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Asynchronous JavaScript and XML (Ajax), a browser feature accessible in JavaScript, allows a script to make an HTTP request to a website behind the scenes, without the need for a page reload.

Ajax has been around more than a decade. Though the name includes XML, you can transfer nearly anything in an Ajax request. The most commonly used data is JSON, which is close to JavaScript syntax and consumes less bandwidth.

Browser's use a policy of same-origin. That is ajax requests can only be made to the same server from where http request originated.

To overcome this a technique known as JSONP came up.

JSONP stands for JSON with Padding. (very poorly named technique)

Instead of using XMLHttpRequest we have to use script HTML tags, the ones you usually use to load JS files, in order for JS to get data from another domain.

script = document.createElement("script");
script.type = "text/javascript";
script.src = "";

Notice my_callback function over there? So - when JSONP server receives your request and finds callback parameter - instead of returning plain JS array it'll return this:

my_callback({['some string 1', 'some data', 'whatever data']});
See where the profit is: now we get automatic callback (my_callback) that'll be triggered once we get the data.

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