firebase.database.DataSnapshot

interface   static

A DataSnapshot contains data from a Database location.

Any time you read data from the Database, you receive the data as a DataSnapshot. A DataSnapshot is passed to the event callbacks you attach with on() or once(). You can extract the contents of the snapshot as a JavaScript object by calling the val() method. Alternatively, you can traverse into the snapshot by calling child() to return child snapshots (which you could then call val() on).

A DataSnapshot is an efficiently generated, immutable copy of the data at a Database location. It cannot be modified and will never change (to modify data, you always call the set() method on a Reference directly).

Properties

key

(string or null)

The key (last part of the path) of the location of this DataSnapshot.

The last token in a Database location is considered its key. For example, "ada" is the key for the /users/ada/ node. Accessing the key on any DataSnapshot will return the key for the location that generated it. However, accessing the key on the root URL of a Database will return null.

Examples 

// Assume we have the following data in the Database:
{
  "name": {
    "first": "Ada",
    "last": "Lovelace"
  }
}

var ref = firebase.database().ref("users/ada");
ref.once("value")
  .then(function(snapshot) {
    var key = snapshot.key; // "ada"
    var childKey = snapshot.child("name/last").key; // "last"
  });
var rootRef = firebase.database().ref();
rootRef.once("value")
  .then(function(snapshot) {
    var key = snapshot.key; // null
    var childKey = snapshot.child("users/ada").key; // "ada"
  });

ref

non-null firebase.database.Reference

The Reference for the location that generated this DataSnapshot.

Methods

child

child(path) returns firebase.database.DataSnapshot

Gets another DataSnapshot for the location at the specified relative path.

Passing a relative path to the child() method of a DataSnapshot returns another DataSnapshot for the location at the specified relative path. The relative path can either be a simple child name (for example, "ada") or a deeper, slash-separated path (for example, "ada/name/first"). If the child location has no data, an empty DataSnapshot (that is, a DataSnapshot whose value is null) is returned.

Parameter

path

string

A relative path to the location of child data.
Returns

non-null firebase.database.DataSnapshot 

Example 

// Assume we have the following data in the Database:
{
  "name": {
    "first": "Ada",
    "last": "Lovelace"
  }
}

// Test for the existence of certain keys within a DataSnapshot
var ref = firebase.database().ref("users/ada");
ref.once("value")
  .then(function(snapshot) {
    var name = snapshot.child("name").val(); // {first:"Ada",last:"Lovelace"}
    var firstName = snapshot.child("name/first").val(); // "Ada"
    var lastName = snapshot.child("name").child("last").val(); // "Lovelace"
    var age = snapshot.child("age").val(); // null
  });

exists

exists() returns boolean

Returns true if this DataSnapshot contains any data. It is slightly more efficient than using snapshot.val() !== null.

Returns

boolean 

Example 

// Assume we have the following data in the Database:
{
  "name": {
    "first": "Ada",
    "last": "Lovelace"
  }
}

// Test for the existence of certain keys within a DataSnapshot
var ref = firebase.database().ref("users/ada");
ref.once("value")
  .then(function(snapshot) {
    var a = snapshot.exists();  // true
    var b = snapshot.child("name").exists(); // true
    var c = snapshot.child("name/first").exists(); // true
    var d = snapshot.child("name/middle").exists(); // false
  });

exportVal

exportVal() returns any type

Exports the entire contents of the DataSnapshot as a JavaScript object.

The exportVal() method is similar to val(), except priority information is included (if available), making it suitable for backing up your data.

Returns

any type The DataSnapshot's contents as a JavaScript value (Object, Array, string, number, boolean, or null).

forEach

forEach(action) returns boolean

Enumerates the top-level children in the DataSnapshot.

Because of the way JavaScript objects work, the ordering of data in the JavaScript object returned by val() is not guaranteed to match the ordering on the server nor the ordering of child_added events. That is where forEach() comes in handy. It guarantees the children of a DataSnapshot will be iterated in their query order.

If no explicit orderBy*() method is used, results are returned ordered by key (unless priorities are used, in which case, results are returned by priority).

Parameter

action

function(non-null firebase.database.DataSnapshot)

A function that will be called for each child DataSnapshot. The callback can return true to cancel further enumeration.
Returns

boolean true if enumeration was canceled due to your callback returning true.

