JavaScript (sometimes abbreviated JS) is a scripting language commonly implemented as part of a web browser in order to create enhanced user interfaces and dynamic websites, but also used in other contexts (such as server-side JavaScript).
This is not the same as Java. People constantly get confused about that.
JavaScript was originally developed at Netscape by Brendan Eich (who later became Mozilla CEO... very briefly), where it was originally called LiveScript while under development, but became JavaScript (with the name licensed from Sun) in order to capitalize on the popularity of Java, though the languages aren't really related (although there are some similarities in syntax). Later, a Microsoft implementation designed to be (more or less) compatible was called JScript, and an attempt at a formally standardized version of the language was published by ECMA as ECMAScript.
The node.js runtime environment is (mostly) implemented in JavaScript, as are applications running within it.
Due to its status as the only scripting language for browsers, JavaScript skyrocketed in popularity in the 2000's and 2010's and as of 2017 serves as a lingua franca of Web development. Arguably, it is currently the world's most popular programming language (it tops GitHub and StackOverflow currently).
File types
.js