I am originally coming from Java programming language as background. Java is strongly typed and quite explicit (I mean by that, you have to write things out, you can't just omit them). One thing, that will never get in my head is how that implicit parameter passing to javascript works...
as an example:
const observer = {
next: console.log,
error: console.error,
test: console.table,
}
observer.next('HI World!')
in next, I specify console.log which is a function, but I never say to accept a value, but in Javascript I apparently can just throw anything to functions as suffix and it takes that as an argument to its function.
Thats also how pipelining or currying basically works, it takes the remaining return values as parameters..why is that so?
Secondly, e.g in express, I have a function signature like so:
app.get('/', function (req, res) {
res.send('GET request to the homepage');
});
I actually get req and res out of my function callback is that right? those are not parameters that I pass to my callback function?
Edit: Can someone explain to me again where the REQand RES parameters are coming from? How does that construct work? Because I am defining the callback function myself, but instead of passing req and res as parameters to my callback, it seems they get passed from somewhere back to inside my callback function?!?!
A function in JavaScript is an object just like any other. This means a function can be passed around, can be assigned to variables, and essentially can be used anywhere a value can be used. This is known as first-class functions.
For example, given a simple function like this one:
function log(msg) {
console.log(msg);
}
These two functions are conceptually equivalent:
const log1 = log;
// same as
function log1(msg) {
log(msg);
}
As you can see, we can directly assign the function log to a variable like log1, ,and then we can use log1 just like we use log:
log1('foobar');
Another usage of first-class functions is to be able to pass them as arguments to other functions. A function that takes a callback like in your Express example is known as a higher-order function. The arguments of the callback are provided by the caller of that callback. In your case Express provides the req and res arguments.
Using callbacks is a form of inversion of control. You are passing control of that part of the program to the caller.
Related
I have a question about asynchronous/synchronous when we write code in JS and I have done google search but I am still a bit confused.
I understand that we use callback functions when we want to make sure that the callback function is executed only after the asynchronous task (such as accessing database) in the outer function is complete. I understand how deserialize and serialize user works.
I am confused why when we do serialize user or deserialize user with passport.js we need a callback function like this ?
passport.serializeUser((user, done) => {
done(null, user.id);
});
If all we want is that the inner arrow function that is passed as an argument to serializeUser() to be executed only after serializeUser() is finished. Or why do we need to pass it as a callback function instead of calling that arrow function below serializeUser() ? I thought JS is synchronous so it will execute the arrow function after serializeUser() is completed anyway ?
I only found serializeUser() documentation in passport documentation on how to use it, but not its implementation so I am also confused whether serializeUser() or deserializeUser()( or any other passport functions) are asynchronous functions ?
Thank you !
It is a fragment of this function on github (https://github.com/jaredhanson/passport/blob/08f57c2e3086955f06f42d9ac7ad466d1f10019c/lib/authenticator.js).
As you can see this function takes a fn parameter. Then it checks if it is a function. If yes it pushes it to this._serializers. Later it does 'some magic' on it, I think it's not important now.
As you can read in jsdoc, serializeUser purpose is to register a function used to serialize user objects. So you pass some function which is invoked later in a code with 2 arguments which labels on that particular function level are user and done.
They left you some space which you can fill with your own code. You can tell how this will behave. They gave you an opportunity to implement your own logic.
Simple example:
function doSomeMagic(fn) {
// I do some my staff
const a = 5;
const b = 10;
// and now I let you decide what to do
// you can implement your own logic in my function
const result = fn(a, b);
console.log(result);
}
doSomeMagic((a, b) => {
return a * b;
});
doSomeMagic((a, b) => {
return a + b;
});
//output:
// 50
// 15
That's exactly what they've done. You can take my function, I give you two arguments do whatever you want. You can multiply, add, substract, whatever you want. I don't have to write separated functions to do math, I can write one, pass values as arguments and let you decided what operation you want to do. And it doesn't mean my code is going to run in an async way.
