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leonard.l..

JSONP and randomly generated callback function

in Using jQuery • 10 years ago

Hello, I've several doubt about the JSONP usage. I'm using JQuery to make JSONP requests and the documentation is quite confusing to me. I hope my question is not too long.

In the specs it's written a JSONP call is always async. Is it a jQuery choice or the nature of JSONP?

async:false would be simply ignored?

If the jsonpCallback parameter is specified, this function will be executed when the data are retrieved. But right after, also the success callback will be executed. Can two callbacks co-exists? Is it the best usage pattern or should I just remove the success in this case?
If the jsonpCallback is not specified a random callback function will be created and attached to the window object. Something like jQuery1360574548776335413_1776656584447. How does it work? Does it call the success callback passing to it its return data?
Is the success the only available helper? The error and complete aren't available?

Here's my code:

(function($) {
var url = "https://www.googleapis.com/books/v1/volumes/zyTCAlFPjgYC";
$.ajax({
type: 'GET',
url: url,
// JSONP always async?
async: false,
jsonp: "callback",
jsonpCallback: 'jsonCallback',
contentType: "application/json",
dataType: 'jsonp',
//success always called even when jsonpCallback is specified?
success: function(json) {
console.dir(json);
},
// Error never called?
error: function(e) {
console.log(e.message);
}
});
})(jQuery);
function jsonCallback(json) {
$(".test").html(json.volumeInfo.title);
}

Thanks

Replies(5)

jakecigar

Re: JSONP and randomly generated callback function

10 years ago

1. yes

2. using jsonpCallback can be a pain. Save it for advanced usage.

3. jQuery makes up the name, sends it to the request, and the request uses it. jQuery doesn't really use it again.

4. yes.

try

(function($) {
var url = "https://www.googleapis.com/books/v1/volumes/zyTCAlFPjgYC?callback=?";
$.ajax({
type: 'GET',
url: url,
dataType: 'json',
success: function(json) {
console.dir(json);
$(".test").html(json.volumeInfo.title);
},
// Error never called!!!!
error: function(e) {
console.log(e.message);
}
});
})(jQuery);

JΛ̊KE

leonard.l..

Re: Re: JSONP and randomly generated callback function

10 years ago

Thank you jake.

Where can I read more about the point 2?

About your answer of the point 3: "jQuery makes up the name, sends it to the request, and the request uses it. jQuery doesn't really use it again". I'm a bit confused:

-- the request uses it? If for example the functions is called jQuery136057454877633 it means the response will be jQuery136057454877633({JSON_DATA}). Then what happens? It just disappears, won't be called?

-- Who is calling the success function? How my JSON_DATA are passed to it? Should I care about the jQuery136057454877633 random generated function or just ignore it and use the success?

-- How do I handle the errors if the error helper isn't available?

My questions are focused not only in make things work but also HOW things work behind. I found this part of documentation a little bit tricky.

Thanks again

jakecigar

Re: JSONP and randomly generated callback function

10 years ago

First thing is JSONP is NOT AJAX. It is a dynamically generated script.

I could do that JSONP without jQuery,if I wanted to run it as soon as possible. And didn't care that it would freeze the browser for a moment.

<script>
function callMe(data){
console.log(data)
}
</script>
<script src="https://www.googleapis.com/books/v1/volumes/zyTCAlFPjgYC?callback=callMe"></script>

jQuery does all the nasty parts for you. It saves off your success callback as the name it made up. jQuery also cleans up after itself.

You can catch the error(fail) like this: http://jsfiddle.net/jakecigar/DEfbL/28/

JΛ̊KE

leonard.l..

Re: Re: JSONP and randomly generated callback function

10 years ago

First of all, thank you again Jake, you have been very helpful. I will try to explain what I have just learnt to other people on my blog.

I know JSONP is not ajax that is why I've asked confirmation on the async always true. If I wanted I could make a sync call with the script tag, so why jQuery force me to make only async JSONP calls? Is it because jQuery has to make the request when the page is loaded, and adding the script tag after having loaded the page is actually in result an async anyway? (just a guess)

Also my english is not perfect so when you say "It saves off your success callback as the name it made up. " means that success is just an alias of the random_whatever_generate_function or it is more complicated than that?

Thanks a lot, really!

jakecigar

Re: JSONP and randomly generated callback function

10 years ago

jQuery.ajax() | jQuery API Documentation defines

jsonpCallback
Type: String or Function()
Specify the callback function name for a JSONP request. This value will be used instead of the random name automatically generated by jQuery. It is preferable to let jQuery generate a unique name as it'll make it easier to manage the requests and provide callbacks and error handling. You may want to specify the callback when you want to enable better browser caching of GET requests. As of jQuery 1.5, you can also use a function for this setting, in which case the value of jsonpCallback is set to the return value of that function.

JΛ̊KE

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