Hermes engine target

[oblador]

Hi, I'm using esbuild with react-native via react-native-esbuild and would like to add support for the Hermes engine that is announced to overtake JSC as the default engine. Unfortunately its language feature set is all over the place with many newer features, while some features stuck at es5 level.

If I bundle with target es5, esbuild errors out on many features supported by hermes, but at the same time when using es6 esbuild itself will produce incompatible code (primarily re-exports IIRC). Would you accept a PR adding hermes as an engine?

[evanw]

No. Each target is a very heavyweight thing. It's not just a single set of supported features. The target setting lets you specify any past version, so esbuild needs a support matrix for every version and every feature. I have some primary data sources for this for browsers and node, but I have also had to comb through browser bug reports and other random web pages too. This is an ongoing maintenance burden. While I believe it's worth it for browser runtimes, I'm not interested in maintaining additional targets for less-used JavaScript runtimes like Hermes. Instead, I will add the ability for you to specify the support matrix yourself at the individual feature level.

[evanw]

I just implemented custom per-feature support overrides. In the next release, you will now be able to e.g. specify --supported:class=false to forbid classes in your code. So I'm closing this issue as what you are asking for will now be possible.

[oblador]

That makes a lot of sense, thank you!

[evanw]

Sorry, I didn't realize that Hermes was listed in the data source I already use for browser compatibility data. So it shouldn't be hard to add this after all. So I can add a hermes target in the next release. I'm not sure how accurate or up-to-date that data is but you can always use --supported to override any issues.