• firebase.
• database.
• ThenableReference

The relative path can either be a simple child name (for example, "ada") or a deeper slash-separated path (for example, "ada/name/first").

example

var usersRef = firebase.database().ref('users');
var adaRef = usersRef.child('ada');
var adaFirstNameRef = adaRef.child('name/first');
var path = adaFirstNameRef.toString();
// path is now 'https://sample-app.firebaseio.com/users/ada/name/first'

Using startAt(), startAfter(), endBefore(), endAt() and equalTo() allows you to choose arbitrary starting and ending points for your queries.

The ending point is inclusive, so children with exactly the specified value will be included in the query. The optional key argument can be used to further limit the range of the query. If it is specified, then children that have exactly the specified value must also have a key name less than or equal to the specified key.

You can read more about endAt() in Filtering data.

example

// Find all dinosaurs whose names come before Pterodactyl lexicographically.
// Include Pterodactyl in the result.
var ref = firebase.database().ref("dinosaurs");
ref.orderByKey().endAt("pterodactyl").on("child_added", function(snapshot) {
  console.log(snapshot.key);
});

Using startAt(), startAfter(), endBefore(), endAt() and equalTo() allows you to choose arbitrary starting and ending points for your queries.

The ending point is exclusive. If only a value is provided, children with a value less than the specified value will be included in the query. If a key is specified, then children must have a value lesss than or equal to the specified value and a a key name less than the specified key.

example

// Find all dinosaurs whose names come before Pterodactyl lexicographically.
// Do not include Pterodactyl in the result.
var ref = firebase.database().ref("dinosaurs");
ref.orderByKey().endBefore("pterodactyl").on("child_added", function(snapshot) {
  console.log(snapshot.key);
});

@param value The value to end before. The argument
  type depends on which `orderBy*()` function was used in this query.
  Specify a value that matches the `orderBy*()` type. When used in
  combination with `orderByKey()`, the value must be a string.
@param key The child key to end before, among the children with the
  previously specified priority. This argument is only allowed if ordering by
  child, value, or priority.

Using startAt(), startAfter(), endBefore(), endAt() and equalTo() allows you to choose arbitrary starting and ending points for your queries.

The optional key argument can be used to further limit the range of the query. If it is specified, then children that have exactly the specified value must also have exactly the specified key as their key name. This can be used to filter result sets with many matches for the same value.

You can read more about equalTo() in Filtering data.

example

// Find all dinosaurs whose height is exactly 25 meters.
var ref = firebase.database().ref("dinosaurs");
ref.orderByChild("height").equalTo(25).on("child_added", function(snapshot) {
  console.log(snapshot.key);
});

Two Reference objects are equivalent if they represent the same location and are from the same instance of firebase.app.App.

Two Query objects are equivalent if they represent the same location, have the same query parameters, and are from the same instance of firebase.app.App. Equivalent queries share the same sort order, limits, and starting and ending points.

example

var rootRef = firebase.database.ref();
var usersRef = rootRef.child("users");

usersRef.isEqual(rootRef);  // false
usersRef.isEqual(rootRef.child("users"));  // true
usersRef.parent.isEqual(rootRef);  // true

example

var rootRef = firebase.database.ref();
var usersRef = rootRef.child("users");
var usersQuery = usersRef.limitToLast(10);

usersQuery.isEqual(usersRef);  // false
usersQuery.isEqual(usersRef.limitToLast(10));  // true
usersQuery.isEqual(rootRef.limitToLast(10));  // false
usersQuery.isEqual(usersRef.orderByKey().limitToLast(10));  // false

The limitToFirst() method is used to set a maximum number of children to be synced for a given callback. If we set a limit of 100, we will initially only receive up to 100 child_added events. If we have fewer than 100 messages stored in our Database, a child_added event will fire for each message. However, if we have over 100 messages, we will only receive a child_added event for the first 100 ordered messages. As items change, we will receive child_removed events for each item that drops out of the active list so that the total number stays at 100.

You can read more about limitToFirst() in Filtering data.

example

// Find the two shortest dinosaurs.
var ref = firebase.database().ref("dinosaurs");
ref.orderByChild("height").limitToFirst(2).on("child_added", function(snapshot) {
  // This will be called exactly two times (unless there are less than two
  // dinosaurs in the Database).

