What does this mean? (function (x,y)){...}){a,b); in JavaScript

Question

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I saw a this function

(function (x, y, data, lbl, dot) {
            var timer, i = 0;
            rect.hover(function () {
                clearTimeout(leave_timer);
                var side = "right";
                if (x + frame.getBBox().width > width) {
                    side = "left";
                }
                var ppp = this.popup(x, y, label, side, 1);
                //var ppp = r.popup(x, y, label, side, 1);
                frame.show().stop().animate({path: ppp.path}, 200 * is_label_visible);
                label[0].attr({text: data + " hit" + (data == 1 ? "" : "s")}).show().stop().animateWith(frame, {translation: [ppp.dx, ppp.dy]}, 200 * is_label_visible);
                dot.attr("r", 6);
                is_label_visible = true;
            }, function () {
                dot.attr("r", 4);
                leave_timer = setTimeout(function () {
                    frame.hide();
                    label[0].hide();
                    is_label_visible = false;
                }, 1);
            });
        })(x, y, data[i], labels[i], dot);

what is this? a function? why function definition in ()?

Answer 1

It is a self anonymous invoking function. The function is defined and executed immediately. The parenthesis that wrap the function ensure that it is treated as a function expression instead of a function declaration. The final pair of parenthesis invoke the function and pass the arguments.

Answer 2

Its a bit vague don't you think? Maybe its some library minimized code from a framework but you can't really expect anyone to just look at this and tell you what it does.

Please be a bit more specific?

Answer 3

In javascript you can have anonymous and self invoking functions.

function add(a, b)
{
   return a + b;
}

is same as

var add = function (a, b) {
             return a + b;
          }

and you call these as

add(10, 20)

You can define the function and call it immediately as

(
   function(a, b)
   {
      return a + b;
   }
)(10, 20);

The

   (
       function(a, b)
       {
          return a + b;
       }
    )

part defines a function, and the (10, 20) immediately after it calls the function just defined, with 10 and 20 as arguments to it.

Since the function does not have a name, it cannot be used later in the code.

The code in your question is probably minified, and creates a function in a similar way and calls it immediately.

Answer 4

A self-executing anonymous function would be a pretty accurate description.

Answer 5

The function inside the parenthesis is an anonymous function. I cannot say why this is done the way it is done, but the programmer defines and anonymous function, and calls it immediately. You could probably do the same thing by simply substituting its arguments with the values passed in, and removing the function definition.

Answer 6

function() {} is a definition of an anonymous function and (function() {})() is a call of that anonymous function.

This works since functions can be passed like data. So window.alert is the known alert function itself and window.alert() will call that function.

This technique is often used to keep the current variable scope clean as the function has its own variable scope.

Answer 7

It is a self invoking function, it is executed right away. Self invoking functions are effective for avoiding creation of global variables, jQuery uses this very effectively.

Answer 8

var funct = function(x,y) { }
funct(1,2)

is the same as

(function(x,y){ })(1,2);

it defines a self invoking anonymous function. It gets executed and then thrown away. It is a way of tidying your code (although this is a bad example) and having private variables in only that scope. It also won't get stored within the closure.