Question

Curious what I’m doing wrong here :

employee_ids = $('[data-employee_id="'+employee+'"]');
timestamp_ids = $('[data-scheduled_on="'+timestamp+'"]');

var common = $.grep(timestamp_ids, function(element) {
  $.each(employee_ids, function(idx, item) {
     if ( item === element ) { console.log ("omg!") };
  });
});

This returns just the list of timestamp_ids and not that array compared against employee_ids looking for a single match.

Answer 1

You are not using .grep correctly. Each iteration of grep should return a boolean: true to add it to the result array, false to ignore it.

var listA = [1, 2, 3];
var listB = [2, 3, 4];

var union = $.grep(listA, function (element) {
    return listB.indexOf(element) !== -1;
}); 

Note that IE does not support .indexOf on Arrays, you will have to implement the comparison some other way.

EDIT: if you are trying to find a single item of an array that matches some criteria, i would suggest just using a regular for loop:

var result;
for (var i = 0; i < yourArray.length; i++) {
    if (yourArray[i].id === employee_ID) { // whatever conditions you have
        result = yourArray[i];
        break;
    }
}

if (result) {
    // do whatever
} else {
    // no match
}