What PhiLho said is correct but if you're using the Processing IDE, it isn't the whole story.  The Processing IDE's Javascript Mode has very specific limitations on how you can mix the Processing API with Javascript core (don't worry, there's good news at the end!).

If you are in a PDE tab and using the Processing API, the only Javascript code you can mix in is simple variable declarations such as:

1. var myVar = 6; 

and functions that do not have any variables passed into them:

1. function someFunction(){
2.       //some function that
3.       //you cannot pass in variables into
4. };

You can however, create a new tab with the name myTab.js.  This tab will have access to all of Javascript, but it will not have access to any of the processing API.  As long as you keep them separate, you can use variables, functions, objects that you create in myTab.js in sketch.pde like so:

In a tab labeled myJavascript.js (note need for .js suffix):

1. var employees = [
2. 
   
3. { "firstName":"John" , "lastName":"Doe" }, 
4. { "firstName":"Anna" , "lastName":"Smith" }, 
5. { "firstName":"Peter" , "lastName": "Jones" }
6. 
   
7. ];
8. 
   
9. var myJSONObj = JSON.parse(employees);
10. 
    
11. function addition(x, y){
12.   return x + y;
13. }

Then in a tab called myPDE (no need to add suffix as it's presumed)

i can however tell you what you'll need to learn about and that is AJAX and XMLHttpRequest.  The two go together so searching for an AJAX tutorial will include the XMLHttpRequest.  These two things are so fundamental to web programming that there will be no shortage of tutorials. the other thing to be on the look out for is understanding the asynchronous nature of javascript.  when you make an AJAX request the rest of your code will continue to execute even though the request isn't finished it.  therefore, you'll need to learn a little bit about call back functions as well.

good times, eh?

btw.  you can also use loadStrings() to get json but in PJS you won't be able to put that into a JSONObject so that you can access it's elements very easily.

i'll keep tabs on this thread for sure, hopefully i can come up with something because i'd love to know to!

no problem.  glad i was able to point you in the right direction.  It's kind of a non-answer answer but sometimes those are helpful when you don't know what you don't know yet.