View difference between Paste ID: kB2FDp1B and WsNyFyXH

 // This is an improved version of https://stackoverflow.com/a/5835533/1175714
 
 function slideLoop(currMarg, selector){
     let contStyle = document.querySelector(selector).style;
 
     return function(){
         currMarg = (currMarg == 1800) ? (0) : (currMarg + 600);
         contStyle.marginLeft = "-%dpx".replace("%d", curMarg);
     }
 }
 
 function slide() {
     return setInterval(slideLoop(0, "#container"), 3000);
 }
 
 
 // Notes:
 // Use document.querySelector(css_selector) instead of document.getElementById(id)
 // Always track setInterval(fn, interval)'s return value so it can later be closed with clearInterval(interval_id)
 // Put the interval's callback into its own formal function for readability and maintainability
 // If state is kept with the callback, the callback should be returned from another function (slideLoop) to form a closure.
 // Use proper whitespace. Logical blocks that do different things should be separated by one empty line
 // Avoid "string" + value + "string" + ... Make a format function or use String.prototype.replace(string/regex, new_string)
 // Always surround the arguments of your ternary expressions with parens. It aids readability and prevents bugs.