[SOLVED] In what circumstances is it allowed to have a function body be an expression NOT surrounded by curly brackets?

In what circumstances is it allowed to have a function body be an expression NOT surrounded by curly brackets?

I try to understand how Protovis works, and I stumbled upon code like this:

force.node.add(pv.Dot)
    .size(function(d) (d.linkDegree + 4) * Math.pow(this.scale, -1.5)) // notice this
    .fillStyle(function(d) d.fix ? "brown" : colors(d.group)) // and this
    .strokeStyle(function() this.fillStyle().darker()) // and even this
    .lineWidth(1)
    .title(function(d) d.nodeName)
    .event("mousedown", pv.Behavior.drag())
    .event("drag", force);

I tried rolling my own short functions, like this:

(function(a) a+2)

I am NOT asking about anonymous functions declared like function(){stuff();}. The code in question looks like function() stuff; and it works. I want to know why. I don't want to learn about constructs like myvar = function(a){return a+1;}, but about constructs like myvar = (function(a) a+1). Please look at the above code more carefully.

But, as I suspected, it threw a syntax error.

How can such code work?

(Note: the protovis code does work as intended.)

Solution

This is a Expression Closure that was introduced in JavaScript 1.8. It is an extension to ECMAScript.

https://developer.mozilla.org/en/JavaScript/New_in_JavaScript/1.8