Examples 

// Assume we have the following data in the Database:
{
  "users": {
    "ada": {
      "first": "Ada",
      "last": "Lovelace"
    },
    "alan": {
      "first": "Alan",
      "last": "Turing"
    }
  }
}

// Loop through users in order with the forEach() method. The callback
// provided to forEach() will be called synchronously with a DataSnapshot
// for each child:
var query = firebase.database().ref("users").orderByKey();
query.once("value")
  .then(function(snapshot) {
    snapshot.forEach(function(childSnapshot) {
      // key will be "ada" the first time and "alan" the second time
      var key = childSnapshot.key;
      // childData will be the actual contents of the child
      var childData = childSnapshot.val();
  });
});
// You can cancel the enumeration at any point by having your callback
// function return true. For example, the following code sample will only
// fire the callback function one time:
var query = firebase.database().ref("users").orderByKey();
query.once("value")
  .then(function(snapshot) {
    snapshot.forEach(function(childSnapshot) {
      var key = childSnapshot.key; // "ada"

      // Cancel enumeration
      return true;
  });
});

getPriority

getPriority() returns (string, number, or null)

Gets the priority value of the data in this DataSnapshot.

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

Returns

(string, number, or null) 

hasChild

hasChild(path) returns boolean

Returns true if the specified child path has (non-null) data.

Parameter

path

string

A relative path to the location of a potential child.
Returns

boolean true if data exists at the specified child path; else false.

Example 

// Assume we have the following data in the Database:
{
  "name": {
    "first": "Ada",
    "last": "Lovelace"
  }
}

// Determine which child keys in DataSnapshot have data.
var ref = firebase.database().ref("users/ada");
ref.once("value")
  .then(function(snapshot) {
    var hasName = snapshot.hasChild("name"); // true
    var hasAge = snapshot.hasChild("age"); // false
  });

hasChildren

hasChildren() returns boolean

Returns whether or not the DataSnapshot has any non-null child properties.

You can use hasChildren() to determine if a DataSnapshot has any children. If it does, you can enumerate them using forEach(). If it doesn't, then either this snapshot contains a primitive value (which can be retrieved with val()) or it is empty (in which case, val() will return null).

Returns

boolean true if this snapshot has any children; else false.

Example 

// Assume we have the following data in the Database:
{
  "name": {
    "first": "Ada",
    "last": "Lovelace"
  }
}

var ref = firebase.database().ref("users/ada");
ref.once("value")
  .then(function(snapshot) {
    var a = snapshot.hasChildren(); // true
    var b = snapshot.child("name").hasChildren(); // true
    var c = snapshot.child("name/first").hasChildren(); // false
  });

numChildren

numChildren() returns number

Returns the number of child properties of this DataSnapshot.

Returns

number 

Example 

// Assume we have the following data in the Database:
{
  "name": {
    "first": "Ada",
    "last": "Lovelace"
  }
}

var ref = firebase.database().ref("users/ada");
ref.once("value")
  .then(function(snapshot) {
    var a = snapshot.numChildren(); // 1 ("name")
    var b = snapshot.child("name").numChildren(); // 2 ("first", "last")
    var c = snapshot.child("name/first").numChildren(); // 0
  });

toJSON

toJSON() returns Object

Returns a JSON-serializable representation of this object.

Returns

nullable Object A JSON-serializable representation of this object.

val

val() returns any type

Extracts a JavaScript value from a DataSnapshot.

Depending on the data in a DataSnapshot, the val() method may return a scalar type (string, number, or boolean), an array, or an object. It may also return null, indicating that the DataSnapshot is empty (contains no data).

Returns

any type The DataSnapshot's contents as a JavaScript value (Object, Array, string, number, boolean, or null).

Examples 

// Write and then read back a string from the Database.
ref.set("hello")
  .then(function() {
    return ref.once("value");
  })
  .then(function(snapshot) {
    var data = snapshot.val(); // data === "hello"
  });
// Write and then read back a JavaScript object from the Database.
ref.set({ name: "Ada", age: 36 })
  .then(function() {
   return ref.once("value");
  })
  .then(function(snapshot) {
    var data = snapshot.val();
    // data is { "name": "Ada", "age": 36 }
    // data.name === "Ada"
    // data.age === 36
  });