what is API

• JavaScript — A high-level scripting language built into browsers that allows you to implement functionality on web pages/apps. Note that JavaScript is also available in other programming environments, such as Node.
• Browser APIs — constructs built into the browser that sit on top of the JavaScript language and allow you to implement functionality more easily.
• Third party APIs — constructs built into third-party platforms (e.g. Twitter, Facebook) that allow you to use some of those platform's functionality in your own web pages (for example, display your latest Tweets on your web page).
• JavaScript libraries — Usually one or more JavaScript files containing custom functions that you can attach to your web page to speed up or enable writing common functionality. Examples include jQuery, Mootools and React.
• JavaScript frameworks — The next step up from libraries, JavaScript frameworks (e.g. Angular and Ember) tend to be packages of HTML, CSS, JavaScript, and other technologies that you install and then use to write an entire web application from scratch. The key difference between a library and a framework is “Inversion of Control”. When calling a method from a library, the developer is in control. With a framework, the control is inverted: the framework calls the developer's code.