Methods to search for user accounts by email address in Firebase

1_using_queries.js

/***************************************************
 * Simple and elegant, no code complexity
 * Disadvantages: Requires warming all data into server memory (could take a long time for MBs of data or millions of records)
 * (This disadvantage should go away as we add optimizations to the core product)
 ***************************************************/
 
var fb = firebase.database.ref();

/**
 * @param {string} emailAddress
 * @return {Object} the object contains zero or more user records, the keys are the users' ids
 */
function findUsersMatchingEmail( emailAddress, callback ) {
    fb.child('user').orderByChild('emailAddress').equalTo(emailAddress).once('value', function(snap) {
        callback( snap.val() );
    });
}

2_using_an_index.js

/***************************************************
 * Useful for MBs or more of data, or lists of thousands or more records
 * Disadvantages: Slight code complexity due to two queries (one for key, another for record); escaping emails is annoying
 ***************************************************/

var fb = firebase.database.ref();

/**
 * Looks up a user id by email address and invokes callback with the id or null if not found
 * @return {Object|null} the object contains the key/value hash for one user
 */
function getUserIdByEmail( emailAddress, callback ) {
   fb.child('emails_to_ids/'+emailToKey(emailAddress)).once('value', function(snap) {
       callback( snap.val() );
   });
}

/**
 * Creates a new user record and also updates the index
 */
function createNewUser( userRecord ) {
   var uid = fb.child('user').push().key();
   
   // do a multi-path write!
   var mergedData = {};
   mergedData['users/' + uid] = userRecord;
   mergedData['emails_to_ids/'+emailToKey(userRecord.email)] = uid;
   fb.update(mergedData);
   
   return id;
}

/**
 * Firebase keys cannot have a period (.) in them, so this converts the emails to valid keys
 */
function emailToKey(emailAddress) {
   return emailAddress.replace(/[.]/g, '%20');
}

@mikemurray Addressed comments and updated for Firebase queries. Thanks!
@jondthompson world writable/readable isn't necessary. You can make paths editable only by the owner's uid.

@al-the-x hashes are a great answer but btoa isn't necassarily available cross-platform. It just requires including a hash function of some sort or a polyfill for btoa, so still a great choice.

This is awesome, thanks!

var uid = fb.child('user').push().key();
For me it's working as a prop and not a method, so .key instead of .key()

@kristijanmatic Thanks for pointing it out. Also check out this https://developer.mozilla.org/en/docs/Web/API/WindowBase64/Base64_encoding_and_decoding

Now you can just do the following:

admin.auth().getUserByEmail(email)
  .then(function(userRecord) {
    // See the UserRecord reference doc for the contents of userRecord.
    console.log("Successfully fetched user data:", userRecord.toJSON());
  })
  .catch(function(error) {
    console.log("Error fetching user data:", error);
  });

See https://firebase.google.com/docs/auth/admin/manage-users for details.

@nderkach it works!
thanks man