There is an article I've seen about the callbacks in javascript. https://codeburst.io/javascript-what-the-heck-is-a-callback-aba4da2deced I know that I can understand it by reading the article. However, I'm getting confused of the callback while studying the module export in node.js
Callback - A callback is a function that is to be executed after another function has finished executing
Callback in javascript
function doHomework(subject, callback) {
console.log(`Starting my ${subject} homework.`);
callback();
}
doHomework('math', function() {
console.log('Finished my homework');
});
Module export in node.js
//app.js
const logger = require('./logger');
logger.log(10, 10);
//logger.js
const multiply = require('./multiplication');
function log(valueOne, valueTwo) {
multiply('The result is ', valueOne, valueTwo);
}
module.exports.log = log;
//
function multiply(speech, valueOne, valueTwo) {
let result = valueOne * valueTwo;
return console.log(speech + result);
}
module.exports = multiply;
and ran the node app.js on my terminal.
The result that I got from running the node app.js is The result is 100 and that is correct.
But my question is
Does the approach that I did on the node app is consider as callback as well?
Callback - A callback is a function that is to be executed after another function has finished executing
That's not a correct definition of "callback." Unless that definition had major caveats on it you didn't quote, I wouldn't continue to use whatever resource you got that from.
A fairly broad definition of a callback is:
A callback is a function you pass to something else (as a function argument, property value, etc.) that the other thing will call when its defined criteria for calling the function are met.
Some might argue for a narrower definition:
A callback is a function you pass to another function for that other function to call back when its defined criteria for doing so are met.
Examples of callbacks:
DOM event handlers, although we usually call them "handlers" rather than callbacks. (Broad definition.)
The function you pass to Array.prototype.sort to compare array elements. (Both the broad and narrower definitions.)
The function you pass to new Promise to start the asynchronous operation the promise will observe (called the "promise executor function"). (Both the broad and narrower definitions.)
The function you pass to Array.prototype.map to transform elements. (Both the broad and narrower definitions.)
The function you pass to a promise's then, catch, or finally method.
The function you pass to fs.openFile that Node.js will call when the file has been opened (or the operation has failed). (Both the broad and narrower definitions.)
...and many others.
Notice that many of those (2, 3, and 4) are called before the function calling them has finished executing.
Does the approach that I did on the node app is consider as callback as well?
No. Although you use multiply in log, it's just a function you call from log, not a callback. This would be a callback:
function multiply(a, b, cb) {
cb(a * b);
}
function showResult(msg) {
console.log(msg);
}
multiply(7, 6, showResult);
showResult is used as a callback when calling multiply.
I don't entirely understand your question. However, from what I gather, module.exports does not make a function a callback function explicitly. The purpose of module.exports is to allow access to that function when requiring the relevant .js file...as seen in your example.
Your log() function is a not a callback as you are simply passing in parameters and then using those values to call the multiply function and output the result.
When you call the multiply function you are simply calling it like so:
multiply('some text', 10, 10)
For this to be a callback it would have to take a function as it's final parameter, i.e.:
multiply('some text', 10, 10, function(err, data) {
// ...
})
This also goes for the log function, and any for that matter.
So, unless the final parameter of a function is a function, it is a not a callback. module.exports purely allows access to that function or the functions you specify in the object, for example:
module.exports = {
functionOne: someFunctionName,
functionTwo,
functionThree
}
If the name of the function is the same name as what you are trying to export you do not need to specify a value to the key.
I understand the essence of callback functions in that the function is executed again after being passed as the parameter to another function. However, I'm confused as to where the variables inside the callback function come from as shown in the following node.js example:
router.get('/', function(req, res){
res.render('index', {});
});
How do the variables req and res get populated? An example explaining how I can just call res.render(...) without declaring res myself would be greatly appreciated.
They come from the same place they come from when a normal non callback function is invoked, at invocation time.
If you have this function,
function add (a, b) {
return a + b
}
You're fine with knowing that a and b come from when you invoke add,
add(1,2)
and it's the same principle with callbacks, don't let your brain get all twisted just because it's getting invoked later.
