Develop Reference

can AJAX do anything else than load a JSON file?

I'm want to (or think I need to) use AJAX to accomplish what I intend.
When clicking on a specific link in a list of links, I want to fill the HTML markup below with content of specific subpages. The data is naturally somewhere in the database and actually easily accessible with the CMS's API (I'm using Processwire).
I'm quite new to coding and especially AJAX and all documentation I find online only mention it in combination with a JSON file that would be loaded via AJAX.
However, I don't have a JSON file on the server, that means, according to my understanding, I would need to
store the data I need in a multidimensional php array,
use json_decode to create and then save that JSON-file on the server,
load that file via AJAX and process through more JS.
Let alone keep that JSON-file updated (or create a new one and delete the old one?) since new content will arrive periodically. It seems unnecessarily complicated to me, but what do I know.
There's got to be a better way…
Any help is appreciated.

AJAX is simply a way to make a request to the web server for information.
When you make an AJAX request you ask for a response from a file on a server. So, you can send an AJAX request to a PHP script for-instance.
The PHP script could return anything, JSON is common and very widely used response format, but XML might be another one you've encountered.
So, your request for information is made using AJAX, and the response you get back is JSON.
You don't need to store a JSON file on your server. You just need to make an AJAX request that returns current data in JSON format.

AJAX allows you to do asynchronous HTTP requests.
You can of course ask for a json file, but you can also (for example) call an API.
I suggest you start by reading the the getting started guide for AJAX in MDN:
https://developer.mozilla.org/en-US/docs/Web/Guide/AJAX/Getting_Started

How to retrieve info from database to display with Chrome extension

I am trying to write my first chrome extension. The workflow goes something like this -When the extension is installed and active if a user hovers over a specific product/ID displayed on the page, the extension retrieves related vendor data about the product with the ID.
This is how I thought about this:
Use jQuery attr to access the ID on mouse over.
Post this ID to a retrieve.php file with .post() method
The retrieve.php file retrieves the data from database
Display the data in a tool tip on the web page.
I have some queries for the above process:
I am able to get this working on a local XAMPP server but how will it work online as the chrome extension will not have access to server. What is the way around to retrieve data without using PHP?
I am able to get the logic working but am unable to place these in respective files - Will all my logic reside in background.js ?
Any suggestions on getting this started will be much appreciated.

You could build a very simple API on your server that responds with JSON to any request it receives after processing it. Like this:
{"firstVar":"foo","secondVar":"bar" }
Your chrome extension can then make an xmlhttp request to this server and and process the returned data.(You could also use JSONP and wrap the response in a callback function which will execute as soon as you have the reponse)
The JS extension will be able to deal with the JSON nicely as it can understand that format so you can then choose to display the data in whatever way you want.
Essentially, what you want is a server that can take an ID posted to it and return the corresponding date in a nice and readable format. And a chrome extension that can make an request to a server and then process the response. Build and test them separately (keep positing an ID to the server and see the response and for your JS side at first instead of making requests to your unfinished API just set a static response to begin with which will be the same as an expected response.

Insert data into DB with Symfony2 via JavaScript, without a form

I'm currently working on a Symfony2 project where a user can comment on parts of a webpage via contenteditable spans. I now want to save these comments, that are assembled in an array of JSON objects, into the database, with a reference to the page and the user id. This is where I'm stuck.
What steps are needed to put the data from the JavaScript code into the specific DB table?
I already created a Comment class in the Entity folder. And based on this
answer I added the following code in my saveComment() in JavaScript:
$.post('saveComments.php', {
page_id : getPageId(),
user_id : getUserId(),
comments : getJSONcomments(),
});
What next...?
Ok, let's create a saveComments.php ... but where in the bundle? Or could this be done with a Controller? If so, how and what would I need to replace the url ("saveComments.php") in the $.post(...) call with?

