Definitely Maybe with Proxy

definitely-maybe.js

const unwrapSymbol = Symbol();

function Definitely(x) {

    if (x == null || !(unwrapSymbol in x))
        return x;
    
    return x[unwrapSymbol];
}

const Nothing = new Proxy(Object.seal(function() {}), {

    get(target, key) { return key === unwrapSymbol ? void 0 : Nothing },
    set() {},
    has() { return true },
    apply() { return Nothing },
    construct() { return Nothing },
});

const maybeHandler = {

    get(target, key) { return key === unwrapSymbol ? target : Maybe(target[key]) },
    has(target, key) { return key === unwrapSymbol || key in target },
    
    apply(target, thisArg, argumentList) { 
        if (thisArg !== Nothing)
            thisArg = Definitely(thisArg);
        return Maybe(target.apply(thisArg, argumentList));
    },
};

function Maybe(x) {

    return x === Nothing || x == null ? Nothing : new Proxy(Object(x), maybeHandler);
}

// alert(Definitely(Maybe(self).document.scripts[0].parentNode.nodeName))
// alert(Definitely(Maybe(self).document.querySelectorAll("div").length))

That's neat. Why do you return true line 15 (has() { return true },)? Because ("foo" in Maybe(null)) === true sounds weird to me.

Florent

Also I get this error (using Firefox):

>>> Maybe(self).document.querySelectorAll("foo")
TypeError: 'querySelectorAll' called on an object that does not implement interface Document.

I don't know if there is anything to do to prevent this error?

Ah yes. We can't use the "thisArg" supplied to the apply hook in that case, because a Proxy won't work there. Instead, we just call Definitely on the "thisArg" to unwrap it first. Should work now : )

For Nothing's "has" hook, I was thinking that it "has" everything, with the value of all properties being Nothing.