Develop Reference

Detect when a function is getting called in JavaScript

There are several elements on HTML page which triggers a js function HardCoded().
I cannot modify HardCoded() function.
I want to run some custom js code after the HardCoded() function is getting called. How can I do that? Is there any handlers for js functions?
I'm building a chrome extension that's why I cannot modify page source code.
I have access to JQuery.
One way is to find all elements who are calling HardCoded() and attach events to those elements but I would like to avoid this method.

You could do something like this:
var oldFn = HardCoded;
window.HardCoded = function(){
var res = oldFn.apply(this, arguments);
// New Code ....
return res;
}
What this does is to create a reference to the HardCoded function, redefine this function and then call the old implementation using the previously created reference.

Callbacks usage with instanced objects in Javascript

I was facing a problem with callbacks in Javascript. I solved my problem using what I would call an ugly property of Javascript (so to say, something that would logically be forbiden and never work in other languages than Javascript). So my question: Is there an ELEGANT way, to do the same thing.
I will so begin with the beginning. My goal was to wrap, in some manner, the Web Audio API. In the architecture, I implemented a class, lets call it AudioRessource, which is destined to be an interface (abstraction) in some manner of the AudioBuffer object of the Web Audio API.
This class (AudioRessource) have a prototype member function that must simply take an url as argument to automatically load audio data, decode it, handle errors, etc and finally hold the resulting AudioBuffer object in a "pseudo-private" member:
function AudioRessource()
{
this._aBuffer = null; // future reference to `AudioBuffer` object
this._loadStatus = 2;
};
AudioRessource.prototype.loadData = function(url) {
/* deal here with async functions to
provides audio data loading automation */
}
The main problem here, is that this will be an object instance (of AudioRessource) which will create the callback functions, using only local references, and must be able to pass the final AudioBuffer object to itself.
To load the raw audio data, this is pretty simple, I use the XMLHttpRequest object, with an extra property set as member of the XMLHttpRequest object, like this:
AudioRessource.prototype.loadData = function(url) {
let req = new XMLHttpRequest();
req.extraProperty = this; // reference to `AudioRessource` instance
req.onload = function(){
// retrive instance reference within the callback
this.extraProperty._loadStatus = 0;
}
req.onerror = function(){
// retrive instance reference within the callback
this.extraProperty._loadStatus = -1;
}
req.open('GET', url, true);
req.send(null);
this._loadStatus = 1;
}
The big problem appear when we have to decode the coded raw audio data into PCM data, that is, an Web Audio API AudioBuffer object instance. Indeed, the Web Audio API provides only one function to achieve this, and this function is asynchronous, and takes a callback that simply recieve the resulting buffer as argument: how to "catch" this resulting buffer to assign it to the proper AudioRessource instance (the one who lauched the process) ? This work that way:
AudioCtx.decodeAudioData(rawData,
function(result){
// do something with result },
function(error){
// do something with error });
My first naive approach, was to think like we were in C/C++ : I simply put an AudioRessource instance function "pointer" (reference) as callback, this way, the AudioRessource instance will directly recieve the buffer:
// where 'this' is an `AudioRessource` instance
AudioCtx.decodeAudioData(rawData,
this._handleDecodeSuccess,
this._handleDecodeError);
However, this does not work, because in javascript, this is not a "function pointer" that is passed into the decodeAudioData, but if I well undstand, an literal expression, that is, the "ASCII content" of the function... So the 'this' reference is lost !
I spent some time to try understand how this kind of asynchronous function is attended to work, since to me, coming from C/C++, this is simply an heresy: The function does not take any extra argument, no way to pass any external reference... "What is that thing ?". Then I finaly decided to try the "Illogical Javascript logic" way... And I found the solution :
// Create local variable which stores reference to 'this'
let thisInstReference = this;
// Use the local variable to write our callback
AudioCtx.decodeAudioData(rawData,
function(resut){
thisInstReference._aBuffer = result;
thisInstReference._loadStatus = 0;
},
function(resut){
thisInstReference._loadStatus = -3;
});
To be honnest, to me, this is simply freaking. First of all, I even don't understand what realy happen: HOW a local variable (to a object instance's member function), that stores a reference to an object instance (this), can be used "as this" in a callback function ? I do not even understand how a language can allow this kind of thing. Secondly, to me, this not a "proper way" to code something: this code is simply illogical, dirty, this works but this appear as an ugly hack that takes advantage of Javascript misdesign.
So here is my question: How to achieve this, in a elegant way ?

