callbackStyle.js

gistfile1.js

var userTable = {
	'123': {
		id: '123',
		name: 'Elon',
		usage: 5000
	}
};

var getUser = function(userId, callback) {
	callback(null, {
		id: userId,
		usage: usageTable[userId]
	});
};

var calculageUsage = function(user, callback) {
	callback(null, user.usage + 50);
};

var save = function(user, callback) {
	userTable[user.id] = user;
	callback(null, 1);
};

var errorHandler = function(err) {
	alert('sorry there as an error' + err);
};

// This is what a program looks like when you use the callback style
var updateUsage = function(userId, callback) {
	// promise style
	getUser(123, function(err, user) {
		if (err) return errorHandler(err);

		updateUsage(user, function(err, usage) {
			if (err) return errorHandler(err);
			user.usage = usage;
			
			save(user, function(err, updateCount) {
				if (err) return errorHandler(err);
				alert('user ' + userId + 'usage has been updated');
				alert('userTable:' + JSON.stringify(userTable));
				callback();
			});
		});
	});
};