  // It will also get fired again if one of the first two dinosaurs is
  // removed from the data set, as a new dinosaur will now be the second
  // shortest.
  console.log(snapshot.key);
});

The limitToLast() method is used to set a maximum number of children to be synced for a given callback. If we set a limit of 100, we will initially only receive up to 100 child_added events. If we have fewer than 100 messages stored in our Database, a child_added event will fire for each message. However, if we have over 100 messages, we will only receive a child_added event for the last 100 ordered messages. As items change, we will receive child_removed events for each item that drops out of the active list so that the total number stays at 100.

You can read more about limitToLast() in Filtering data.

example

// Find the two heaviest dinosaurs.
var ref = firebase.database().ref("dinosaurs");
ref.orderByChild("weight").limitToLast(2).on("child_added", function(snapshot) {
  // This callback will be triggered exactly two times, unless there are
  // fewer than two dinosaurs stored in the Database. It will also get fired
  // for every new, heavier dinosaur that gets added to the data set.
  console.log(snapshot.key);
});

Detach a callback previously attached with on(). Note that if on() was called multiple times with the same eventType and callback, the callback will be called multiple times for each event, and off() must be called multiple times to remove the callback. Calling off() on a parent listener will not automatically remove listeners registered on child nodes, off() must also be called on any child listeners to remove the callback.

If a callback is not specified, all callbacks for the specified eventType will be removed. Similarly, if no eventType is specified, all callbacks for the Reference will be removed.

example

var onValueChange = function(dataSnapshot) {  ... };
ref.on('value', onValueChange);
ref.child('meta-data').on('child_added', onChildAdded);
// Sometime later...
ref.off('value', onValueChange);

// You must also call off() for any child listeners on ref
// to cancel those callbacks
ref.child('meta-data').off('child_added', onValueAdded);

example

// Or you can save a line of code by using an inline function
// and on()'s return value.
var onValueChange = ref.on('value', function(dataSnapshot) { ... });
// Sometime later...
ref.off('value', onValueChange);

This is the primary way to read data from a Database. Your callback will be triggered for the initial data and again whenever the data changes. Use off( ) to stop receiving updates. See Retrieve Data on the Web for more details.

value event

This event will trigger once with the initial data stored at this location, and then trigger again each time the data changes. The DataSnapshot passed to the callback will be for the location at which on() was called. It won't trigger until the entire contents has been synchronized. If the location has no data, it will be triggered with an empty DataSnapshot (val() will return null).

child_added event

This event will be triggered once for each initial child at this location, and it will be triggered again every time a new child is added. The DataSnapshot passed into the callback will reflect the data for the relevant child. For ordering purposes, it is passed a second argument which is a string containing the key of the previous sibling child by sort order, or null if it is the first child.

child_removed event

This event will be triggered once every time a child is removed. The DataSnapshot passed into the callback will be the old data for the child that was removed. A child will get removed when either:

• a client explicitly calls remove() on that child or one of its ancestors
• a client calls set(null) on that child or one of its ancestors
• that child has all of its children removed
• there is a query in effect which now filters out the child (because it's sort order changed or the max limit was hit)

child_changed event

This event will be triggered when the data stored in a child (or any of its descendants) changes. Note that a single child_changed event may represent multiple changes to the child. The DataSnapshot passed to the callback will contain the new child contents. For ordering purposes, the callback is also passed a second argument which is a string containing the key of the previous sibling child by sort order, or null if it is the first child.

child_moved event

This event will be triggered when a child's sort order changes such that its position relative to its siblings changes. The DataSnapshot passed to the callback will be for the data of the child that has moved. It is also passed a second argument which is a string containing the key of the previous sibling child by sort order, or null if it is the first child.

example

Handle a new value:

ref.on('value', function(dataSnapshot) {
  ...
});

example

Handle a new child:

ref.on('child_added', function(childSnapshot, prevChildKey) {
  ...
});

example

Handle child removal:

ref.on('child_removed', function(oldChildSnapshot) {
  ...
});

example

Handle child data changes:

ref.on('child_changed', function(childSnapshot, prevChildKey) {
  ...
});

example

Handle child ordering changes:

ref.on('child_moved', function(childSnapshot, prevChildKey) {
  ...
});

This is equivalent to calling on(), and then calling off() inside the callback function. See on() for details on the event types.

example

// Basic usage of .once() to read the data located at ref.
ref.once('value')
  .then(function(dataSnapshot) {
    // handle read data.
  });

Queries can only order by one key at a time. Calling orderByChild() multiple times on the same query is an error.

Firebase queries allow you to order your data by any child key on the fly. However, if you know in advance what your indexes will be, you can define them via the .indexOn rule in your Security Rules for better performance. See the .indexOn rule for more information.

