Question

For a project of mine I need to do multiple calls to a (remote) API using JSONP for processing the API response. All calls use the same callback function. All the calls are generated dynamically on the client’s side using JavaScript.

The problem is as follows: How do I pass additional parameters to that callback function in order to tell the function about the request parameters I used. So, e.g., in the following example, I need the myCallback function to know about id=123.

<script src="http://remote.host.com/api?id=123&jsonp=myCallback"></script>

Is there any way to achieve this without having to create a separate callback function for each of my calls? A vanilla JavaScript solution is preferred.

EDIT:

After the first comments and answers the following points came up:

• I do not have any control over the remote server. So adding the parameter to the response is not an option.
• I fire up multiple request concurrently, so any variable to store my parameters does not solve the problem.
• I know, that I can create multiple callbacks on the fly and assign them. But the question is, whether I can avoid this somehow. This would be my fallback plan, if no other solutions pop up.

Answer 1

Your options are as follows:

1. Have the server put the ID into the response. This is the cleanest, but often you cannot change the server code.
2. If you can guarantee that there is never more than one JSONP call involving the ID inflight at once, then you can just stuff the ID value into a global variable and when the callback is called, fetch the id value from the global variable. This is simple, but brittle because if there are every more than one JSONP call involving the ID in process at the same time, they will step on each other and something will not work.
3. Generate a unique function name for each JSONP call and use a function closure associated with that function name to connect the id to the callback.

Here’s an example of the third option.

You can use a closure to keep track of the variable for you, but since you can have multiple JSON calls in flight at the same time, you have to use a dynamically generated globally accessible function name that is unique for each successive JSONP call. It can work like this:

Suppose your function that generate the tag for the JSONP is something like this (you substitute whatever you’re using now):

function doJSONP(url, callbackFuncName) {
   var fullURL = url + "&" + callbackFuncName;
   // generate the script tag here
}

Then, you could have another function outside of it that does this:

// global var
var jsonpCallbacks = {cntr: 0};


function getDataForId(url, id, fn) {
    // create a globally unique function name
    var name = "fn" + jsonpCallbacks.cntr++;

    // put that function in a globally accessible place for JSONP to call
    jsonpCallbacks[name] = function() {
        // upon success, remove the name
        delete jsonpCallbacks[name];
        // now call the desired callback internally and pass it the id
        var args = Array.prototype.slice.call(arguments);
        args.unshift(id);
        fn.apply(this, args);
    }

    doJSONP(url, "jsonpCallbacks." + name);
}

Your main code would call getDataForId() and the callback passed to it would be passed the id value like this followed by whatever other arguments the JSONP had on the function:

getDataForId(123, "http://remote.host.com/api?id=123", function(id, /* other args here*/) {
    // you can process the returned data here with id available as the argument
});