fastest way to detect if a value is in a group of values in Javascript

You can use an object like so:

// prepare a mock-up object
setOfValues = {};
for (var i = 0; i < 100; i++)
  setOfValues["example value " + i] = true;

// check for existence
if (setOfValues["example value 99"]);   // true
if (setOfValues["example value 101"]);  // undefined, essentially: false

This takes advantage of the fact that objects are implemented as associative arrays. How fast that is depends on your data and the JavaScript engine implementation, but you can do some performance testing easily to compare against other variants of doing it.

If a value can occur more than once in your set and the "how often" is important to you, you can also use an incrementing number in place of the boolean I used for my example.

Use a hash table, and do this:

// Initialise the set

mySet = {};

// Add to the set

mySet["some string value"] = true;

...

// Test if a value is in the set:

if (testValue in mySet) {
     alert(testValue + " is in the set");
} else {
     alert(testValue + " is not in the set");
}

A comment to the above mentioned hash solutions. Actually the {} creates an object (also mentioned above) which can lead to some side-effects. One of them is that your "hash" is already pre-populated with the default object methods.

So "toString" in setOfValues will be true (at least in Firefox). You can prepend another character e.g. "." to your strings to work around this problem or use the Hash object provided by the "prototype" library.