title: Group elements of an array by some property
body: <p>I have an array of objects with property <code>date</code>.</p>
  
  <p>What I want is to create array of arrays where each array will contain objects with the same date.</p>
  
  <p>I understand, that I need something like <code>.filter</code> to filter objects, and then <code>.map</code> to add every thing to array.</p>
  
  <p>But how to tell <code>.map</code> that I want separate array for each group from filtered objects and that this array must be added to "global" array and how to tell <code>.filter</code> that I want objects with the same date ? </p>