JavaScript Throttle Function

throttle.js

//source: https://remysharp.com/2010/07/21/throttling-function-calls 

/* Let us work on performance issues while making resource-intensive things */
  /* Will wait every `limit` miliseconds until the callback is executed  */
  function _throttle(fn, threshold, scope) {
    threshold || (threshold = 250);
    var last,
        deferTimer;
    return function () {
      var context = scope || this;

      var now = +new Date,
          args = arguments;
      if (last && now < last + threshold) {
        // hold on to it
        clearTimeout(deferTimer);
        deferTimer = setTimeout(function () {
          last = now;
          fn.apply(context, args);
        }, threshold);
      } else {
        last = now;
        fn.apply(context, args);
      }
    };
  }


// Usage
  $(window).on('scroll', _throttle(function() {
    if ($(this).scrollTop() > $(this).height()) {
      $("#scroll-to-top").addClass('visible');
    } else {
      $("#scroll-to-top").removeClass('visible');
    }
  }, 500));