i don't understand something with javascript callback. I'd like to do something like a DAO in Java where I manipulate my data then call the DAO object in my controller. But in javascript I have something like this :
DAO
function DAO() {
this.getData = function(arg1, arg2, callbackSuccess, callbackError) {
var data = null;
// do something to get data. If ok then call callbackSuccess
callbackSuccess(data);
}
var callbackSuccess = function(data) {
// HERE I HAVE THE DATA. HOW TO RETURN IT TO <DATA> ?
}
var callbackError = function(data) {
// ERROR
}
}
Somewhere else where i need to fetch data ...
var dao = new DAO();
var <DATA> = dao.getData(var1, var2, callbackSuccess, callbackError);
I can probably put the callback in my controller and send them to the getData method but that's seem strange to me...
You need to understand the difference between synchronous and asynchronous code.
In synchronous code, every statement you execute is evaluated and its result returned to you. No other statement is evaluated until you finish evaluating the current statement. As such, statements are evaluated always in order.
var a = syncStatement1();
var b = syncStatement2();
console.log("finished!!!");
In the code above syncStatement2() would never be evaluated before syncStatement1(). And the log "finished!!!" will only be printed when all the code within the two preceding functions has been completed.
In asynchronous code, though, the order of execution is different. When you ask an asynchronous statement to be evaluated, the execution engine simply schedule it for evaluation at some point later and continues executing your program.
var a = asyncStatement1();
var b = asyncStatement2();
console.log("finished???");
In this second example above, the execution engine will not wait for every statement function to evaluate before executing the next statement. By the time you reach the logging line saying "finished???" you cannot be sured that the previous functions have finished executing.
The executing engine has simply schedule them for execution, and at some point it will execute them and "return" whatever they return, but that did not happen when you initially invoked them.
This has several implications, the first is that you cannot ask for a result of an asynchronous function like a = asyncFunc() because what should be the value of a if the function has not even run yet?. Most likely undefined, since there is no result available yet. However, depending on how you are creating the asynchronous function, you may get back a promise object, that you can also use to cancel the asynchronous action, or to ask, later on, if there is a result available.
The second implication is that, since the code will execute later on, within a context you have no control whatsoever, the only way to know what happened is by asking the function itself to notify you of the results, and that is why you are forced to register callbacks with your asynchronous functions, like:
function asyncFunc(args, success, error) {
//do something here
if(success) success(data);
}
There is no way to go around this and pretend the code is actually running in synchronous mode. You are forced to change the way you are thinking.
As I said above, depending on the framework that you are using, you may get a promise from your asynchronous function and change the things a bit to look slightly better as in:
var promise = asynchFunc(args);
promise.success(function(data) {
//do something with data when successful in async.
});
promise.error(function(error){
//handle the error that happened in async.
});
console.log("Most likely not finished yet!!!");
But this is still asynchronous execution and by the time you reach the log, you most likely have not yet handled any callback.
Some third party libraries like async can make your life slightly simpler when dealing with this type of code, in case you want to give them some consideration.
Finally, to address your question, you cannot do this
var dao = new DAO();
var <DATA> = dao.getData(var1, var2, callbackSuccess, callbackError);
But you could do this:
var <DATA> = null;
function onSuccess(data){
//act on your data and update controller
<DATA> = data;
}
function onError(error){
//handle error
}
dao.getData(var1, var2, onSuccess, onError);
Related
Here is my code
var x = 0
data.results[0].taxonomies.some(function(e){
if(taxo.indexOf(e.code)!=-1){ //if at least one code from the data base is in my list of codes
callback(validLic(data.results[0].taxonomies)) //return true after some other validations
return true
}else{
x++
return false
}
})
if(x==data.results[0].taxonomies.length){callback(false)}//if the entire array was processed, and I didn't find anything I was looking for, return false
I'd like someone to confirm that due the async nature of node, the last if statement is at some point, bound to fire off before I'm done processing the array.
How can I better manage this situation without the help of some sync or parallel library?
The reason I ask it that way is because I'm under the impression that if I can't write something to be completely async, then I'm not writing it efficiently, right?
EDIT:
Based on Luc Hendirks' logic, I have changed my code to this:
var flag = data.results[0].taxonomies.some(function(e){
if(taxo.indexOf(e.code)!=-1){ //if at least one code from the data base is in my list of codes
return true
}else{
return false
}
})
if(flag==true){
callback(validLic(data.results[0].taxonomies))
}else{
callback(false)
}
Because this follows the sync traits outlined below, I shouldn't have an issue with flag being undefined before the callback is called now right?
