是什么

Question

I came across the following Node.js snippet in a code golf IRC room, and it is confusing me.

for (let x = 100; 1^ <!-- x; x++)
    console.log(`${x} bottles of beer on the wall.`);
    console.log("The beer goes", x - <!-- (3, 5) % (
        - "outputs"
        * "fizz"
        * "buzz"
        * "fizzbuzz"
    ));

The code doesn't seem to be correct -- when I try running it in Node, I get an infinite loop. But to my surprise, it does compile and run!

It seems to be using several features I haven't seen before. The use of ^1 of it reminds me of the upcoming C# range index syntax , and the list almost looks like Markdown , and it seems like some kind of weird currying of the modulo operator is going on.

However, I'd particularly like to understand what's going on with the <!-- operator. The first instance is being used in a loop, with the potential index operator, which seems like some kind of range iteration syntax or something. But I can't even guess what's happening in the second instance.

What is the <!-- operator in Node.js and how should it be used?

Answer 1

Here's the thing... you may not think that it's HTML, but that's where the syntax comes from.

I put exactly that code into a file named test.js , then node test.js :

100 bottles of beer on the wall.
The beer goes NaN
100 bottles of beer on the wall.
The beer goes NaN
100 bottles of beer on the wall.
The beer goes NaN
--- ad infinitum

So what's going on here? And what's with the syntax? It looks like there's an extra parenthesis at the end! Also, there is no { } , so the second line shouldn't be called each iteration! Right? Wrong .

Here's what the source looks like:

for (let x = 100; 1^ <!-- x; x++)
    console.log(`${x} bottles of beer on the wall.`);
    console.log("The beer goes", x - <!-- (3, 5) % (
        - "outputs"
        * "fizz"
        * "buzz"
        * "fizzbuzz"
    ));

Now, this isn't quite the <!-- comment opener in HTML that would require a closing --> . Instead, of treating it like /* , JavaScript uses these like // : they comment the remainder of that line .

for (let x = 100; 1^ // x; x++)
    console.log(`${x} bottles of beer on the wall.`);
    console.log("The beer goes", x - // (3, 5) % (
        - "outputs"
        * "fizz"
        * "buzz"
        * "fizzbuzz"
    ));

Hm.. This is starting to make a bit more sense. Let's kill the comments and adjust the newlines:

for (let x = 100;
     1^ console.log(`${x} bottles of beer on the wall.`);
     console.log("The beer goes", x - - "outputs" * "fizz" * "buzz" * "fizzbuzz")
    );

AHA! Now our infinite loop is starting to make sense. The termination condition of the loop always evaluates truthy:

1^ console.log(`${x} bottles of beer on the wall.`);

Our incrementing "operator" is a log message with nonsense instead of a number (ie the reason it prints NaN ):

console.log("The beer goes", x - - "outputs" * "fizz" * "buzz" * "fizzbuzz")

And the loop body is empty:

for ( ...
    );

---

I'm not sure what this code was intended to do, but at least you know why it was doing exactly what you observed it to do and why it "worked".