promises

callback.js

function getData(callback) {
	var result = {};
 
	getJSONAsync('/users', 
		function(data) {
			result.users = data;
			getJSONAsync('/companies', 
				function(data) {
					result.companies = data;
					getJSONAsync('/contacts', 
						function(data) {
							result.contacts = data;
							callback(result);
						}, 

						function(error) {
							// error handler
						}
					);
				}, 

				function(error) {
					// error handler
				}
			);
		}, 

		function(error) {
			// error handler
		}
	);
}

promises.js

function getData() {
	var result = {};

	return getJSON('/users').then(
		function(data) {
			result.users = data;
			return getJSON('/contacts');
		}
	).then(
		function(data) {
			result.companies = data;
			return getJSON('/companies');
		}
	).then(
		function(data) {
			result.contacts = data;
			return result;
		}
	).then(
		null,
		function() {
			// error handler
		}
	);
}

sync.js

function getData() {
	var result = {};

	try {
		result.users = getJSON('/users');
		result.companies = getJSON('/companies');
		result.contacts = getJSON('/contacts');

		return result;
	} catch (e) {
		// error handler
	}
}