Question

How can I edit this to run only once after user scroll up:

(function () {
    var previousScroll = 0;

    $(window).scroll(function(){
       var currentScroll = $(this).scrollTop();
       if (currentScroll < previousScroll){
           alert('up');
       }
       previousScroll = currentScroll;
    });
}()); //run this anonymous function immediately

Answer 1

if you want the user to be notified only once, then you could just use a boolean to keep track whether the notice is done.

(function () {
var previousScroll = 0;
var isAlerted = false;
$(window).scroll(function(){
   if(isAlerted) return;                    
   var currentScroll = $(this).scrollTop();
   if (currentScroll < previousScroll){
       console.log('up');
       isAlerted = true;
   }
   previousScroll = currentScroll;
});
})();​

Also note the change in the last line. Use the self-executing anonymous function properly as

(function(){})();