ES2015 changes the semantics of Arrays and certain other classes to create subclass instances from certain methods, and allow a new interception point, Symbol.species, to control whether it's actually the subclass or another class which is instantiated. For example, Array.prototype.map constructs instances of the subclass as its output, with the option to customize this by changing Symbol.species
Docs: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/species