Javascript (and Node) are single threaded, meaning it has only 1 CPU available. If the functions you call only require CPU time, making it async is useless. If you need to call a function and the CPU has to wait (do nothing), then making it async is very useful. Because while it is waiting until the function is finished it can do something else.
A function that checks if a url is valid with a regular expression can be synchronous, as the CPU needs to do some calculations and you get the result. If the function actually does a GET request and checks the response code, the CPU has to wait until the response is received. In the meantime it could do something else, so this function should be made asynchronous.
The difference of a synchronous and asynchronous function is that a synchronous function returns a value:
function(a) { return a; }
and an asynchronous function returns the result using a callback function (this is an actual function that you put in as a function argument):
function(callback){
// Do something that takes time but not CPU, like an API call...
callback('Some result');
}
A synchronous function is called like this:
var a = something();
Asynchronous like this:
something(function(result){
console.log(result);
});
So to answer your question, if some() is an asynchronous function, then the last if statement can be executed before the some function is finished. Because: the CPU does not want to wait. It can do other stuff while waiting. That's the power of asynchronous programming. t
It is also important to know that "parallel" does not exist in Javascript/Node. There is only 'doing stuff instead of waiting', like executing multiple API calls at the same time. That is not parallel computing as in using multiple threads.
Here is some more info: What is the difference between synchronous and asynchronous programming (in node.js)
I have been looking for assistance with setting up an IndexedDB for web storage, and I have run into a problem that I cannot find a good answer to. After I have successfully setup/opened the database I am having trouble passing the variable that contains the database information to access it later. Here is my code, I have been following a guide from MDN :
const DB_NAME = 'database-name';
const DB_VERSION = 2;
const DB_STORE_NAME = 'users';
var db;
function openDb() {
console.log('openDb ...')
var request = indexedDB.open(DB_NAME, DB_VERSION);
request.onsuccess = function(event) {
db = request.result;
console.log("openDb DONE");
};
request.onerror = function(event) {
console.log(request.errorCode);
};
request.onupgradeneeded = function(event) {
console.log('openDb.onupgradeneeded');
var store = event.currentTarget.result.createObjectStore(DB_STORE_NAME, { keyPath: 'id', autoIncrement: true });
store.createIndex('age', 'age', { unique: false });
};
}
function getObjectStore(store_name, mode) {
var tx = db.transaction(store_name, mode);
return tx.objectStore(store_name);
}
When getObjectStore is called the variable db is undefined. My knowledge of javascript is very limited and some concepts I don't get. The guide doesn't not show anything special being done and their demo works as is. Some other guides have mentioned implementing a callback, but they don't show how its done nor do I understand the concept of callbacks. Any help is appreciated. Thanks.
Unfortunately you need to learn about a relatively complicated concept usually referred to as asynchronous JavaScript before proceeding to use indexedDB. There are already several thousand questions related to AJAX on stackoverflow. I am trying to think of the most polite way to say this, but basically, the answer you are looking for is already provided by these other questions, and by many other websites. Nevertheless, here are some quick tips.
First, your approach is never going to work. You cannot skip over learning about async.
Second, do not use the setTimeout trick to get it to work. That is horrible advice.
Third, at a general level, a callback is simply a word used to describe a function when the function is used in a particular way. Specifically, a callback refers to a function that is passed as an argument to another function, where the other function then maybe calls the function at some later point in time. More specifically, a callback is generally a function that is called at the end of the function it is passed to, when the function has completed.
For example:
function a(b) { alert(b); }
function c(d) { d('hi'); }
c(a);
That might look a bit confusing at first but it is the simplest thing I can describe off the top of my head. In the example, the final line calls function c and passes in function a. The effect of the code is that you see 'hi' as a browser alert. In this example, the function a is passed as a parameter/argument to function c. Function c uses the name d for its one and only argument. c calls d with the string 'hi'. When describing this example, we would say that argument d represents a callback function passed to function c. We could also say that function a is the particular callback function used by function c. So that is basically it. When you pass a function in as an argument and the other function calls the passed in argument, you are using a callback.
