
const post = await getPost('post-id-1')

// If the post doesn't exist, post will be undefined.
const doesPostExist = post !== undefined

// If the post exists, post will be an object with the post data.
console.log(post.title) // 'My First Post'

// The post object will also have an id property.
console.log(post.id) // 'post-id-1'

​
Type Safety

If a type parameter is provided, the returned function will be typesafe. Its return type will be a Promise that resolves with the provided type or undefined. 
type Post = {
  title: string
  body: string
}

const getPost = firestoreGet<Post>('posts')

// This return type will be Promise<Post | undefined>
const myPost = await getPost(postId)
The provided type definitions will help you to avoid common mistakes, but they are not realtime validation. You can still retrieve invalid data to Firestore if you don’t use the provided functions correctly.