A couple of weeks ago, I got a great question from a reader of Head First HTML5 Programming:

What is a API? Is it like a constructor function or is it just an object or something else?

We use the term “API” frequently in Head First HTML5 Programming because the book is primarily about the JavaScript APIs you use to program web pages. So, what exactly is an API?

An API is an Application Programming Interface. If you write some software that can be used by another piece of software, the API is what specifies how to do it.

When you use the built-in JavaScript functions, you’re using an API, because you are using functions running in the browser (a program) that are designed to be used by other programs (your JavaScript program). We often call collections of functions that work together to achieve some purpose–like Geolocation–an API, as in “The Geolocation API”. Typically, there is some documentation somewhere (in this case, the HTML5 spec) that tells you *how* to use those Geolocation functions.

The APIs we cover in Head First HTML5 Programming include Geolocation, Web Workers, Web Storage, Canvas and Video. We also talk about using JSON with the Twitter REST API, which is an API that’s designed to be accessed via URLs. All these APIs are now part of JavaScript; each is just a group of functions that allows you to access particular functionality in the browser.

Thanks for a great question!

We wrote the book: with over a 1,000,000 books sold, we’ve created some of the industries top-selling software & technology titles. We teach the experts: you find us teaching on today’s premier training sites, like Lynda.com and Safaribook’s Online, and we’ve created digital experiences for a small, but exceptional group of industry leaders and luminaries. Throughout it all, we’ve applied our wickedly smart philosophy to get you there faster with amazing results. How can we help you?

We wrote the book: with over a 1,000,000 books sold, we’ve created some of the industries top-selling software & technology titles. We teach the experts: you find us teaching on today’s premier training sites, like Lynda.com and Safaribook’s Online, and we’ve created digital experiences for a small, but exceptional group of industry leaders and luminaries. Throughout it all, we’ve applied our wickedly smart philosophy to get you there faster with amazing results. How can we help you?

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