Then things gets way more complicated, because you have to learn about how to read and write asynchronous code. Properly introducing it would take several pages. Here is an extreme crash course.
You have traditionally been writing synchronous code, even if you did not call it that. Synchronous code runs exactly when you expect it to, in the order that you write your statements. Here is a brief example of typical sync code:
function sum(a, b) { return a + b; }
alert(sum(1, 2));
Simple stuff. The next example is code that uses a callback, but is still synchronous.
function doOperation(op, num1, num2) { return op(num1, num2); }
function sumOperation(num1, num2) { return num1 + num2; }
var result = doOperation(sumOperation, 1, 2);
alert(result);
Here we passed the sumOperation function into the doOperation function. sumOperation is the callback function. It is the first argument with the name 'op'. Still pretty simple stuff. Now consider the next example. The point of the next example is to show how we pass control to the function to do something. Kind of like how goto/labels work.
function doOperation(op, num1, num2) {
var result = op(num1, num2);
alert(result);
return undefined;
}
function sumOperation(num1, num2) { return num1 + num2; }
doOperation(sumOperation, 1, 2);
Notice how doOperation no longer returns a value. It has the logic within its function body. So once we call doOp, the browser starts running the code inside doOperation. So we switched from the outer context into the body of the function. Also, because doOperation doesn't return anything, we cannot do anything with its return value. The logic is locked inside the body of the doOperation function. The code still works about the same, its just that now we are not returning anything from doOperation, and now the logic is inside doOperation instead of outside in the main/global context.
Now an example that uses setTimeout. This is completely unrelated to the suggestion of using setTimeout.
function doOperation(op, num1, num2) {
setTimeout(function runLater() {
var result = op(num1, num2);
alert(result);
return undefined;
}, 1000);
return undefined;
}
function sumOperation(num1, num2) { return num1 + num2; }
doOperation(sumOperation, 1, 2);
The point here is to understand that we use a callback (named runLater in this example), and that the code inside the callback does not run immediately. Therefore, we can no longer say it runs synchronously. We instead refer to the statements constituting the body of the callback function as asynchronous. So now an alert appears after 1 second. Notice how we cannot return anything from runLater. Also notice how we cannot return anything from doOperation. There is nothing to return. There is no way to get the value in the 'result' variable out of the scope of runLater. It is locked in there.
Let's try almost the same thing, but try to have runLater set a variable. Also, I am going to omit 'return undefined' because that is what every function without a explicit return statement returns.
var aGlobalResult = null;
function doOperation(op, num1, num2) {
setTimeout(function runLater() {
aGlobalResult = op(num1, num2);
}, 1000);
}
function sumOperation(num1, num2) { return num1 + num2; }
doOperation(sumOperation, 1, 2);
alert(aGlobalResult);
Hopefully you are catching on to the problem. First off, runLater does not return anything, so doOperation does not return anything, so we could not even try to do something like aGlobalResult = doOperation(...);, because that would not make any sense. Second, the result here is that you will see an alert 'undefined' because the alert statement executes prior to the statement that assigns a value to aGlobalResult. This is even though you wrote the assignment statement higher up (earlier) in the code, and the alert is later. This is the brick wall some newer developers run into right here. This is indeed confusing for some. aGlobalResult is undefined here because setTimeout does not set it until later. Even if we passed in 0 milliseconds to setTimeout, it is still 'later', meaning that the assignment happens at a later point in time, after the alert. The alert message is always going to be undefined. There is absolutely nothing you can do to avoid the way this works. Nothing. Period. Stop trying. Learn it, or give up entirely.
So, how does one typically write code that behaves or involves asynchronous stuff? By using callbacks. Which again means that you can no longer use return statements to assign values to outer scope variables. You instead want to write functions and pass around control to various functions. In other words, instead of:
function a() {}
function b() {}
function c() {}
a(); b(); c();
you write code like this:
function a(callback) {
var asdf = 1+2; // do some stuff in a
alert('a finished');
// a has now completed, call its callback function, appropriately named callback
callback();
}
function b(callback) {
var asdfasdfasdf = 3 + 4;
alert('b finished');
// call the callback
callback();
}
a( function(){ b(function() { alert('both a and b finished'); }); });
This is more formally known as continuation passing style, or CPS.
