Question

The interface through which browser JavaScript can make HTTP requests is called XMLHttpRequest (note the inconsistent capitalization). It was designed by Microsoft, for its Internet Explorer browser, in the late 1990s. During this time, the XML file format was very popular in the world of business software—a world where Microsoft has always been at home. In fact, it was so popular that the acronym XML was tacked onto the front of the name of an interface for HTTP, which is in no way tied to XML.

The name isn’t completely nonsensical, though. The interface allows you to parse response documents as XML if you want. Conflating two distinct concepts (making a request and parsing the response) into a single thing is terrible design, of course, but so it goes.

When the XMLHttpRequest interface was added to Internet Explorer, it allowed people to do things with JavaScript that had been very hard before. For example, websites started showing lists of suggestions when the user was typing something into a text field. The script would send the text to the server over HTTP as the user typed. The server, which had some database of possible inputs, would match the database entries against the partial input and send back possible completions to show the user. This was considered spectacular—people were used to waiting for a full page reload for every interaction with a website.

The other significant browser at that time, Mozilla (later Firefox), did not want to be left behind. To allow people to do similarly neat things in its browser, Mozilla copied the interface, including the bogus name. The next generation of browsers followed this example, and today XMLHttpRequest is a de facto standard interface.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.