At some point the function you pass to router.get is going to be invoked, and when it does, it will receive req and res.
Let's pretend the definition for router.get looks like this
router.get = function(endpoint, cb){
//do something
var request = {}
var response = {}
cb(request, response) // invocation time
}
In the case of your example, it's just up to node to pass your function request and response whenever .get is invoked.
The whole point of the callback is that the invoked function calls it back.
In the case of router.get, it will insert the route (path, method, callback) in a lookup table; when a request comes in, Express will construct the response object, match the request's path and method against all the entries in the lookup table, take the callback from the matching entry and invoke callback(request, response) (passing the detected request and created response).
They get populated by whatever code is calling the callback. In your example, this is something inside the Express framework, though Express uses the node http library under the hood and adds additional functionality to the request and response objects provided by it.
But in code you write you can create a callback function signature that takes whatever params you want.
I am fairly used to RX having used it in .NET and Java, I am expecting to be able to do the following:
Rx.Observable.fromCallback(websocket.onmessage)
.map(...)
.subscribe(...);
however, the console has the following:
Uncaught TypeError: Rx.Observable.fromCallback(websocket.onmessage).map is not a function
which would appear to indicate that the fromCallback is not returning an Observable.
What am I doing wrong here? Have I misunderstood what fromCallback is doing and I need to use a Subject? Can I not wrap some arbitrary handler in an observable?
You are actually looking for fromEvent or fromEventPattern:
Rx.Observable.fromEvent(websocket, 'message').map(/*...*/).subscribe();
Rx.Observable.fromEventPattern(
function add(h) { websocket.addEventListener(h); },
function remove(h) { websocket.removeEventListener(h); })
.map(/*...*/)
.subscribe();
The first one will attempt to use some of the standard ways of subscribing to an event emitter, which WebSocket is. However, if that fails you can use fromEventPattern instead to specify how handlers are added or removed from your object.
One additional note, JavaScript does not pass along an implicit reference to the instance of the object you are using as C# and Java do, so your code fromCallback(websocket.onmessage) is not passing along websocket, it is passing along the reference to the method from the function prototype. this will be determined at execution time.
Rx.Observable.fromCallback is for functions whose last argument is a callback function which is a standard pattern for asynchronous JavaScript code. Further, the fromCallback method does not return an Observable it returns a function that when called returns an Observable i.e.
function methodWithCallback(arg0, arg1, cb) {
setTimeout(function() {
cb(arg0 + arg1);
}, 2000);
}
var newMethod = Rx.Observable.fromCallback(methodWithCallback);
//[After 2 seconds] 3
newMethod(1, 2).subscribe(console.log.bind(console));
Someone please help me. I have been reading a lot of javascript documentation and fiddling with javascript.
I am able to use the functions but I don't quite get this essential syntactic jazz going on here
var http = require('http');
http.createServer(function (req, res) {
res.writeHead(200, {'Content-Type': 'text/plain'});
res.end('Hello World\n');
}).listen(1337, 'myhost');
console.log('Server running at http://myhost:1337/');
I cant figure out why its okay to use req and res in the anonymous function above. It's like they live somewhere inside of http. They are not declared anywhere! They are made up variable names in an anonymous function that reference inner objects or something. It's crazy.
How does a callback function like this?
or like
stream.on('data', function(data){
// use data... i dont know where its coming from
// or how it got there, but use it
});
If you could post a small example that mimics this process and syntax or explain how these callback functions can work like this or how I can pass these undeclared variables into functions like this I would greatly appreciate it.
