Going functional with JavaScript

JS has evolved through the years, and the version we'll be using is (informally) called JS8, and (formally) ECMAScript 2017, usually shortened to ES2017 or ES8; this version was finalized in June 2017. The previous versions were:

• ECMAScript 1, June 1997
• ECMAScript 2, June 1998, basically the same as the previous version
• ECMAScript 3, December 1999, with several new functionalities
• ECMAScript 5 appeared only in December 2009 (and no, there never was an ECMAScript 4, because it was abandoned)
• ECMAScript 5.1 was out in June 2011
• ECMAScript 6 (or ES6; later renamed ES2015) in June 2015
• ECMAScript 7 (also ES7, or ES2016) was finalized in June 2016
• ECMAScript 8 (ES8 or ES2017) was finalized in June 2017

ECMA originally stood for European Computer Manufacturers Association, but nowadays the name isn't considered an acronym anymore. The organization is responsible for more standards other than JS, including JSON, C#, Dart, and others. See its site at www.ecma-international.org/.

You can read the standard language specification at www.ecma-international.org/ecma-262/7.0/. Whenever we refer to JS in the text without further specification, ES8 (ES2017) is meant. However, in terms of the language features that are used in the book, if your were just to use ES2015, you'd have no problems with this book. 

No browsers fully implement ES8; most provide an older version, JavaScript 5 (from 2009), with a (always growing) smattering of ES6, ES7, and ES8 features. This will prove to be a problem, but fortunately, a solvable one; we'll get to this shortly, and we'll be using ES8 throughout the book. 

In fact, there are only a little differences between ES2016 and ES2015, such as the Array.prototype.includes method and the exponentiation operator **. There are more differences between ES2017 and ES2016 – such as async and await, some string padding functions, and more – but they won't impact our code.