main.js

function initComparisons() {
    var x, i;
    /* Find all elements with an "overlay" class: */


    x = document.getElementsByClassName("img-comp-overlay");
    for (i = 0; i < x.length; i++) {
      /* Once for each "overlay" element:
      pass the "overlay" element as a parameter when executing the compareImages function: */
      compareImages(x[i]);
    }


    function compareImages(img) {
      var slider, img, clicked = 0, w, h;
      /* Get the width and height of the img element */
      w = img.offsetWidth;
      h = img.offsetHeight;
      /* Set the width of the img element to 50%: */
      img.style.width = (w / 2) + "px";
      /* Create slider: */
      slider = document.createElement("DIV");
      slider.setAttribute("class", "img-comp-slider");
      /* Insert slider */
      img.parentElement.insertBefore(slider, img);
      /* Position the slider in the middle: */
      slider.style.top = (h / 2) - (slider.offsetHeight / 2) + "px";
      slider.style.left = (w / 2) - (slider.offsetWidth / 2) + "px";
      /* Execute a function when the mouse button is pressed: */
      slider.addEventListener("mousedown", slideReady);
      /* And another function when the mouse button is released: */
      window.addEventListener("mouseup", slideFinish);
      /* Or touched (for touch screens: */
      slider.addEventListener("touchstart", slideReady);
       /* And released (for touch screens: */
      window.addEventListener("touchend", slideFinish);
      function slideReady(e) {
        /* Prevent any other actions that may occur when moving over the image: */
        e.preventDefault();
        /* The slider is now clicked and ready to move: */
        clicked = 1;
        /* Execute a function when the slider is moved: */
        window.addEventListener("mousemove", slideMove);
        window.addEventListener("touchmove", slideMove);
      }
      function slideFinish() {
        /* The slider is no longer clicked: */
        clicked = 0;
      }
      function slideMove(e) {
        var pos;
        /* If the slider is no longer clicked, exit this function: */
        if (clicked == 0) return false;
        /* Get the cursor's x position: */
        pos = getCursorPos(e)
        /* Prevent the slider from being positioned outside the image: */
        if (pos < 0) pos = 0;
        if (pos > w) pos = w;
        /* Execute a function that will resize the overlay image according to the cursor: */
        slide(pos);
      }
      function getCursorPos(e) {
        var a, x = 0;
        e = (e.changedTouches) ? e.changedTouches[0] : e;
        /* Get the x positions of the image: */
        a = img.getBoundingClientRect();
        /* Calculate the cursor's x coordinate, relative to the image: */
        x = e.pageX - a.left;
        /* Consider any page scrolling: */
        x = x - window.pageXOffset;
        return x;
      }
      function slide(x) {
        /* Resize the image: */
        img.style.width = x + "px";
        /* Position the slider: */
        slider.style.left = img.offsetWidth - (slider.offsetWidth / 2) + "px";
      }
    }
  }