document automatic semicolon insertion (ASI) hazard?

[dckc]

What exactly is the hazard / risk?

I filed tgrospic/rnode-client-js#16 asking @tgrospic to avoid it, but when I did so, I struggled to come up with any argument other than authority; i.e. Agoric says so (and Crockford used to say so). @tgrospic is (quite reasonably, as far as I can tell) pushing back.

[erights]

Given the stated rules -- never start a line with [, (, , +, *, /, -, ,, .,` -- and given high quality IDE support, which is plausibly as good as he says, I can believe that the hazard goes away. But that's a lot of work and narrow set of assumptions for questionable benefit. I'd rather invest my IDE engineering on real problems, of which we have plenty.

[erights]

To answer your question, "What exactly is the hazard/risk?", it is that the programmer thought they wrote a program whose parse would give it one meaning but the machine runs it as a program with a different meaning based on a different parse. Without his restrictions and IDE support there are plenty of examples caused by ASI. Given his restrictions and IDE support, I don't know if there are any examples where ASI causes such ambiguity.

Here's a tricky non-ASI example. What does the following code do as a script? As a CJS module? As an EcmaScript module? As an EcmaScript module packaged up by your favorite packer, say rollup?

let x = 8;
let y = 9;
let z = y <!--x;
console.log(z);

[dckc]

OK. Thanks. I recall that it simplifies writing a Jessie parser, which is probably enough motivation for this scope.