Your problem is simply due the the nature of how this works in javascript. The value of this is not bound at compile time nor at runtime but instead very late at call time.
In the following code:
AudioCtx.decodeAudioData(rawData,
this._handleDecodeSuccess,
this._handleDecodeError);
.. the value of this inside _handleDecodeSuccess and _handleDecodeError is not determined at object creation time but instead at the time they are called. And it is the decodeAudioData method that will eventually call them when decoding is complete. This causes the value of this to become something else (depending on how the functions are called).
The modern solution is to statically bind this to the functions:
AudioCtx.decodeAudioData(rawData,
this._handleDecodeSuccess.bind(this),
this._handleDecodeError.bind(this));
Note: the .bind() method creates a new function that wraps your function with this permanently bound to the argument you pass to it.
The traditional solution is to capture this inside a closure like what you have done.

Javascript * is not a function (prototype function)

Coming from a C++ background, trying to work with an OO language that doesn't have explicit typing is a little more than a headache.
So I have dynamic elements for a webpage that are "controlled" by objects since there are tons of stuff I need to manage on each for it to work. The element is just the visual output of the data inside of the object itself, that's all I really need it for.
Except that I need the object to perform an internal function when it's clicked. That seems to be the biggest source of my headache thus far.
Javascript:
function onClick(file) //The external onClick function I use to try to get it to call from.
{
file.state = INUSE;
file.checkState();
}
function fileObject () { //The file object itself
this.element;
this.newElement();
//initialize stuff for the object
}
fileObject.prototype.newElement = function() { //creates a new element and sets its event listener
this.element.click(function() {onClick(this)});
}
fileObject.prototype.checkState = function() {/*does stuff*/} //apparently this is "not a function"
The error I get exactly is "file.checkState is not a function" from Firefox's console panel.
I'm still new to javascript, but after doing some debugging, I've come to find out that it's explicitly the onClick(this) function that is causing all of the errors. When used with something else, the onClick function works perfectly, but for some reason, the this keyword doesn't appear to actually be sending the reference to the fileObject since all checks show file being undefined when inside of the onClick scope.
Is there something fundamentally wrong about the way I'm trying to do this or am I just missing a step (or adding something that I don't need) that will help get this snippet working.

So you know, your initial problem isn't actually handling the action, but listening to it. click will trigger a synthetic click event, rather than liste for one.
You want ... .element.addEventListener("click", callback); that said, you face a second problem, immediately thereafter.
I will leave my example code as you've written it to not confuse the matter...
But when you see click( ) know that I mean subscribing with addEventListener, if element really does mean a browser DOM element. If it's not a standard browser element, and your own API, then ignore the previous portion, and carry on.
this is dynamically bound at the invocation time of the function (not at definition time).
The nearest function, scoped above, is your callback function that you are passing into .click( ... ).
Which is entirely different than the this which you mean outside of the callback.
Whatever is on the left-hand side of the dot is the this context for the duration of that particular invocation.
Needless to say, click() doesn't know enough to bind the this you mean, to the left-hand side of your callback.
The solution (or one of many) is to use lexical scoping and/or closure to retain the value of the object you mean.
// easy but messier
var fileObject = this;
... .click(function () { onClick(fileObject); });
// Cleaner with thunks:
function clickHandler (onClick, obj) {
return function () { onClick(obj); };
}
... .click(clickHandler(this));

Coming from c++ the way Javascript handles this will seem a little crazy, it looks like here you need to tell the function you've defined what this is - like so:
this.element.click(function() {onClick(this)}.bind(this));

