JavaScript was first released in 1995 and confirms to ECMAScript (ES) specification.
Here is the timeline of JavaScript.
1995 - First released
2009 - ES5 released
2015 - ES6 released
2016 - ES7 released
2017 - ES8 released
2018 - ES9 released
2019 - ES10 released
ES6 and onward specifications provide many new features but some browsers will not support new features.
In such scenario, we use transpiling and polyfill to write code in newer version and then
that code is converted into older specification.
Each browser has a javascript engine that is used to execute the JavaScript code.