Question

I have a hidden field on my page that stores space separated list of emails.
I can have maximum 500 emails in that field.

What will be the fastest way to search if a given email already exists in that list?
I need to search multiple emails in a loop

1. use RegEx to find a match

2. use indexOf()

3. convert the list to a
   javascript dictionary and then
   search

If this is an exact duplicate, please let me know the other question.
Thanks

EDIT:
Thanks everyone for your valuable comments and answers.
Basically my user has a list of emails(0-500) in db.
User is presented with his own contact list.
User can then choose one\more emails from his contact list to add to the list.
I want to ensure at client side that he is not adding duplicate emails.
Whole operation is driven by ajax, so jsvascript is required.

Answer 1

Instead of looking for the fastest solution, you first need to make sure that you’re actually having a correct solution. Because there are four cases an e-mail address can appear and a naive search can fail:

1. Alone: [e-mail]
2. At the begin: [e-mail] ...
3. At the end: ... [e-mail]
4. In between: ... [e-mail] ...

Now let’s analyze each variant:

1. To allow arbitrary input, you will need to escape the input properly. You can use the following method to do so:

   RegExp.quote = function(str) {
       return str.toString().replace(/(?=[.?*+^$[\]\\(){}-])/g, "\\");
   };
   

   To match all four cases, you can use the following pattern:

   /(?:^|\ )user@example\.com(?![^\ ])/
   

   Thus:

   var inList = new RegExp("(?:^| )" + RegExp.quote(needle) + "(?![^ ])").test(haystack);
   

2. Using indexOf is a little more complex as you need to check the boundaries manually:

   var pos = haystack.indexOf(needle);
   if (pos != -1 && (pos != 0 && haystack.charAt(pos-1) !== " " || haystack.length < (pos+needle.length) && haystack.charAt(pos+needle.length) !== " ")) {
       pos = -1;
   }
   var inList = pos != -1;
   

3. This one is rather quite simple:

   var dict = {};
   haystack.match(/[^\ ]+/g).map(function(match) { dict[match] = true; });
   var inList = dict.hasOwnProperty(haystack);
   

Now to test what variant is the fastest, you can do that at jsPerf.