Read and Write Data on the Web

(Optional) Prototype and test with Firebase Local Emulator Suite

Before talking about how your app reads from and writes to Realtime Database, let's introduce a set of tools you can use to prototype and test Realtime Database functionality: Firebase Local Emulator Suite. If you're trying out different data models, optimizing your security rules, or working to find the most cost-effective way to interact with the back-end, being able to work locally without deploying live services can be a great idea.

A Realtime Database emulator is part of the Local Emulator Suite, which enables your app to interact with your emulated database content and config, as well as optionally your emulated project resources (functions, other databases, and security rules). Note that the Local Emulator Suite doesn't yet support emulated Cloud Storage.

Using the Realtime Database emulator involves just a few steps:

  1. Adding a line of code to your app's test config to connect to the emulator.
  2. From the root of your local project directory, running firebase emulators:start.
  3. Making calls from your app's prototype code using a Realtime Database platform SDK as usual, or using the Realtime Database REST API.

A detailed walkthrough involving Realtime Database and Cloud Functions is available. You should also have a look at the Local Emulator Suite introduction.

Get a database reference

To read or write data from the database, you need an instance of firebase.database.Reference:

var database = firebase.database();

Write data

This document covers the basics of retrieving data and how to order and filter Firebase data.

Firebase data is retrieved by attaching an asynchronous listener to a firebase.database.Reference. The listener is triggered once for the initial state of the data and again anytime the data changes.

Basic write operations

For basic write operations, you can use set() to save data to a specified reference, replacing any existing data at that path. For example a social blogging application might add a user with set() as follows:

function writeUserData(userId, name, email, imageUrl) {
  firebase.database().ref('users/' + userId).set({
    username: name,
    email: email,
    profile_picture : imageUrl

Using set() overwrites data at the specified location, including any child nodes.

Read data

Listen for value events

To read data at a path and listen for changes, use the on() oronce() methods of firebase.database.Reference to observe events.

Event Typical usage
value Read and listen for changes to the entire contents of a path.

You can use the value event to read a static snapshot of the contents at a given path, as they existed at the time of the event. This method is triggered once when the listener is attached and again every time the data, including children, changes. The event callback is passed a snapshot containing all data at that location, including child data. If there is no data, the snapshot will return false when you call exists() and null when you call val() on it.

The following example demonstrates a social blogging application retrieving the star count of a post from the database:

var starCountRef = firebase.database().ref('posts/' + postId + '/starCount');
starCountRef.on('value', (snapshot) => {
  const data = snapshot.val();
  updateStarCount(postElement, data);

The listener receives a snapshot that contains the data at the specified location in the database at the time of the event. You can retrieve the data in the snapshot with the val() method.

Read data once

Read data once with get()

The SDK is designed to manage interactions with database servers whether your app is online or offline.

Generally, you should use the value event techniques described above to read data to get notified of updates to the data from the backend. The listener techniques reduce your usage and billing, and are optimized to give your users the best experience as they go online and offline.

If you need the data only once, you can use get() to get a snapshot of the data from the database. If for any reason get() is unable to return the server value, the client will probe the local storage cache and return an error if the value is still not found.

Unnecessary use of get() can increase use of bandwidth and lead to loss of performance, which can be prevented by using a realtime listener as shown above.

database.child("users").child(userId).get().then(function(snapshot) {
  if (snapshot.exists()) {
  else {
    console.log("No data available");
}).catch(function(error) {

Read data once with an observer

In some cases you may want the value from the local cache to be returned immediately, instead of checking for an updated value on the server. In those cases you can use once() to get the data from the local disk cache immediately.

This is useful for data that only needs to be loaded once and isn't expected to change frequently or require active listening. For instance, the blogging app in the previous examples uses this method to load a user's profile when they begin authoring a new post:

var userId = firebase.auth().currentUser.uid;
return firebase.database().ref('/users/' + userId).once('value').then((snapshot) => {
  var username = (snapshot.val() && snapshot.val().username) || 'Anonymous';
  // ...

