javascript html dom-events jquery-ui-slider

Create Jquery UI Slider from scratch

I am new to Javascript and trying to create Jquery UI custom range slider on my own. Below is html and CSS code for this task.

#slider {
  position: relative;
  height: 15px;
  width: 400px;
  margin: 10px;
  border: 1px solid #c5c5c5;
}

.ui-slider {
  position: absolute;
  width: 20px;
  height: 20px;
  background-color: rgb(246, 246, 246);
  border: 1px solid #c5c5c5;
  left: 50%;
  top: -4.5px;
  border-radius: 3px;
}

.ui-slider:hover {
  background-color: rgb(237, 237, 237);
}

<div id="slider">
  <span class="ui-slider"></span>
</div>

Now I think some Javascript event should be attached to span element to make it slide back and forth. Should I use mouseDown or onDrag event listeners for this task? Please help me to write Javascript code to make this slider working actually. You can find slider that I want to build on this link.

Solution

var sliderSpan = document.getElementById("slider-span");
var slider = document.getElementById("slider");

sliderSpan.onmousedown = function(event) {
  var sliderCoords = getCoords(slider);
  var sliderSpanCoords = getCoords(sliderSpan);
  var shift = event.pageX - sliderSpanCoords.left;

  document.onmousemove = function(event) {
    var left =
      ((event.pageX - shift - sliderCoords.left) / sliderCoords.width) * 100;
    if (left < 0) left = 0;
    if (left > 100) left = 100;
    sliderSpan.style.left = left + "%";
  };
  document.onmouseup = function() {
    document.onmousemove = document.onmouseup = null;
  };
  return false;
};

function getCoords(elem) {
  var box = elem.getBoundingClientRect();
  return {
    top: box.top + pageYOffset,
    left: box.left + pageXOffset,
    width: box.right - box.left
  };
}

#slider {
  position: relative;
  height: 15px;
  width: 400px;
  margin: 10px;
  border: 1px solid #c5c5c5;
}

.ui-slider {
  position: absolute;
  width: 20px;
  height: 20px;
  background-color: rgb(246, 246, 246);
  border: 1px solid #c5c5c5;
  left: 0%;
  top: -4.5px;
  border-radius: 3px;
  margin-left: -10px;
}

.ui-slider:hover {
  background-color: rgb(237, 237, 237);
}

<div id="slider">
  <span class="ui-slider" id="slider-span"></span>
</div>