Enforce no duplicate emails in Firebase

no_dup_emails.js

/**
 * My Firebase data structure:
 * 
 *   /email_index/$email/user_id
 *   /user/$user_id/email
 */

var fb = new Firebase(URL);

function isDuplicateEmail(email, callback) {
  fb.child('email_index/'+escapeEmail(email)).once('value', function(snap) {
     callback( snap.val() !== null );
  });
}

function updateUser(user_id, email) {
   fb.child('user/'+user_id).set({email: email});
   fb.child('email_index/'+escapeEmail(email)).set(user_id);
}

function escapeEmail(email) {
  return (email || '').replace('.', ',');
}

What exactly is escapeEmail() doing to the string? What I see is "Return either the same email or nothing with periods replaced with commas..." why commas?

escapeEmail() is used to manipulate email so that i conforms to Firebase "child path" naming convention. Here escapeEmail() replaces the dot at email string with an allowed character, ie comma.
It will return empty string or for valid email string will give us that email which dot is replaced with comma.
See https://www.firebase.com/docs/creating-references.html

To replace all occurrences.

function escapeEmail(email) { return (email || '').replace(/\./g, ','); }