@@can.proto

The @@can.proto symbol is placed on an object to reference its prototype. Since the non-standard __proto__ is deprecated, this can serve as a prototype chain reference without invoking Object.getPrototypeOf() every time.

function Foo() {}
Foo.prototype = {
    bar: "baz"
};

var obj = new Foo();
obj[canSymbol.for("can.proto")] = Foo.prototype;