ES6 (ES2015) and Beyond: Understanding JavaScript Versioning

As programming languages go, JavaScript’s development has been positively frantic in the last few years. With each year now seeing a new release of the ECMAScript specification, it’s easy to get confused about JavaScript versioning, which version supports what, and how you can future-proof your code.
To better understand the how and why behind this seemingly constant stream of new features, let’s take a brief look at the history of the JavaScript and JavaScript versioning, and find out why the standardization process is so important.
The Early History of JavaScript Versioning
The prototype of JavaScript was written in just ten days in May 1995 by Brendan Eich. He was initially recruited to implement a Scheme runtime for Netscape Navigator, but the management team pushed for a C-style language that would complement the then recently released Java.
JavaScript made its debut in version 2 of Netscape Navigator in December 1995. The following year, Microsoft reverse-engineered JavaScript to create their own version, calling it JScript. JScript shipped with version 3 of the Internet Explorer browser, and was almost identical to JavaScript — even including all the same bugs and quirks — but it did have some extra Internet Explorer-only features.
The Birth of ECMAScript
The necessity of ensuring that JScript (and any other variants) remained compatible with JavaScript motivated Netscape and Sun Microsystems to standardize the language. They did this with the help of the European Computer Manufacturers Association, who would host the standard. The standardized language was called ECMAScript to avoid infringing on Sun’s Java trademark — a move that caused a fair deal of confusion. Eventually ECMAScript was used to refer to the specification, and JavaScript was (and still is) used to refer to the language itself.
The working group in charge of JavaScript versioning and maintaining ECMAScript is known as Technical Committee 39, or TC39. It’s made up of representatives from all the major browser vendors such as Apple, Google, Microsoft and Mozilla, as well as invited experts and delegates from other companies with an interest in the development of the Web. They have regular meetings to decide on how the language will develop.
When JavaScript was standardized by TC39 in 1997, the specification was known as ECMAScript version 1. Subsequent versions of ECMAScript were initially released on an annual basis, but ultimately became sporadic due to the lack of consensus and the unmanageably large feature set surrounding ECMAScript 4. This version was thus terminated and downsized into 3.1, but wasn’t finalized under that moniker, instead eventually evolving into ECMAScript 5. This was released in December 2009, 10 years after ECMAScript 3, and introduced a JSON serialization API, Function.prototype.bind, and strict mode, amongst other capabilities. A maintenance release to clarify some of the ambiguity of the latest iteration, 5.1, was released two years later.

Do you want to dive deeper into the history of JavaScript? Then check out chapter one of JavaScript: Novice to Ninja, 2nd Edition.

ECMAScript 2015 and the Resurgence of Yearly Releases
With the resolution of TC39’s disagreement resulting from ECMAScript 4, Brendan Eich stressed the need for nearer-term, smaller releases. The first of these new specifications was ES2015 (originally named ECMAScript 6, or ES6). This edition was a large but necessary foundation to support the future, annual JavaScript versioning. It includes many features that are well-loved by many developers today, such as:

Classes
Promises
Arrow functions
ES Modules
Generators and Iterators

ES2015 was the first offering to follow the TC39 process, a proposal-based model for discussing and adopting elements.
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