So, that is an example of the very basics of writing callback functions and basic asynchronous code. Now you can start to use indexedDB. The first thing you will notice is that the function indexedDB.open is documented as asynchronous. So, how can we use it? Like this:
var someGlobalVariable = null;
var openRequest = indexedDB.open(...);
openRequest.onsuccess = function openRequestOnSuccessCallbackFunction(event) {
// Get a reference to the database variable. If this function is ever called at some later
// point in time, the database variable is now defined as the 'result' property of the
// open request. There are multiple ways to access the result property. Any of the following
// lines works 100% the same way. Use whatever you prefer. It does not matter.
var databaseConnection = openRequest.result;
var databaseConnection = this.result;
var databaseConnection = event.target.result;
var databaseConnection = event.currentTarget.result;
// Now that we have a valid and defined databaseConnection variable, do something with it
// e.g.:
var transaction = databaseConnection.transaction(...);
var objectStore = transaction.objectStore(...);
// etc.
// DO NOT DO THE FOLLOWING, it does not work. Why? Review the early descriptions. First off
// this onsuccess callback function does not return anything. Second off this function gets
// called at some *later* point in time, who knows when. It could be a nanosecond later.
someGlobalVariable = databaseConnection;
};
Hopefully that sets you on the path.
Edit: I thought I would add a bit more of an intro. A related concept you need to learn that I did not explain clearly enough regarding control is the difference between imperative and declarative programming.
Imperative programming involves executing a series of statements in the order you wrote. You are the caller and are in control. Imperative code looks like this (fictional code):
var dbConn = dbFactory.newConnection('mydb');
var tx = dbConn.newTransaction();
var resultCode = tx.put(myObject);
if(resultCode == dbResultConstants.ERROR_PUT_KEY_EXISTS) {
alert('uhoh');
}
Declarative programming is subtly different. With a declarative approach, you write functions, and then you register (aka hook or bind) the functions to the JavaScript engine, and then at some point later, when it is appropriate, the engine runs your code. The engine is the caller and is in control, not you. Declarative programming involves callbacks and looks like this (fictional code):
dbFactory.newConnection(function onConnect(dbConn) {
dbConn.newTransaction(function onNewTransaction(tx) {
tx.put(myObject, function onPut(resultCode) {
if(resultCode == dbResultConstants.ERROR_PUT_KEY_EXISTS) {
alert('uhoh');
}
});
});
});
In this example, the only thing you called was the fictional dbFactory.newConnection function. You passed in a callback function. You did not call the callback function yourself. The engine calls the callback function. You cannot call the callback function yourself. This is kind of the whole idea behind why JavaScript engines can allow you to write asynchronous code. Because you don't get to control the order of execution of statements/functions. The engine gets to control it. All you get to do is write your functions, register them, and then start a chain of callbacks (the sole imperative line, the starting statement).
So this is why a function like getObjectStore in your question will not work. You are trying to call the function yourself, but that is backwards. You can only write a function and register it (somehow hook it up as a callback somewhere) and then the engine, not you, calls it at some later point in time.
Hopefully this is not more confusing, but you could actually write your function getObjectStore if you really wanted to by passing in the database variable to the function as its first argument. This leads to the logical next question, how to get a valid database variable to pass into the function. You cannot get one in a global context (reliably). Because the connection variable is only valid within the context of the onOpen callback function. So you would have to make your call to this function from within the onOpen function. Something like:
function getObjectStore(db, name, mode) {
var tx = db.transaction(name, mode);
var store = tx.objectStore(name);
return store;
}
var openRequest = indexedDB.open(...);
openRequest.onsuccess = function onOpen(event) {
// get the connection variable. it is defined within this (onOpen) function and open.
var db = this.result;
// call our simple imperative helper function to get the users store. only call it from
// within this onOpen function because that is the only place we can get the 'db' variable.
var usersStore = getObjectStore(db, 'users', 'readwrite');
// do something here with usersStore, inside this function only.
};
I'm not even sure how to phrase this question so I'll have to go with examples. This might not look like useful code here, and indeed it isn't, but it's an example of a problem I've just encountered, stripped down to the bare essentials.
Lets' assume I have
function myObject(params) {
.. do some time-consuming asynchronous stuff
with AJAX and the like ...
return (before all the above is completed);
}
function myFunction(params) {
var doTheSlowStuff = new myObject(params);
}
myFunction(firstParams);
myFunction(moreParams);
What happens to the first myObject when I make the second call to myFunction()? Does it get a chance to complete its work (and if so will it be garbage collected when it has)? Or does it get unceremoniously dumped before it has a chance to finish what it started?
