Adding a zoom attr to a container
Let's remove all if/else checks:


Set direction to 1 or -1 based on the button class and multiply it by ZOOM_OFFSET
Use Math.max, Math.min to constrain zoom value in the range
Use .toggleClass('.foo', condition) to disable the buttons conditionally
Set the attribute just once, in the end: no need to treat the absence of the attribute specially, just use a fallback parseInt(foo) || MIN_ZOOM


And fix some inconsistencies:


Use camelCase names for normal variables, which is a standard JavaScript convention
Use single or double quotes for strings consistently
Prefer .text(foo) over .html(foo) in case of a simple text without HTML tags




$('.schedule-zoom .zoom-btn:not(.disable)').click(function() {
  var direction = $(this).hasClass('zoom-in') ? 1 : -1;
  var container = $('.fixed-container');

  var zoom = parseInt(container.attr('zoom')) || MIN_ZOOM;
  zoom = Math.max(MIN_ZOOM, Math.min(MAX_ZOOM, zoom + direction * ZOOM_OFFSET));

  container.attr('zoom', zoom).text(zoom);
  $('.schedule-zoom .zoom-in').toggleClass('disable', zoom == MAX_ZOOM);
  $('.schedule-zoom .zoom-out').toggleClass('disable', zoom == MIN_ZOOM);
});