Async function calls (like timer or event callbacks) are not executed from the current call stack, but pushed to the end of the queue, creating their own new call stack.

gistfile1.js

function stuff() {
	var start = new Date();
	// notice how 'stuff' is in the the call stack
	debugger;
	setTimeout(function timeout() {
		var end = new Date()
		  , diff = end - start;
		// notice how 'stuff' is NOT in the call stack
		debugger;
                console.log( diff + 'ms have passed' );	
	}, 500);
	while( new Date() - 1000 < start ) {}
}

stuff();