Notes

Realtime Database
See the Realtime Database docs for web.
Set a ref
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function writeUserData(userId, name, email, imageUrl) {
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  firebase
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    .database()
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    .ref('users/' + userId)
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    .set({
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      username: name,
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      email: email,
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      profile_picture: imageUrl,
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    });
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}
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Value events
Value events fire with the entire data payload for any and all changes
Listen to ongoing events
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var starCountRef = firebase.database().ref('posts/' + postId + '/starCount');
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starCountRef.on('value', function(snapshot) {
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  updateStarCount(postElement, snapshot.val());
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});
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Listen to a single event and stop listening
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var userId = firebase.auth().currentUser.uid;
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return firebase
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  .database()
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  .ref('/users/' + userId)
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  .once('value')
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  .then(function(snapshot) {
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    var username = (snapshot.val() && snapshot.val().username) || 'Anonymous';
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    // ...
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  });
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Multi-path updates
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function writeNewPost(uid, username, picture, title, body) {
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  // A post entry.
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  var postData = {
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    author: username,
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    uid: uid,
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    body: body,
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    title: title,
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    starCount: 0,
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    authorPic: picture,
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  };
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​
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  // Get a key for a new Post.
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  var newPostKey = firebase
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    .database()
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    .ref()
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    .child('posts')
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    .push().key;
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​
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  // Write the new post's data simultaneously in the posts list and the user's post list.
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  var updates = {};
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  updates['/posts/' + newPostKey] = postData;
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  updates['/user-posts/' + uid + '/' + newPostKey] = postData;
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​
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  return firebase
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    .database()
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    .ref()
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    .update(updates);
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}
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Delete data
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function deleteUser(userId) {
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  return firebase
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    .database()
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    .ref('/users/' + userId)
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    .remove();
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}
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Detach listener
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var starCountRef = firebase.database().ref('posts/' + postId + '/starCount');
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var listener = starCountRef.on('value', function(snapshot) {
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  updateStarCount(postElement, snapshot.val());
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});
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​
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function detachListener() {
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  starCountRef.off('value', listener);
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}
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Transactions
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function toggleStar(postRef, uid) {
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  postRef.transaction(function(post) {
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    if (post) {
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      if (post.stars && post.stars[uid]) {
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        post.starCount--;
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        post.stars[uid] = null;
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      } else {
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        post.starCount++;
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        if (!post.stars) {
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          post.stars = {};
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        }
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        post.stars[uid] = true;
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      }
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    }
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    return post;
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  });
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}
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Child events
child_added: fires once for every existing result and then again for every new result; does not fire for changes or removals, only new records
child_changed: fires when the underlying object or value is changed in any way
child_removed: fires when the entire record is removed
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var commentsRef = firebase.database().ref('post-comments/' + postId);
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commentsRef.on('child_added', function(data) {
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  addCommentElement(postElement, data.key, data.val().text, data.val().author);
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});
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​
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commentsRef.on('child_changed', function(data) {
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  setCommentValues(postElement, data.key, data.val().text, data.val().author);
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});
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​
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commentsRef.on('child_removed', function(data) {
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  deleteComment(postElement, data.key);
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});
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Sort data
orderByChild('childName'): Orders by a child attribute
orderByKey(): Orders by record keys
orderByValue(): Orders by record values; only relevant when values are strings or numbers and not nested objects
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var topUserPostsRef = firebase
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  .database()
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  .ref('user-posts/' + myUserId)
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  .orderByChild('starCount');
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​
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var mostViewedPosts = firebase
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  .database()
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  .ref('posts')
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  .orderByChild('metrics/views');
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Filter data
Assumes that data is ordered by key unless otherwise specified
limitToFirst(count): Sets the maximum number of items to return from the beginning of the ordered list of results.
limitToLast(count): Sets the maximum number of items to return from the end of the ordered list of results.
startAt(value): Return items greater than or equal to the specified key or value, depending on the order-by method chosen.
endAt(value): Return items less than or equal to the specified key or value, depending on the order-by method chosen.
equalTo(value): Return items equal to the specified key or value, depending on the order-by method chosen.
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var first100Days = firebase
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  .database()
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  .ref('days/2018')
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  .orderByChild('dayOfYear')
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  .limitToFirst(100);
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​
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var first10DaysOfFebruary = firebase
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  .database()
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  .ref('days/2018')
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  .orderByChild('dayOfYear')
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  .limitToFirst(10)
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  .startAt(32);
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​
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var last10DaysOfJanuary = firebase
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  .database()
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  .ref('days/2018')
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  .orderByChild('dayOfYear')
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  .limitToLast(10)
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  .endAt(31);
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​
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var first10DaysOfJanuary = firebase
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  .database()
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  .ref('days/2018')
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  .orderByChild('dayOfYear')
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  .limitToFirst(100) // Limit is never hit
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  .endAt(10); // endAt stops the query before it hits the limit
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Authenticate Node.js
Full admin privileges
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var admin = require('firebase-admin');
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​
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// Fetch the service account key JSON file contents
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var serviceAccount = require('path/to/serviceAccountKey.json');
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​
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// Initialize the app with a service account, granting admin privileges
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admin.initializeApp({
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  credential: admin.credential.cert(serviceAccount),
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  databaseURL: 'https://databaseName.firebaseio.com',
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});
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​
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// As an admin, the app has access to read and write all data, regardless of Security Rules
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var db = admin.database();
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var ref = db.ref('restricted_access/secret_document');
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ref.once('value', function(snapshot) {
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  console.log(snapshot.val());
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});
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Initialize Node.js with limited privileges
Set auth token variables to limit access
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// Initialize the app with a custom auth variable, limiting the server's access
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admin.initializeApp({
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  credential: admin.credential.cert(serviceAccount),
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  databaseURL: 'https://databaseName.firebaseio.com',
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  databaseAuthVariableOverride: {
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    uid: 'my-service-worker',
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  },
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});
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Act as an un-authenticated user
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// Initialize the app with a custom auth variable, limiting the server's access
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admin.initializeApp({
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  credential: admin.credential.cert(serviceAccount),
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  databaseURL: 'https://databaseName.firebaseio.com',
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  databaseAuthVariableOverride: null,
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});
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Initialize Node.js with limited privileges