Grammar ambiguity in `for ( async of`

[bakkot]

The token sequence for ( async of can begin two different parses: either for (async of => {};;); or for (async of foo);.

@waldemarhorwat raised this issue at the December 2019 meeting. During that discussion we decided to resolve this by using a lookahead restriction to disallow for-of loops which start with the token async.

[Fenzland]

why allow keyword like async, of and even undefined as variable name?

[ljharb]

Because they’ve always been allowed, and it wouldn’t be web compatible to disallow them now.

[strager]

For reference, the following code prints 42 in Node.js version v12.20.0 and in Chromium version 88.0.4324.150, but emits a SyntaxError in Firefox 84.0.1 and in macOS Safari version 12.1.2 (14607.3.9):

var async;
for (async of [42]) {}
console.log(async);

[syg]

@strager A fix was recently merged (https://chromium-review.googlesource.com/c/v8/v8/+/2683221), and Chrome 90 / V8 9.0 will correctly disallow for ( async of.