async example

gistfile1.js

var n; // a global counter of how many requests hand been processed
var a = [1,2,3,4,5,6,7];

// a handler that increments `n`, and print out each element in a.
function countHandler(req,res,next) {
	// these are new for each request, don't need to protect them.
	var	a,b,c;

	// an asynchronous loop that print out each element in an array
	var i = 0; // this loop would break if you move it outside of the countHandler scope.
	function iter() {
		if(i >= a.length) {
			return;
		} else {
			console.log(a[i]);
			setTimeout(iter,100);
		}
	}
	setTimeout(iter,100);
	
	// in other languages you'll need to lock this, but not in node.
	n++;

	next();
}

app.use(countHandler);