User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:72.0) Gecko/20100101 Firefox/72.0

Steps to reproduce:

I was trying out new language features, as one does, and typed in (void 0) ?? "test" in the console.

Actual results:

The resulting value was undefined.

Expected results:

I think the correct result is "test". Assigning void 0 to a temporary value produces that result, as does the original expression in Chromium.

I'm not sure when an expression like this would ever come up in practice, but I wanted to make a note of it just in case.

It looks like FoldAndOrCoalesce just needs to handle ParseNodeKind::VoidExpr.

[Tracking Requested - why for this release]:
New feature.

mattheww, great catch. Thank you so much for the bug report. I think you saved us some real embarrassment.

The bug is fun. It happens in a part of the JS engine where we examine source code and try to simplify it a bit, "algebraically", before running it, an optimization called constant folding. So for example, if you have a function that does let x = 60 * 60;, we simplify that to let x = 3600; before anything runs -- the function will run faster if we multiply once, up front, rather than every time the function is called. Constant folding also throws away most if (false) { ... } statements, and so on.

Unfortunately, in this case we were erroneously assuming that the only constant expressions that evaluate to null or undefined were null and undefined. As you found, there's one more we need to account for.

https://hg.mozilla.org/mozilla-central/rev/9dbf3a2c8c37