Hey @chenyulun! We really appreciate you taking the time to report an issue. The collaborators on this project attempt to help as many people as possible, but we're a limited number of volunteers, so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack community that typically always has someone willing to help. You can sign-up here for an invite.

Hermes is in compat-table, so we can do this. We already did something similar a couple of months ago, adding support for Deno as you already saw — that PR is a good example to follow.

Since you checked "Would you like to work on this feature?", I'm leaving here some instructions :)

If it is the first time that you contribute to Babel, follow these steps: (you need to have make and yarn available on your machine)

1. Write a comment there to let other possible contributors know that you are working on this bug.
2. Fork the repo
3. Run git clone https://github.com/<YOUR_USERNAME>/babel.git && cd babel
4. Run yarn && make bootstrap
5. Wait ⏳
6. Add Hermes to packages/babel-compat-data. All the JSON files are generated using make build-compat-data.
7. Run make watch (or make build whenever you change a file)
8. Add support for Hermes in packages/babel-helper-compilation-targets
9. Add a test in packages/babel-preset-env (only input.js; output.js will be automatically generated)
10. yarn jest preset-env to run the tests
    • If some test outputs don't match but the new results are correct, you can delete the bad output.js files and run the tests again
    • If you prefer, you can run OVERWRITE=true yarn jest [name-of-the-package-to-test] and they will be automatically updated.
11. If it is working, run make test to run all the tests
12. Run git push and open a PR!