Is this factory method a good approach?
When using callbacks, the first parameter is the error. So instead of throwing the error, you should pass it as the first argument. The signature of the callback should look like this function(err, all, other, arguments).

So instead of this

var fs = require('fs');

module.exports.getHandler = function(type,callback) {
    if(!type) {
        throw new Error('Please specifiy a type');
    }
    var nodeModule = type.ucFirst()+'FieldHandler';
    var file = __dirname+'/'+nodeModule+'.js';
    fs.exists(file, function(exists) {
        if (exists) {
            var handler = require('./'+nodeModule);
            var instance = new handler;
            callback(instance);
        } else {
            throw new Error('Handler for "'+type+'" does not exist. ' +
                'Expect file of handler to have the name "'+file+'"');
        }
    });
};


You should have something like

var fs = require('fs');

module.exports.getHandler = function(type,callback) {
    if(!type) {
        callback(new Error('Please specifiy a type');
    }
    var nodeModule = type.ucFirst()+'FieldHandler';
    var file = __dirname+'/'+nodeModule+'.js';
    fs.exists(file, function(exists) {
        if (exists) {
            var handler = require('./'+nodeModule);
            var instance = new handler;
            callback(null, instance);
        } else {
            callback(new Error('Handler for "'+type+'" does not exist. ' +
                'Expect file of handler to have the name "'+file+'"');
        }
    });
};


Then it should become pretty easy to test. As a bonus, you can have a look at https://github.com/cjohansen/Sinon.JS. It's a good module for mocks, stubs and spies.

Also, I realized that you are doing a synchronous for loop calling asynchronous functions. var aString = handleField(field); will never do what you are expected. handleField does not return anything. You should expect it in a callback. If you want a tip to handle the flow properly, use https://github.com/caolan/async. With this module, you will stay away of the mythical "callback-hell".

Enjoy, node.js!