Updating or deleting data

Update specific fields

To simultaneously write to specific children of a node without overwriting other child nodes, use the update() method.

When calling update(), you can update lower-level child values by specifying a path for the key. If data is stored in multiple locations to scale better, you can update all instances of that data using data fan-out.

For example, a social blogging app might create a post and simultaneously update it to the recent activity feed and the posting user's activity feed using code like this:

function writeNewPost(uid, username, picture, title, body) {
  // A post entry.
  var postData = {
    author: username,
    uid: uid,
    body: body,
    title: title,
    starCount: 0,
    authorPic: picture

  // Get a key for a new Post.
  var newPostKey = firebase.database().ref().child('posts').push().key;

  // Write the new post's data simultaneously in the posts list and the user's post list.
  var updates = {};
  updates['/posts/' + newPostKey] = postData;
  updates['/user-posts/' + uid + '/' + newPostKey] = postData;

  return firebase.database().ref().update(updates);

This example uses push() to create a post in the node containing posts for all users at /posts/$postid and simultaneously retrieve the key. The key can then be used to create a second entry in the user's posts at /user-posts/$userid/$postid.

Using these paths, you can perform simultaneous updates to multiple locations in the JSON tree with a single call to update(), such as how this example creates the new post in both locations. Simultaneous updates made this way are atomic: either all updates succeed or all updates fail.

Add a Completion Callback

If you want to know when your data has been committed, you can add a completion callback. Both set() and update() take an optional completion callback that is called when the write has been committed to the database. If the call was unsuccessful, the callback is passed an error object indicating why the failure occurred.

firebase.database().ref('users/' + userId).set({
  username: name,
  email: email,
  profile_picture : imageUrl
}, (error) => {
  if (error) {
    // The write failed...
  } else {
    // Data saved successfully!

Delete data

The simplest way to delete data is to call remove() on a reference to the location of that data.

You can also delete by specifying null as the value for another write operation such as set() or update(). You can use this technique with update() to delete multiple children in a single API call.

Receive a Promise

To know when your data is committed to the Firebase Realtime Database server, you can use a Promise. Both set() and update() can return a Promise you can use to know when the write is committed to the database.

Detach listeners

Callbacks are removed by calling the off() method on your Firebase database reference.

You can remove a single listener by passing it as a parameter to off(). Calling off() on the location with no arguments removes all listeners at that location.

Calling off() on a parent listener does not automatically remove listeners registered on its child nodes; off() must also be called on any child listeners to remove the callback.

Save data as transactions

When working with data that could be corrupted by concurrent modifications, such as incremental counters, you can use a transaction operation. You can give this operation an update function and an optional completion callback. The update function takes the current state of the data as an argument and returns the new desired state you would like to write. If another client writes to the location before your new value is successfully written, your update function is called again with the new current value, and the write is retried.

For instance, in the example social blogging app, you could allow users to star and unstar posts and keep track of how many stars a post has received as follows:

function toggleStar(postRef, uid) {
  postRef.transaction((post) => {
    if (post) {
      if (post.stars && post.stars[uid]) {
        post.stars[uid] = null;
      } else {
        if (!post.stars) {
          post.stars = {};
        post.stars[uid] = true;
    return post;

Using a transaction prevents star counts from being incorrect if multiple users star the same post at the same time or the client had stale data. If the transaction is rejected, the server returns the current value to the client, which runs the transaction again with the updated value. This repeats until the transaction is accepted or you abort the transaction.

Work with data offline

If a client loses its network connection, your app will continue functioning correctly.

Every client connected to a Firebase database maintains its own internal version of any active data. When data is written, it's written to this local version first. The Firebase client then synchronizes that data with the remote database servers and with other clients on a "best-effort" basis.

As a result, all writes to the database trigger local events immediately, before any data is written to the server. This means your app remains responsive regardless of network latency or connectivity.

Once connectivity is reestablished, your app receives the appropriate set of events so that the client syncs with the current server state, without having to write any custom code.

We'll talk more about offline behavior in Learn more about online and offline capabilities..

Next steps