ECMA Spec

JS is built according to the ECMAScript standard.

An update to the standard describing new features is published every year, then JavaScript engine projects (browser and runtime manufacturers) put them in place.

Adoption of APIs Is Not Universal

This statement may seem scary, but the vast majority of JavaScript APIs work in the most common browser platforms. Still, implementers don’t adopt every language feature or API at the same rate. Some never adopt certain features (although this is rare). In general if you want to use a newer feature of the language, you should understand how well it will work in your target browser(s) using a resource like caniuse.com. 

If a new feature isn’t implemented natively, there’s often a bit of code written to temporarily serve the purpose of that missing feature. This temporary fill-in code is called a polyfill.