That's the key point

As of ECMAScript 2015 (ECMA-262 6th Edition / June 2015), Maps (the spec and MDN docs) can be used to store objects —or any other "arbitrary ECMAScript language values"— as keys.

Array.filter() is therefore pure.

React ES5 to ES6

So what's the difference between bar and quux?

In summary:

1. Class decorators are functions that wrap a constructor function
2. Property decorators are functions that take a prototype and property name as input and can modify the property
3. Method decorators are functions that take a prototype, method name and method descriptor as input and return an updated descriptor