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Comment by Leszek | Hacker News Reader

Fun fact -- in V8 this is the only case which requires three token lookahead, to distinguish `<!-foo` meaning "less-than not negative foo" from `<!--` meaning "start of HTML comment". There's a whole rewinding mechanism in the scanner which wouldn't have to exist if not for this syntax.

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Source:

https://github.com/v8/v8/blob/main/src/parsing/scanner-inl.h...

and

https://github.com/v8/v8/blob/main/src/parsing/scanner.cc#L3...

And also this nice article:

https://v8.dev/blog/scanner

"The scanner chooses a specific scanner method or token based on a maximum lookahead of 4 characters, the longest ambiguous sequence of characters in JavaScript"

That's extra funny considering `<!-foo` is an extraordinarily useless expression.

In useful code sure. But `<!-f` with its dynamic casting implications may be useful in code golf. I haven’t golfed in years so I’m not sure, but I can imagine it having some benefit I haven’t though of.

You may be right. Unless it's true that `<!-f` is always equivalent to `<!f`.

Then again, I'd be even more horrified if the above statement is not true anyways.

Had to try it! Here is some horror for your day: https://codepen.io/eyelidlessness/pen/abVrjQq

this is <-!, not <!-

It's 0<!-f in the code, just not in the html text

Thanks both, fixed! That’s what I get for coding on my phone

For anyone confused, this is actually (semi) sane behavior by javascript standards, because it relies on `-"f"` evaluating to NaN.

Of course, because javascript, there are other string values of `f` for which the two statements are equal as well. (e.g for empty string both statements are true because `-"" === -0`, and for "1" both values are false)

https://codepen.io/LordDragonfang/pen/KKyLjLg

While we’re at it, here’s some more madness… this time with booleans and numbers!

https://codepen.io/eyelidlessness/pen/yLPdLey

This one really gets to me.

    f = 0 (type: object, constructor: Number)
    0<!f = false
    0<!-f = true

I’ve tried three times to write why I think this is, and each time I get lost in my reasoning. Alas, I’ve been on mobile for this whole thread and I don’t have any more brain cells to try to code like this tonight lol

It's because `f = new Number(0)` is a boxed number (therefore it's an object), and `!f` calls ToBoolean on this object which unconditionally returns true (https://tc39.es/ecma262/#table-toboolean-conversions). `!-f` first applies unary minus, which first performs a ToNumber on the f object (which returns unboxed 0), that's negated to -0, and then ToBoolean of -0 is false.