Don't post to a PHP page as this breaks the Symfony 2 model. Instead post to a route, like save/comment or along these lines.
Configure the route to point to an action in a controller.
Implement the controller and the action so that it does not take any parameters. Inside the action unpack the posted data the usual PHP way using (because you won't have a form to bind your data to):
$_POST
Just to get it working, echo a var_dump() to the console and see what you get. This is an example on how to write to the console.
Decode the JSON data with the serializer.
The decoded data will be a simple associative PHP array. Interpret the data you received and act accordingly (don't forget to handle security and all that stuff, too -- you don't want to open up security holes through AJAX).
Best is, you check that the route you chose falls into the security tier you need and probably already have configured in the Symfony 2 application. This way you don't need to handle security manually in the action.
Once this is done return an HTTP response with code 200 OK.
return new Response('', Response::HTTP_OK); // no response text at all
If there's any error reply with a 500 class HTTP server error:
return new Response('', Response::HTTP_INTERNAL_SERVER_ERROR); // 500
Also, don't forget to handle client-side errors with the .error() method.

Read/write json from js to file on server (not server app)

I'm working with a .js client and have and object that I need to write out to a file on the server. A couple of questions - file i/o with JavaScript is new to me... I was planning on using jquery and json. I'm using java serverside. I don't have a problem reading what I get back from my servlet, but the file i/o is killing me! A couple of questions:
I can open a file I generated myself via the .js with an $.ajax call, but it's not handling my json syntax (I tried both an $.getJson and $.ajax - handwritten json, so I might (probably) are doing something wrong with it). I used firebug's console and it looks ok...
How can I write my object to a file on the server?
Then, when I want to read it, what do I need to do to process it? Right now I'm using a jsonFilter function (uses JSON.parse if that's available, otherwise eval) to process data that I'm getting from the servlet.
The object I'm writing isn't simple, but it's not super complex either. There's an array that contains an array, but that shouldn't make a difference if the software is both reading/writing it.
Thanks for any help! I'm at a loss - tried alot of different things.

You can open a file located on the server via ajax by querying the file and loading it into a JSON object. You might want to LINT your JSON
You can not write to an object on the server via the client. This is a severe security breach.
Common practice is to change the JSON data and then send it via ajax to server-side code. The server will then do the file IO.
Yes using JSON.parse otherwise eval is indeed correct. I would recommend json2.js
The data should be fine as long as it passes JSONLint.
Your main issue is that it's impossible to write to the server from the client. Get the client to load the data through ajax change it and then query the server to update the file.

js don't have i/o property;
you should use ajax or http request to send message to server,and tell server to do de i/o action...

Ajax /jQuery finding if user completed the download

Here is what I am trying to do: I am making a custom text file containing a test. This test is unique to the user and well I don't want my server to stack all those text files.
Is there a way to use Ajax/JavaScript/jQuery to find if the user has already finished the download and if they have get a return value (1 if finished) so the response can be sent back to the PHP file and it can delete that file off the server (real-time like)?
I know there are plenty of ways to do this using PHP. Sort of like run clean up upon user log out and so on but I wanted to try using the method above since it can have many other applications that might be cool to use. I tried most search engines but they have nothing close to what I need.

Why do you need to store them in a file? Just use a PHP script or such that creates the test and outputs it directly to the user. That's how it will get deleted when the download is complete.

If it's important you may want the user to return to your server with the hash of the downloaded file. If the hash matches you know two things:
1. The user downloaded the file successfully
2. It's now ok to delete the file

Well it is very simple. I don't know how to make a PHP webpage send itself to the user other than to make the PHP make a text file and force send that to the user. This creates the problem of having so many text files in a temporary folder.
Now if the test required say 15 chapters each having a text or HTML format file then the script neatly zips all those files and sends them to the user. Again falling on the same problem if the user is finished downloading I am trying to get any type of script to delete the temporary zip or text file out of the temporary directory in somewhat real time.
If I could MD5 a downloaded file using JavaScript I welcome it and it would be a hack solution to the problem but how will the JavaScript gain access to the root access of the download folder of the user? There are security issues there if I am not mistaken. Hope this helps round the question a bit more.

I have a good solution for you here using the jQuery File Download plugin I created. It allows for you to get the behavior of performing an Ajax file download (not actually possible possible) complete with Success and Failure callbacks. In a nutshell you can just use the Success callback (that indicates the file download was successful) to perform an Ajax post back to the server to delete the file. Take a look at the blog post for an example on how to use the Success callback option or a demo which uses those callbacks in the context of showing modals to inform the user of what is going on.