Is it possible to catch exceptions thrown in a JavaScript async callback?

The reason it won't catch anything in your example is because once the geocode() callback is called, the try/catch block is over. Therefore the geocode() callback is executed outside the scope of the try block and thus not catchable by it.

As far as I know, it is not possible to catch exceptions thrown in JavaScript callbacks (at least, not in any straightforward manner).

Yes, you can override the default behaviour of window.onerror:

window.onerror = function(message, file, lineNumber) {
  // all errors will be caught here
  // you can use `message` to make sure it's the error you're looking for
  // returning true overrides the default window behaviour
  return true; 
};

You can indeed catch exceptions that fire within a JavaScript callback function.

The key is to set up the try/catch block within the callback code, as any try/catch blocks outside the callback code will have already exited by the time the callback code is executed. So while your try/catch block above won't be able to catch any exceptions that get thrown when the callback function is called, you can still do something like this:

// this will cause an exception ing google.maps.Geocoder().geocode() 
// since it expects a string.
var zipcode = 30045; 
var map = new google.maps.Map(document.getElementById('map_canvas'), {
    zoom: 5,
    center: new google.maps.LatLng(35.137879, -82.836914),
    mapTypeId: google.maps.MapTypeId.ROADMAP
});
// exception in callback:
var geo = new google.maps.Geocoder().geocode({ 'address': zipcode }, 
   function(geoResult, geoStatus) {
      try {
          if (geoStatus != google.maps.GeocoderStatus.OK) console.log(geoStatus);
      } catch(e){
          alert("Callback Exception caught!");
      }
   }
);

and you'll be able to capture the exception when it is thrown. I wasn't 100% sure whether that would be the case or not, so I wrote some test code to verify. The exception is captured as expected on Chrome 19.0.1055.1 dev.

If you can use Promises, it can be solved as shown in sample code below:

function geocode(zipcode) {
  return new Promise((resolve, reject) => {
    const g = new google.maps.Geocoder().geocode({ 'address': zipcode },  function(geoResult, geoStatus) {
      if (geoStatus != google.maps.GeocoderStatus.OK) {
        reject(new Error("Callback Exception caught"));
      } else {
        resolve(g);
      };
    });
  });
}

Executing the function and catching the error using catch():

geocode(zipcode)
  // g will be an instance of new google.maps.Geocoder().geocode..
  .then( g => console.log ( g ) )
  .catch( error => console.log( error ) );

Executing the function and catching the error using async/await:

try {
  // g will be an instance of new google.maps.Geocoder().geocode..
  // you can resolve with desired variables
  const g = await geocode(zipcode);
} catch( e ) {
  console.log(e);
}

I have detected the error by monkey patching the console logs.

if(window.console && console.error){
    var old = console.error;
    console.error = function(){
        if(arguments[0].indexOf('Google Maps API error')!=-1){
            alert('Bad Google API Key '+ arguments[0]);
        }
        Array.prototype.unshift.call(arguments);

        old.apply(this, arguments);
    }
}

Here's my approach:

// the purpose of this wrapper is to ensure that any
// uncaught exceptions after a setTimeout still get caught
function callbackWrapper(func) {
    return function() {
        try {
            func();
        } catch (err) {
            // callback will reach here :)
            // do appropriate error handling
            console.log("error");
        }
    }
}

try {
    setTimeout(callbackWrapper(function() {throw "ERROR";}), 1000);
} catch (err) {
    // callback will never reach here :(
}