A similar example to the answer posted below.
var b = {
somefunction : function( data ){
var x = 100;
x = x+1; // 101
return data(x); // returns the callback function data w/ value x
}
};
b.somefunction(function(foo){
foo++; // 101 + 1
console.log(foo); // prints 102
});
The main issue is that Javascript is a functional language so you can pass functions as parameters to other functions. In other languages you may have experienced passing a pointer or handle to a function, for example. Consider a simple cases in which you have some math functions:
function add(a,b) {return (a+b)};
function subtract(a,b) {return (a-b)}:
Now I could create a new function:
function longWayAroundTheBarn(num1,num2,theFunc)
{
// invoke the passed function with the passed parameters
return theFunc(num1,num2);
}
And call it like this:
console.log(longWayAroundTheBarn(1,2,add));
> 3
Or even like this:
console.log(longWayAroundTheBarn(longWayAroundTheBarn(2,2,subtract),4,add);
> 4
Obviously this would be a silly use callbacks, but you can imagine generally that the ability to 'plug-in' a function this way can be pretty powerful.
Consider if you couldn't pass functions. This might be one way you would implement this:
function reallyLongWayAround(num1,num2,funcName)
{
if(funcName==='add')
return add(num1 ,num2);
else if (funcName === 'subtract')
return subtract(num1, num2);
}
You can imagine that besides being really tedious code to have to write and maintain, it's not nearly so powerful because the reallyLongWayAround function could only ever call code it knew about. By passing functions, my longWayAroundTheBarn doesn't care if I create new functions and pass it to it. Note that because of weak typing, it doesn't even need to care about the parameters it is passing. Maybe you want to implement something like
function businessDaysBetween(d1,d2)
{
// implementation left as a reader exercise
};
It would work just fine to call:
longWayAroundTheBarn(new Date(2014,1,15), new Date(2014,1,22),businessDaysBetween)
Returning to the specific case you raised, req and res are not 'local variables' as one answer indicates - they are called parameters or arguments. You aren't passing anything into them. They are being passed to you by the calling function. You could actually call them fred and barney if you wanted, although it would be a terrible idea. The main point is that they will be called populated with request and response objects.
You actually don't even have to have the parameters in your function signature, you could just have a callback as below, and make use of the second parameter passed to your function by reading the arguments array (Note, it's not actually an array but behaves similarly in many respects). This would be a terrible, terrible idea, but again, trying to illustrate the point.
var http = require('http');
http.createServer(function () {
arguments[1].writeHead(200, {'Content-Type': 'text/plain'});
arguments[1].end('Hello World\n');
}).listen(1337, 'myhost');
The req and res are actually local variables of the anonymous function.
I have an example below similar to the codes posted in your question.
// static class
var HelperClass = {
"doSomething" : function ( callback ) {
// do something here, for example ajax call to a server
var data = ajaxCallFromServer();
// return the data via callback
callback ( data );
};
};
// you codes here calling the HelperClass
// calling .doSomething method with an anonymous function callback
// that expects 1 parameter. This parameter is returned by the above
// code via callback ( data ). And in the anonymous function
// i called it as retData, you can call it whatever you like
HelperClass.doSomething( function ( retData ) {
// do soemthing with your retData here
});
You should use the documentation.
So for example for createServer, follow this page - http://nodejs.org/api/http.html#http_http_createserver_requestlistener
http.createServer([requestListener])# Returns a new web server object.
The requestListener is a function which is automatically added to the
'request' event.
Then, you check the 'request' event -
Event: 'request'# function (request, response) { }
Emitted each time there is a request. Note that there may be multiple
requests per connection (in the case of keep-alive connections).
request is an instance of http.IncomingMessage and response is an
instance of http.ServerResponse.
Regarding the stream, exactly same. The docs here - http://nodejs.org/api/stream.html#stream_writable_stream
Look for
Event: 'data'
The point is that http.createServer function takes an argument that is not a variable, but a function, if that makes sense. In javascript you can do that. And that function expects arguments that are specified in it's API. You can make it anonymous, like in your example, or declared like below:
function serverFunction(req, res) {
res.writeHead(200, {'Content-Type': 'text/plain'});
res.end('Hello World\n');
}
var http = require('http');
http.createServer(serverFunction).listen(1337, 'myhost');
but in the end it does not matter, it just behaves accordingly to what is specified in its the API.