Creates a Query with the specified ending point.
Using startAt(), 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.
const ref = firebase.database().ref('users');
const snapshot = await ref.orderByKey().endAt('Ada Lovelace').once('value');
The value to end at. 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.
Optionalkey: stringThe child key to end at, among the children with the previously specified priority. This argument is only allowed if ordering by child, value, or priority.
Creates a Query with the specified ending point.
Using startAt(), 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.
const ref = firebase.database().ref('users');
const snapshot = await ref.orderByChild('age').equalTo(30).once('value');
The value to match for. 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.
Optionalkey: stringThe child key to start at, among the children with the previously specified priority. This argument is only allowed if ordering by child, value, or priority.
Returns whether or not the current and provided queries represent the same location, have the same query parameters.
Two Reference objects are equivalent if they represent the same location and are from the same instance of @firebase/app!FirebaseApp. Equivalent queries share the same sort order, limits, and starting and ending points.
const ref1 = firebase.database().ref('users').orderByKey().endAt('Ada Lovelace');
const ref2 = firebase.database().ref('users').orderByKey();
console.log(ref1.isEqual(ref2)); // false
const ref1 = firebase.database().ref('users').orderByKey().endAt('Ada Lovelace');
const ref2 = firebase.database().ref('users').endAt('Ada Lovelace').orderByKey();
console.log(ref1.isEqual(ref2)); // true
The query to compare against.
By calling keepSynced(true) on a location, the data for that location will automatically
be downloaded and kept in sync, even when no listeners are attached for that location.
const ref = firebase.database().ref('users');
await ref.keepSynced(true);
Pass true to keep this location synchronized, pass false to stop synchronization.
Generates a new Query limited to the first specific number of children.
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.
const snapshot = firebase.database().ref('users').orderByKey().limitToFirst(2).once('value');
console.log(snapshot.numChildren()); // 2
The maximum number of nodes to include in this query.
Generates a new Query object limited to the last specific number of children.
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.
const snapshot = firebase.database().ref('users').orderByKey().limitToLast(2).once('value');
console.log(snapshot.numChildren()); // 2
The maximum number of nodes to include in this query.
Detaches a callback previously attached with on().
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.
const ref = firebase.database().ref('settings');
const onValueChange = function(snapshot) { ... };
const onChildAdded = function(snapshot) { ... };
ref.on('value', onValueChange);
ref.child('meta-data').on('child_added', onChildAdded);
// Sometime later...
ref.off('value', onValueChange);
ref.child('meta-data').off('child_added', onChildAdded);
OptionaleventType: EventTypeOne of the following strings: "value", "child_added", "child_changed", "child_removed", or "child_moved." If omitted, all callbacks for the Reference will be removed.
Optionalcallback: (a: FirebaseDatabaseTypes.DataSnapshot, b?: string | null) => voidThe callback function that was passed to on() or undefined to remove all callbacks.
Optionalcontext: Record<string, any>The context that was passed to on().
Listens for data changes at a particular location.
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..
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:
remove() on that child or one of its ancestorsset(null) on that child or one of its ancestorschild_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.
One of the following strings: "value", "child_added", "child_changed", "child_removed", or "child_moved."
A callback that fires when the specified event occurs. The callback will be passed a DataSnapshot. For ordering purposes, "child_added", "child_changed", and "child_moved" will also be passed a string containing the key of the previous child, by sort order, or null if it is the first child.
OptionalcancelCallbackOrContext: Record<string, any> | ((a: Error) => void) | nullAn optional callback that will be notified if your event subscription is ever canceled because your client does not have permission to read this data (or it had permission but has now lost it). This callback will be passed an Error object indicating why the failure occurred.
Optionalcontext: Record<string, any> | nullIf provided, this object will be used as this when calling your callback(s).
Listens for exactly one event of the specified event type, and then stops listening.
This is equivalent to calling on(), and then calling off() inside the callback function. See on() for details on the event types.
// Promise
const snapshot = await firebase.database().ref('users').once('value');
// Callback
firebase.database().ref('users).once('value', (snapshot) => {
console.log(snapshot.val());
});
One of the following strings: "value", "child_added", "child_changed", "child_removed", or "child_moved."
OptionalsuccessCallback: (a: FirebaseDatabaseTypes.DataSnapshot, b?: string | null) => anyA callback that fires when the specified event occurs. The callback will be passed a DataSnapshot. For ordering purposes, "child_added", "child_changed", and "child_moved" will also be passed a string containing the key of the previous child by sort order, or null if it is the first child.
OptionalfailureCallbackContext: Record<string, any> | ((a: Error) => void) | nullAn optional callback that will be notified if your client does not have permission to read the data. This callback will be passed an Error object indicating why the failure occurred.
Generates a new Query object ordered by the specified child key.
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.
You can read more about orderByChild() in Sort data.
const snapshot = await firebase.database().ref('users').orderByChild('age').once('value');
snapshot.forEach((snapshot) => {
console.log('Users age:', snapshot.val().age);
});
The child path node to order by.
Generates a new Query object ordered by key.
Sorts the results of a query by their (ascending) key values.
You can read more about orderByKey() in Sort data.
const snapshot = await firebase.database().ref('users').orderByKey().once('value');
snapshot.forEach((snapshot) => {
console.log('User:', snapshot.val());
});
Generates a new Query object ordered by priority.
Applications need not use priority but can order collections by ordinary properties (see Sort data for alternatives to priority).
Generates a new Query object ordered by value.
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.
await firebase.database().ref('scores').orderByValue().once('value');
Creates a Query with the specified starting point.
Using startAt(), 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.
await firebase.database().ref('users').orderByChild('age').startAt(21).once('value');
The value to start at. 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.
Optionalkey: stringThe child key to start at. This argument is only allowed if ordering by child, value, or priority.
Returns a JSON-serializable representation of this object.
Gets the absolute URL for this location.
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.
const ref1 = firebase.database().ref();
const ref2 = firebase.database().ref('users').orderByValue();
ref1.toString(); // https://sample-app.firebaseio.com/
ref2.toString(); // https://sample-app.firebaseio.com/users
A Query sorts and filters the data at a Database location so only a subset of the child data is included. This can be used to order a collection of data by some attribute (for example, height of dinosaurs) as well as to restrict a large list of items (for example, chat messages) down to a number suitable for synchronizing to the client. Queries are created by chaining together one or more of the filter methods defined here.
Just as with a
Reference, you can receive data from a Query by using the on() method. You will only receive events andDataSnapshots for the subset of the data that matches your query.