You can read more about orderByChild() in Sort data.

example

var ref = firebase.database().ref("dinosaurs");
ref.orderByChild("height").on("child_added", function(snapshot) {
  console.log(snapshot.key + " was " + snapshot.val().height + " m tall");
});

Sorts the results of a query by their (ascending) key values.

You can read more about orderByKey() in Sort data.

example

var ref = firebase.database().ref("dinosaurs");
ref.orderByKey().on("child_added", function(snapshot) {
  console.log(snapshot.key);
});

Applications need not use priority but can order collections by ordinary properties (see Sort data for alternatives to priority.

If the children of a query are all scalar values (string, number, or boolean), you can order the results by their (ascending) values.

You can read more about orderByValue() in Sort data.

example

var scoresRef = firebase.database().ref("scores");
scoresRef.orderByValue().limitToLast(3).on("value", function(snapshot) {
  snapshot.forEach(function(data) {
    console.log("The " + data.key + " score is " + data.val());
  });
});

This is the most common pattern for adding data to a collection of items.

If you provide a value to push(), the value is written to the generated location. If you don't pass a value, nothing is written to the database and the child remains empty (but you can use the Reference elsewhere).

The unique keys generated by push() are ordered by the current time, so the resulting list of items is chronologically sorted. The keys are also designed to be unguessable (they contain 72 random bits of entropy).

See Append to a list of data
See The 2^120 Ways to Ensure Unique Identifiers

example

var messageListRef = firebase.database().ref('message_list');
var newMessageRef = messageListRef.push();
newMessageRef.set({
  'user_id': 'ada',
  'text': 'The Analytical Engine weaves algebraical patterns just as the Jacquard loom weaves flowers and leaves.'
});
// We've appended a new message to the message_list location.
var path = newMessageRef.toString();
// path will be something like
// 'https://sample-app.firebaseio.com/message_list/-IKo28nwJLH0Nc5XeFmj'

Any data at child locations will also be deleted.

The effect of the remove will be visible immediately and the corresponding event 'value' will be triggered. Synchronization of the remove to the Firebase servers will also be started, and the returned Promise will resolve when complete. If provided, the onComplete callback will be called asynchronously after synchronization has finished.

example

var adaRef = firebase.database().ref('users/ada');
adaRef.remove()
  .then(function() {
    console.log("Remove succeeded.")
  })
  .catch(function(error) {
    console.log("Remove failed: " + error.message)
  });

This will overwrite any data at this location and all child locations.

The effect of the write will be visible immediately, and the corresponding events ("value", "child_added", etc.) will be triggered. Synchronization of the data to the Firebase servers will also be started, and the returned Promise will resolve when complete. If provided, the onComplete callback will be called asynchronously after synchronization has finished.

Passing null for the new value is equivalent to calling remove(); namely, all data at this location and all child locations will be deleted.

set() will remove any priority stored at this location, so if priority is meant to be preserved, you need to use setWithPriority() instead.

Note that modifying data with set() will cancel any pending transactions at that location, so extreme care should be taken if mixing set() and transaction() to modify the same data.

A single set() will generate a single "value" event at the location where the set() was performed.

example

var adaNameRef = firebase.database().ref('users/ada/name');
adaNameRef.child('first').set('Ada');
adaNameRef.child('last').set('Lovelace');
// We've written 'Ada' to the Database location storing Ada's first name,
// and 'Lovelace' to the location storing her last name.

example

adaNameRef.set({ first: 'Ada', last: 'Lovelace' });
// Exact same effect as the previous example, except we've written
// Ada's first and last name simultaneously.

example

adaNameRef.set({ first: 'Ada', last: 'Lovelace' })
  .then(function() {
    console.log('Synchronization succeeded');
  })
  .catch(function(error) {
    console.log('Synchronization failed');
  });
// Same as the previous example, except we will also log a message
// when the data has finished synchronizing.

Applications need not use priority but can order collections by ordinary properties (see Sorting and filtering data).

Applications need not use priority but can order collections by ordinary properties (see Sorting and filtering data).

Using startAt(), startAfter(), endBefore(), endAt() and equalTo() allows you to choose arbitrary starting and ending points for your queries.

