this-doesnt-work.js

const window = {
    a: 1
};

global = new Proxy(global, {
    get(target, prop, receiver) {
        if (target[prop] !== undefined) {
            return target[prop];
        }
        return window[prop];
    }
});

// Will be caught by proxy
console.log(global.a);

// Will not be caught by proxy. I'm _guessing_ the
// [[Get]] slot isn't being used when resolving against the
// global environment record?
console.log(a);

this-works.js

// This works as intended. Referencing `a` in the top-level
// calls the getter on the `global` object

Object.defineProperty(global, 'a', {
    get() {
        return 'this works';
    }
});

console.log(a);