[Challenge] log("this") or log("this").withData({})

Yup nice point, I could only figure this one out with setTimeout myself. This was intended as a fun experiment for people. Maybe to understand setTimeout or perhaps finding some quirks in the language, hopefully, if not, just a weird little behavior nonetheless.

This is possible in Python by utilizing the destructor and relying on the fact that Python uses reference counting as its first GC tier:

class log:  # yes this is a class but you can just use it like a function
    def __init__(self, text):
        self.text = text
        self.print_on_dtor = True

    def __del__(self):
        if self.print_on_dtor:
            self.print_on_dtor = False
            print('log:', self.text)

    def with_data(self, data):
        self.print_on_dtor = False
        print('withData:', self.text, data)

JS doesn't have destructors so you can't use that trick there. I'm also not sure if the GC is as predictable as Python's (and with all the various implementations I won't be surprised if it has different rules on different engines...)

Thanks for another fun challenge!

This took quite a bit of thinking, but I ended up working with the fact that setTimeout isn't actual asynchronous.
So here's what my log function looks like:

function log(text) {
    let logger = {
        label: "Log:",
        message: [ text ],
        withData(...data) {
            this.label = "withData:";
            this.message.push(...data);
        }
    };

    setTimeout(() => console.log(logger.label, ...logger.message), 0);

    return logger;
}

And it passes the tests, of course!

log("test"); // > log: test
log("test").withData(2); // > withData: test 2

It can even handle more complicated synchronous behavior!

let logger = log("Here's a");

if (Math.random() > 0.5) {
    logger.withData("number", Math.random());
}
else {
    logger.withData("boolean", Math.random() > 0.5);
}

I guess. The only way I figured it out was making use of setTimeout, but, as you explained in another comment, it's not 100% guaranteed it will always work and can behave in unexpected ways.

Hopefully, somebody comes up with some weird solutions :)