History and Versions

In short: JavaScript is the language; ECMAScript is its official rulebook, kept by a committee called TC39. Brendan Eich wrote the first version in days in 1995; Ecma International standardised it in 1997. The biggest update, ES6 / ES2015, added let, const, arrow functions, classes and more. Since then a smaller edition ships every year, named by year. Editions only add features — they never remove what came before, so old code keeps running in new engines.

Naming a variable the old way and the new way

"JavaScript" is what everyone writes day to day; "ECMAScript" is what browsers and Node.js actually implement. var has worked since 1995 and still works today, alongside the const that arrived twenty years later in ES2015 — nothing broke when const was added:

Both lines run cleanly in any current engine. This is the "editions only add" rule in action: a 1995 keyword and a 2015 keyword sit side by side without conflict.

Converting an edition number to its year

Since ES2015 the numbered and year names line up by a fixed gap: year = edition number + 2009. So ES6 is ES2015, and ES8 is ES2017:

Knowing the gap tells you what a browser from a given year should understand. A browser released well after 2015 accepts const and arrow functions; one from 2012 rejects them with a syntax error, because they simply did not exist yet.

The trap: thinking an edition replaces the last one

New editions only add. They never delete a feature or force old code to change. That is why a codebase can mix var from 1995 with const from 2015 with syntax from last year, all in the same file, and every line still runs.

Quick practice

🎯 Your turn

A friend's code uses const and arrow functions. It works in their new browser but fails in a very old one. Explain why, using the names JavaScript, ECMAScript and ES2015.

Write your explanation and submit it — there's no code to run here, just your reasoning.