Deep reference and object passing
.slice() does not actually do a deep copy, the easiest way to do a deep copy with simple objects is 

array = JSON.parse( JSON.stringify( array ) )


Also, you can sort on a boolean in a simpler way:

function sortWordsByIncorrectAnswers(array)
{
  array =  JSON.parse( JSON.stringify( array || [] ) );
  return array.sort( function(a, b)
  {
    return a.fields.false - b.fields.false;
  });
}