All the time-consuming asynchronous stuff will happen asynchronously :)
That means that the async calls (such as XHR or setTimeout) return instantly and allow execution to continue. In other words, the myObject constructor will return very quickly so there will be no delay between constructing the two myObjects. After both myFunctions return, then finally control will return to the event loop and the JavaScript engine will continue processing events, like mouse clicks, WebSocket events, timers like setTimeout, or XHR requests coming back. Your asynchronous callbacks won't be executed until you return control to the event loop, so don't do anything crazy like
while(true) {
// Check XHR status
}
Don't worry about garbage collection; if you have a DOM event like an AJAX (XHR) request with your myObject in scope then it won't be garbage collected until the event handler itself is garbage collected.
In this specific instance, you will create two instances of myObject which will be retained in memory until your application exits.
You can prove this by running something asynchronously to test this behaviour:
function myObject(params) {
// do something async, like output
// every second ...
var callback = function () {
console.log("I am object " + params);
setTimeout(callback, 1000);
};
callback ();
}
function myFunction(params) {
var doTheSlowStuff = new myObject(params);
}
myFunction(1);
myFunction(2);
// etc.
See a working example at: http://jsbin.com/osEFuWib/1/edit
The asynchronous code has access to a callback function. The callback will run and is independent of any other object. "Losing" the object or creating a new object does not change this. As such, pending asynchronous operations must be explicitly cancelled or the callbacks must be guarded against performing unwanted effects when they are invoked.
The thing about objects in JavaScript is simple: as long as any code - including the callback - can access the object (e.g. assigned to a variable in scope, a window property, or bound to the DOM), they remain accessible. Otherwise, they are unreachable and will be reclaimed at some point.
In tornado we have gen module, that allows us to write constructions like this:
def my_async_function(callback):
return callback(1)
#gen.engine
def get(self):
result = yield gen.Task(my_async_function) #Calls async function and puts result into result variable
self.write(result) #Outputs result
Do we have same syntax sugar in jquery or other javascript libraries?
I want something like this:
function get_remote_variable(name) {
val = $.sweetget('/remote/url/', {}); //sweetget automatically gets callback which passes data to val
return val
}
You describe the function as "my_async_function", but the way you use it is synchronous rather than asynchronous.
Your sample code requires blocking -- if "my_async_function" were truly asynchronous (non-blocking), the following line of code self.write(result) would execute immediately after you called "my_async_function". If the function takes any time at all (and I mean any) to come back with a value, what would self.write(result) end up writing? That is, if self.write(result) is executed before result ever has a value, you don't get expected results. Thus, "my_async_function" must block, it must wait for the return value before going forward, thus it is not asynchronous.
On to your question specifically, $.sweetget('/remote/url/', {}): In order to accomplish that, you would have to be able to block until the ajax request (which is inherently asynchronous -- it puts the first A in AJAX) comes back with something.
You can hack a synchronous call by delaying the return of sweetget until the XHR state has changed, but you'd be using a while loop (or something similar) and risk blocking the browser UI thread or setting off one of those "this script is taking too long" warnings. Javascript does not offer threading control. You cannot specify that your current thread is waiting, so go ahead and do something else for a minute. You could contend with that, too, by manually testing for a timeout threshold.
By now one should be starting to wonder: why not just use a callback? No matter how you slice it, Javascript is single-threaded. No sleep, no thread.sleep. That means that any synchronous code will block the UI.
Here, I've mocked up what sweetget would, roughly, look like. As you can see, your browser thread will lock up as soon as execution enters that tight while loop. Indeed, on my computer the ajax request won't even fire until the browser displays the unresponsive script dialog.
// warning: this code WILL lock your browser!
var sweetget = function (url, time_out) {
var completed = false;
var result = null;
var run_time = false;
if (time_out)
run_time = new Date().getTime();
$.ajax({
url: url,
success: function(data) {
result = data;
completed = true;
},
error: function () {
completed = true;
}
}); // <---- that AJAX request was non-blocking
while (!completed) { // <------ but this while loop will block
if (time_out) {
if (time_out>= run_time)
break;
run_time = new Date().getTime();
}
}
return result;
};
var t = sweetget('/echo/json/');
console.log('here is t: ', t);
Try it: http://jsfiddle.net/AjRy6/
Versions of jQuery prior to 1.8 support sync ajax calls via the async: false setting. Its a hack with limitations (no cross-domain or jsonp, locks up the browser), and I would avoid it if possible.
