wm.js

let doFoo = new WeakMap(),
    doBar = new WeakMap();

class Foo {
    constructor () {
        doFoo.set(this, () => {
            // definition of doFoo for the instance of Foo.
        });

        doBar.set(this, () => {
            // definition of doBar for the instance of Foo.
        });
    }

    publicMethodThatUsesFoo () {
        doFoo.get(this)(); // call doFoo
    }
}