The starting point is exclusive. If only a value is provided, children with a value greater than the specified value will be included in the query. If a key is specified, then children must have a value greater than or equal to the specified value and a a key name greater than the specified key.

example

// Find all dinosaurs that are more than three meters tall.
var ref = firebase.database().ref("dinosaurs");
ref.orderByChild("height").startAfter(3).on("child_added", function(snapshot) {
  console.log(snapshot.key)
});

Using startAt(), startAfter(), endBefore(), endAt() and equalTo() allows you to choose arbitrary starting and ending points for your queries.

The starting point is inclusive, so children with exactly the specified value will be included in the query. The optional key argument can be used to further limit the range of the query. If it is specified, then children that have exactly the specified value must also have a key name greater than or equal to the specified key.

You can read more about startAt() in Filtering data.

example

// Find all dinosaurs that are at least three meters tall.
var ref = firebase.database().ref("dinosaurs");
ref.orderByChild("height").startAt(3).on("child_added", function(snapshot) {
  console.log(snapshot.key)
});

The toString() method returns a URL that is ready to be put into a browser, curl command, or a firebase.database().refFromURL() call. Since all of those expect the URL to be url-encoded, toString() returns an encoded URL.

Append '.json' to the returned URL when typed into a browser to download JSON-formatted data. If the location is secured (that is, not publicly readable), you will get a permission-denied error.

example

// Calling toString() on a root Firebase reference returns the URL where its
// data is stored within the Database:
var rootRef = firebase.database().ref();
var rootUrl = rootRef.toString();
// rootUrl === "https://sample-app.firebaseio.com/".

// Calling toString() at a deeper Firebase reference returns the URL of that
// deep path within the Database:
var adaRef = rootRef.child('users/ada');
var adaURL = adaRef.toString();
// adaURL === "https://sample-app.firebaseio.com/users/ada".

Atomically modify the data at this location. Unlike a normal set(), which just overwrites the data regardless of its previous value, transaction() is used to modify the existing value to a new value, ensuring there are no conflicts with other clients writing to the same location at the same time.

To accomplish this, you pass transaction() an update function which is used to transform the current value into a new value. If another client writes to the location before your new value is successfully written, your update function will be called again with the new current value, and the write will be retried. This will happen repeatedly until your write succeeds without conflict or you abort the transaction by not returning a value from your update function.

Note: Modifying data with set() will cancel any pending transactions at that location, so extreme care should be taken if mixing set() and transaction() to update the same data.

Note: When using transactions with Security and Firebase Rules in place, be aware that a client needs .read access in addition to .write access in order to perform a transaction. This is because the client-side nature of transactions requires the client to read the data in order to transactionally update it.

example

// Increment Ada's rank by 1.
var adaRankRef = firebase.database().ref('users/ada/rank');
adaRankRef.transaction(function(currentRank) {
  // If users/ada/rank has never been set, currentRank will be `null`.
  return currentRank + 1;
});

example

// Try to create a user for ada, but only if the user id 'ada' isn't
// already taken
var adaRef = firebase.database().ref('users/ada');
adaRef.transaction(function(currentData) {
  if (currentData === null) {
    return { name: { first: 'Ada', last: 'Lovelace' } };
  } else {
    console.log('User ada already exists.');
    return; // Abort the transaction.
  }
}, function(error, committed, snapshot) {
  if (error) {
    console.log('Transaction failed abnormally!', error);
  } else if (!committed) {
    console.log('We aborted the transaction (because ada already exists).');
  } else {
    console.log('User ada added!');
  }
  console.log("Ada's data: ", snapshot.val());
});

The values argument contains multiple property-value pairs that will be written to the Database together. Each child property can either be a simple property (for example, "name") or a relative path (for example, "name/first") from the current location to the data to update.

As opposed to the set() method, update() can be use to selectively update only the referenced properties at the current location (instead of replacing all the child properties at the current location).

The effect of the write will be visible immediately, and the corresponding events ('value', 'child_added', etc.) will be triggered. Synchronization of the data to the Firebase servers will also be started, and the returned Promise will resolve when complete. If provided, the onComplete callback will be called asynchronously after synchronization has finished.

A single update() will generate a single "value" event at the location where the update() was performed, regardless of how many children were modified.

Note that modifying data with update() will cancel any pending transactions at that location, so extreme care should be taken if mixing update() and transaction() to modify the same data.

Passing null to update() will remove the data at this location.

See Introducing multi-location updates and more.

example

var adaNameRef = firebase.database().ref('users/ada/name');
// Modify the 'first' and 'last' properties, but leave other data at
// adaNameRef unchanged.
adaNameRef.update({ first: 'Ada', last: 'Lovelace' });