Strange instanceof

In the code below, why does instanceof return true? We can easily see that a is not created by B().

function A() {}
function B() {}

A.prototype = B.prototype = {};

let a = new A();

alert( a instanceof B ); // true

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.