en.javascript.info/1-js/07-object-oriented-programming/11-instanceof/1-strange-instanceof/solution.md

Yeah, looks strange indeed.

But instanceof does not care about the function, but rather about its prototype, that it matches against the prototype chain.

And here a.__proto__ == B.prototype, so instanceof returns true.

So, by the logic of instanceof, the prototype actually defines the type, not the constructor function.