Future

future

var future = function(expectedCalls, action) {
  var counter = 0;
  return function() {
    counter++;
    if (counter === expectedCalls) action();
  }
}

var products = [ {id:1}, {id:2}, {id:3}, {id:4}, {id:5} ];

var notify = future(products.length, function() {
  alert('everything is saved');
});

products.forEach(function(product) {
   $.post("/save", product, function() {})
    .complete( notify );
});

You can try that in webconsole on any website having jQuery.

Nice!
jQuery's Deferred Objects also provide a nice way of doing this:

var products = [ {id:1}, {id:2}, {id:3}, {id:4}, {id:5} ];

var requests = $.map(products, function(val, i) {
  return $.post("/echo/json/", val)
});
$.when(requests).done(function(){
  alert('everything is saved');
});

Thx @dharFr !
Yes, you’re totally right but it’s always cool to "find" those kind of patterns yourself ! :-)
And it’s nice that It takes only a few lines of code.
JavaScript, FTW !