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Send JSONP Cross-domain Requests

16 Dec 2013

Cross-domain Requests

In developing web applications it’s commonplace to send and receive data to APIs (Application Programmable Interfaces) that exist on other domains. This cross-domain communication is often performed directly in the browser, on the client-side using JavaScript. Cross-domain communication can be accomplished using a server-side languages like Python, Nodejs, PHP, etc., however for cases where responses from cross-domain requests are utilized in the browser, JavaScript can save time and simplify development. Addintionaly, JavaScript requests are asynchronous— meaning they can be run in parallel with other processes. For example, JavaScript requests to other domains can be processed asynchronously while the page is still loading.

Same-origin Policy

Same-origin policy is an important security concept in client-side programming languages including JavaScript. This policy is observed by all web browsers and limits scripts running on a site to only sending requests with the same site. Browers use a combination of port number (e.g. 80) and hostname (e.g. my.domain.com) to determine the scope of the ‘site’. There are however exceptions to the same-origin policy, including using the HTML <script> tag to retreive a dynamically generated JavaScript file from an external site. The most prominent of method of exploiting this exception is JSONP.

JSON with Padding (JSONP)

JSONP uses the <script> tag to retreive remote JavaScript code containing formatted JSON data with a function call wrapped around it. The data is not parsed, rather it is evaluated by the JavaScript interpreter.

A typical JSONP request may look like:

<script type="application/javascript"
  src="http://remote.domain.com/data/?callback=jsonpcallback&id=123">
</script>

The dynamically generated JavaScript file may look like:

jsonpcallback({
  "id": "123",
  "comments": "6",
  "name": "sample"
});

The “padding” in JSONP refers to the function call that wraps the formatted JSON object. This padding isn’t always a function call. Though not common, padding could also be a variable assignment:

var data = {
  "id": "123",
  "comments": "6",
  "name": "sample"
};

The purpose of the padding is to allow a means for the formatted data to be put to use in the application.

jQuery for JSONP

The jQuery JavaScript library has functions to make using JSONP a snap.jQuery’s $.ajax() function will automatically handle <script> tag injection and response handling (i.e. calling the ‘callback’ function).

$.ajax({
  //JSONP API
  url: "http://remote.domain.com/data/?callback=jsonpcallback&id=123",
  //the name of the callback function
  jsonp: "jsonpcallback",
  //tell jQuery to expect JSONP
  dataType: "jsonp",
  //tell YQL what we want and that we want JSON
  data: {
    id: "123"
  },
  //work with the response
  success: function(data) {
    console.log(data); //formatted JSON data
  }
});

console.log(data);

{ "id": "123", "comments": "6", "name": "sample" }

Alternatively, if jQuery is not available- which is often the case when developing embeddable widgets- raw JavaScript can be used. The following is minimal JSONP implementation:

//callback function
function jsonpcallback(data) {
  //do stuff with JSON
  console.log(data);
}

//create HTML script tag
var script = document.createElement('script');
script.src = "http://remote.domain.com/data/?callback=jsonpcallback&id=123"

//inject script tag into head
document.getElementsByTagName('head')[0].appendChild(script);

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