Why Ternary Operator doesn't work with using 'hasClass'?

Question

I'm making my own slide plugin using jQuery for practice. The problem is, I don't know why but some reason ternary operator doesn't work. The slider only responses when user clicks prev button which registered as a condition from the code.

I expected it should move to right side when user presses next button, but next button is totally broken.

I could just make 2 different trigger buttons and use multiple if statements for solving this problem, but I want to merge it on single line by using ternary operator and minifise my code as much as I can.

How would I solve this problem?

Code block:

(function($) {
  $.fn.sliderModule = function(options) {
    const defaults = {
      myGallery: $(this)
    };
    var settings = $.extend({
      myItems: null,
      trigger: null,
      current: null
    }, defaults, options);
    $(settings.trigger).on('click', function() {
      if (!(defaults.myGallery.is(':animated'))) {
        var movement = settings.trigger.hasClass('prev') ? -1 : 1
        defaults.myGallery.animate({left: "+=" + (100 / settings.current) * movement  + '%'}); // It doesn't respond the `next` button.
      };
    });
  };
})(jQuery);
$(function() {
  $('#slide-list').sliderModule({
    myItems: '#img-slider',
    trigger: $('#buttons').find('#button'),
    current: 4
  });
})

li {
  list-style-type: none;
}

#slide-container {
  position: relative !important;
  width: 1170px;
  margin: 0 auto;
  overflow: hidden;
  height: auto;
}
#slide-list {
  margin: 0 auto;
  position: relative;
  display: flex;
  width: 1170px;
}
#buttons {
  width: 1170px;
  margin: 0 auto;
  display: flex;
  justify-content: center;
}
#buttons li {
  margin: 0 20px;
}
.slide-l-quarter {
  flex-shrink: 0;
  position: relative;
  white-space: nowrap;
  width: calc(100% / 4);
}
.slide-l-quarter img {
  width: 100%;
  height: auto;
}

<div id="slide-container">
  <ul id="slide-list">
    <li class="slide-l-quarter" id="image-slider">
      <img src="https://i.imgur.com/PVsHlX9.jpg" alt="">
    </li>
    <li class="slide-l-quarter" id="image-slider">
      <img src="https://i.imgur.com/WfWhNnU.jpg" alt="">
    </li>
    <li class="slide-l-quarter" id="image-slider">
      <img src="https://i.imgur.com/eqHdnNs.jpg" alt="">
    </li>
    <li class="slide-l-quarter" id="image-slider">
      <img src="https://i.imgur.com/0jziABY.jpg" alt="">
    </li>
  </ul>
</div>
<ul id="buttons">
  <li id="button" class="prev">Prev</li>
  <li id="button" class="next">Next</li>
</ul>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

Answer 1

There is no problem with using ternary operators. But you made some mistakes along the way.

IDs must be unique

You cannot have this in your HTML:

<li id="button" class="prev">Prev</li>
<li id="button" class="next">Next</li>

If you need multiple elements to have the same selector, use a class instead:

<li class="button prev">Prev</li>
<li class="button next">Next</li>

settings.trigger selects both buttons

When you do this:

settings.trigger.hasClass('prev')

You're refering to both buttons. You only want the one clicked by the user:

$(this).hasClass('prev')

Fixed version

(function($) {
  $.fn.sliderModule = function(options) {
    const defaults = {
      myGallery: $(this)
    };
    var settings = $.extend({
      myItems: null,
      trigger: null,
      current: null
    }, defaults, options);
    $(settings.trigger).on('click', function() {
      if (!(defaults.myGallery.is(':animated'))) {
        var movement = $(this).hasClass('prev') ? -1 : 1;
        defaults.myGallery.animate({left: "+=" + (100 / settings.current) * movement  + '%'});
      };
    });
  };
})(jQuery);
$(function() {
  $('#slide-list').sliderModule({
    myItems: '#img-slider',
    trigger: $('#buttons').find('.button'),
    current: 4
  });
})

li {
  list-style-type: none;
}

#slide-container {
  position: relative !important;
  width: 1170px;
  margin: 0 auto;
  overflow: hidden;
  height: auto;
}
#slide-list {
  margin: 0 auto;
  position: relative;
  display: flex;
  width: 1170px;
}
#buttons {
  width: 1170px;
  margin: 0 auto;
  display: flex;
  justify-content: center;
}
#buttons li {
  margin: 0 20px;
}
.slide-l-quarter {
  flex-shrink: 0;
  position: relative;
  white-space: nowrap;
  width: calc(100% / 4);
}
.slide-l-quarter img {
  width: 100%;
  height: auto;
}

<div id="slide-container">
  <ul id="slide-list">
    <li class="slide-l-quarter" id="image-slider">
      <img src="https://i.imgur.com/PVsHlX9.jpg" alt="">
    </li>
    <li class="slide-l-quarter" id="image-slider">
      <img src="https://i.imgur.com/WfWhNnU.jpg" alt="">
    </li>
    <li class="slide-l-quarter" id="image-slider">
      <img src="https://i.imgur.com/eqHdnNs.jpg" alt="">
    </li>
    <li class="slide-l-quarter" id="image-slider">
      <img src="https://i.imgur.com/0jziABY.jpg" alt="">
    </li>
  </ul>
</div>
<ul id="buttons">
  <li class="button prev">Prev</li>
  <li class="button next">Next</li>
</ul>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>