Regression: if without braces, followed by const or let immediately after the semicolon, produces broken code

[chris-morgan]

This regressed between 1.16.0 and 1.17.0, in 40baf2e.

Minimal test case:

if(0);const e=0

This produces:

if(0){ ;var e=0

Before 40baf2e, it produced this:

if(0){ ; }var e=0

Any whitespace after the semicolon, and it compiles correctly; it’s only if the semicolon is immediately followed by let or const that it breaks.

for and while also exhibit this bug.

I feed some minified code through bublé; I hit this when upgrading from 1.15.2 to the latest and it all stopped working.

[chris-morgan]

Looking at this commit this regressed in, it’s clearly related to upgrading magic-string from 0.14.0 to 0.22.4. It seems to me more likely that this is a bug in magic-string than buble itself, but I’m sufficiently uncertain about it all that this is the best place to file and track it.

[aleclarson]

Why do you minify before Bublé?

[chris-morgan]

@aleclarson Not that it particularly matters—this being a rather severe regression on legitimate code—but there are two main reasons why minified code may be fed through Bublé. I hit both of them.

1. The more likely one: you’re feeding all your code through Bublé, and you happen to have some third-party minified code in there alongside it.

2. For historical reasons of dead code removal: UglifyJS can remove dead code in the presence of declared globals and local constants. This latter used to require you to use const or a /* @const */ comment on var, though some time in the last year it was extended to be able to cope with unadorned var (hence, historical reasons). Therefore, I fed it through UglifyJS to remove dead code, then Bublé to transform all the things (including let and const to var), then UglifyJS again to minify the transformed code.

[adrianheine]

Thanks for your report, I could easily reproduce the issue.

$ echo 'if(0);const e=0' | bin/buble 
if(0){ ;var e=0

I obviously suspected the const transform to be the issue, so I deactivated it, and got valid output:

$ echo 'if(0);const e=0' | bin/buble -n letConst
if(0){ ; }const e=0

So I checked src/program/types/VariableDeclaration.js and found code.overwrite in line 17 which in such cases usually just need a contentOnly option set. The test suite still passes.