let curProject = 3
const fadeTime = 600

$(document).ready(() => {
  const projects = $('.showcase .project')
  $(projects[curProject]).fadeIn(fadeTime)

  // set up the click-event handlers
  $('.arrow-prev').click(() => {
    $(projects[curProject]).fadeOut(fadeTime, () => {
      // callback-function that enables crossfading between project-views

      curProject--
      if (curProject < 0) {
        // Only loop over actual projects
        curProject = projects.length - 1 // go to last
      }
      $(projects[curProject]).fadeIn(fadeTime)
    })
  })

  $('.arrow-next').click(() => {
    $(projects[curProject]).fadeOut(fadeTime, () => {
      // callback-function that enables crossfading between project-views

      curProject++
      if (curProject >= projects.length) {
        // only loop over actual projects
        curProject = 0 // go to first
      }
      $(projects[curProject]).fadeIn(fadeTime)
    })
  })
})

@@ -1,7 +1,7 @@
-let
+var
  cur
@@ -12,21 +12,19 @@
 ect = 3%0A
-const
+var
  fadeTim
@@ -30,16 +30,29 @@
 me = 600
+%0Avar projects
 %0A%0A$(docu
@@ -67,23 +67,23 @@
 ady(
-() =%3E %7B%0A  const
+function () %7B%0A 
  pro
@@ -212,37 +212,43 @@
 ow-prev').click(
-() =%3E
+function ()
  %7B%0A    $(project
@@ -272,37 +272,43 @@
 deOut(fadeTime, 
-() =%3E
+function ()
  %7B%0A      // call
@@ -606,21 +606,27 @@
 ).click(
-() =%3E
+function ()
  %7B%0A    $
@@ -670,13 +670,19 @@
 me, 
-() =%3E
+function ()
  %7B%0A 

Rewrite showcase.js in ES5 for better mobile support.