How to use JSONP with Object Oriented Javascript

I am new to JSONP and had implemented cross domain functionality for my application and everything is working fine. Now i want to change my javascript code to apply object orientation.
My api is
http://localhost:8080/myApplication/getComments?callback=displayComments
CrossDomain.prototype.displayComments = function(data) {
// code to display the comments
}
Now I am getting an error in firebug given below
ReferenceError: displayComments is not defined
I changed the api to
http://localhost:8080/myApplication/getComments?callback=this.displayComments
and found that the function is appended inline to the callback like this
http://localhost:8080/myApplication/getComments?callback=callback=function (jsonData)
{
//code to display the comments
}
this time another error in firebug
SyntaxError: function statement requires a name
I have a doubt whether to use JSONP in object oriented javascript or not.
Please help.
Thanks in advance.

There's no point in defining the function on the prototype of a function unless you are going to create instances of that function, so start by doing that.
var myCrossDomain = new CrossDomain();
Then you have to call the method on the object, not as a global (it isn't a global, so you can't do that anyway)
var uri = "http://localhost:8080/myApplication/getComments?callback=" +
encodeURIComponent("myCrossDomain.displayComments");
In response to edits and comments:
Yes i am creating an instance of this in another js file
Then reference it as shown above.
I changed the api to
http://localhost:8080/myApplication/getComments?callback=this.displayComments
It's JSON-P. It runs by adding a new script element. Everything gets called in the global context. That is going to call this.displayComments which will be the same as window.displayComments.
If you want to call your method directly, then you need to specify the global variable holding the instance explicitly.
If you don't want to call it directly then you can use the more traditional approach of generating a new, anonymous function which has access to said object through a closure, assigning that function to a global variable (with a unique name) and using that name as your callback argument.
and found that the function is appended inline to the callback like this
http://localhost:8080/myApplication/getComments?callback=callback=function (jsonData)
You haven't shown us the code that creates the URI so we can't tell why that might be the case.

javascript asynchronous calls that populate some property

I have an javascript object that includes some image strings as its property. Upon instantiating this object, an AJAX call is made immediately to populate this property.
My view functions, at the same time, tries to read newly instantiated instances and tries to display this object. As expected, the AJAX call may not have finished by then yet, so it will not be able to read the right image url.
One solution to this problem is to pass in some call back function with the AJAX call that modifies the source of image upon completion, but this is introducing a lot of dependency on the model and the view and I'm trying to keep my MVC as separate as possible, so my view function right now just takes that object as an parameter, reads all the properties, and shows it.
Making the AJAX synchronous isn't an option because I have quite a lot of these objects being initialized and only one will be displayed at a time, so making all AJAX calls synchronous will be too expensive of a tradeoff.
What's a good way to solve this problem? The view function is something in the form of:
function displayObj(object) {
var prop1 = object.getProp1();
// this could be the image file that depends on AJAX result
var prop2 = object.getProp2();
}
Ideally, I would like to delegate this task to the getter, so that the view function doesn't have to worry about the interval state of the the object. The getter could handle this by checking whether the image is there yet, if not, wait, then only return if the actual image string is there. However, a blocking wait would for sure block the AJAX process so it's a deadlock, and non-blocking wait will let the getter return and the view function will get null data.
Please shed some light on this situation or suggest alternative ways that I can organize my code. Thank you very much!

I'd be a little bit less strict with the MVC roles and let the model object return the DOM <img> object.
// model object starts loading upon instantiation
var loader = new Loader();
// however the img tag is available immediately.
// the loader will update the src when ajax
// calls complete.
document.body.appendChild(loader.getImg());

Since, from your comments, you are using jQuery I recommend you call the done() method of the request returned from the ajax() method. For example,
var request = $.ajax("example.php");
request.done(notifyWhenSuccess);
request.fail(notifyWhenFailed);
This request returned by jQuery is a superset of the Promise pattern from CommonJS. Also, just because it is in jQuery don't cosider the ajax call part of the view but rather view it as part of the model. Use the done() method to update the model then notify the view with a data changed event.