gistfile1.js

// your function
function calcDistance (start, end) {
		var distance=9999999;
		var request = {
			origin:start, 
			destination:end,
			travelMode: google.maps.DirectionsTravelMode.DRIVING
		};
		directionsService.route(request, function(result, status) {
			if (status == google.maps.DirectionsStatus.OK) {
				distance = result.routes[0].legs[0].distance.value;
				//document.getElementById('distance').innerHTML = "Distance: " + 
				//	distance + " meters";
				// return the distance:
			}
		});
		return distance;
	}

// -----------------------------------------
// how JavaScript executes it
function calcDistance(start, end) ->
    let distance be 9999999
    start a request for the content in the `request` object.
    return the distance // at this point, the request you initiated in the previous line hasn't returned yet

someDistance = calcDistance(0,1) // 9999999, because return is always executed before the request has a chance of begin run