There are several available libraries that provide some syntactic sugar for async operations in Javascript. For example:
https://github.com/caolan/async
https://github.com/coolaj86/futures
...however I don't think anything provides the synchronous syntax you are looking for - there is always a callback involved, because of the way JavaScript works in the browser.
I have a node application that is not a web application - it completes a series of asynchronous tasks before returning 1. Immediately before returning, the results of the program are printed to the console.
How do I make sure all the asynchronous work is completed before returning? I was able to achieve something similar to this in a web application by making sure all tasks we completed before calling res.end(), but I haven't any equivalent for a final 'event' to call before letting a script return.
See below for my (broken) function currently, attempting to wait until callStack is empty. I just discovered that this is a kind of nonsensical approach because node waits for processHub to complete before entering any of the asynchronous functions called in processObjWithRef.
function processHub(hubFileContents){
var callStack = [];
var myNewObj = {};
processObjWithRef(samplePayload, myNewObj, callStack);
while(callStack.length>0){
//do nothing
}
return 1
}
Note: I have tried many times previously to achieve this kind of behavior with libraries like async (see my related question at How can I make this call to request in nodejs synchronous?) so please take the answer and comments there into account before suggesting any answers based on 'just use asynch'.
You cannot wait for an asynchronous event before returning--that's the definition of asynchronous! Trying to force Node into this programming style will only cause you pain. A naive example would be to check periodically to see if callstack is empty.
var callstack = [...];
function processHub(contents) {
doSomethingAsync(..., callstack);
}
// check every second to see if callstack is empty
var interval = setInterval(function() {
if (callstack.length == 0) {
clearInterval(interval);
doSomething()
}
}, 1000);
Instead, the usual way to do async stuff in Node is to implement a callback to your function.
function processHub(hubFileContents, callback){
var callStack = [];
var myNewObj = {};
processObjWithRef(samplePayload, myNewObj, callStack, function() {
if (callStack.length == 0) {
callback(some_results);
}
});
}
If you really want to return something, check out promises; they are guaranteed to emit an event either immediately or at some point in the future when they are resolved.
function processHub(hubFileContents){
var callStack = [];
var myNewObj = {};
var promise = new Promise();
// assuming processObjWithRef takes a callback
processObjWithRef(samplePayload, myNewObj, callStack, function() {
if (callStack.length == 0) {
promise.resolve(some_results);
}
});
return promise;
}
processHubPromise = processHub(...);
processHubPromise.then(function(result) {
// do something with 'result' when complete
});
The problem is with your design of the function. You want to return a synchronous result from a list of tasks that are executed asynchronously.
You should implement your function with an extra parameter that will be the callback where you would put the result (in this case, 1) for some consumer to do something with it.
Also you need to have a callback parameter in your inner function, otherwise you won't know when it ends. If this last thing is not possible, then you should do some kind of polling (using setInterval perhaps) to test when the callStack array is populated.
Remember, in Javascript you should never ever do a busy wait. That will lock your program entirely as it runs on a single process.
deasync is desinged to address your problem exactly. Just replace
while(callStack.length>0){
//do nothing
}
with
require('deasync').loopWhile(function(){return callStack.length>0;});
The problem is that node.js is single-threaded, which means that if one function runs, nothing else runs (event-loop) until that function has returned. So you can not block a function to make it return after async stuff is done.
You could, for example, set up a counter variable that counts started async tasks and decrement that counter using a callback function (that gets called after the task has finished) from your async code.
Node.js runs on A SINGLE threaded event loop and leverages asynchronous calls for doing various things, like I/O operations.
if you need to wait for a number of asynchronous operations to finish before executing additional code
you can try using Async -
Node.js Async Tutorial
You'll need to start designing and thinking asynchronously, which can take a little while to get used to at first. This is a simple example of how you would tackle something like "returning" after a function call.
function doStuff(param, cb) {
//do something
var newData = param;
//"return"
cb(newData);
}
doStuff({some:data}, function(myNewData) {
//you're done with doStuff in here
});
There's also a lot of helpful utility functions in the async library available on npm.