trueObj.js

// a pattern I've found useful for performance:
function trueObj(arr) {
    var o = {};
    for (var i = 0, l = arr.length; i < l; i++) {
      o[arr[i]] = true;
    }
    return o;
}
// when you have a large object of lookup values:
// for instance, ids that you want to filter in
lookup = ['a', 'b', 'c'];
// you can use indexOf, but it operates in linear
// time with respect to the lookup table. that is,
// it can be slow
lookup.indexOf('a') !== -1;
// instead, you can create an object
lookupObj = trueObj(lookup);
// and check membership in it, in usually constant time
lookupObj['a'];