Better error message for missing number exponent after "e"

Feature Request

Is your feature request related to a problem? Consider this (invalid) code:

12.3e

It throws this error:

Invalid number (1:0)

Describe the solution you’d like

It should throw something like

Missing exponent after 'e' in floating-point number

---

If you are interested in fixing this bug, you should introduce a new error message and use it in the “tokenizer”: the part of the parser which takes the input source code as a string and generates a set of corresponding tokens (1.2, foo, +, "baz", var, …).

⚠️ SPOLIER (it’s here, but first try finding it yourself!)

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. Run make watch (or make build whenever you change a file)
7. ~Add a test~ We probably already have tests for this with the “bad” error message
8. Update the code!
9. yarn jest babel-parser to run the tests
   • If some test outputs don’t match but the new results are correct, you can delete the bad output.json files and run the tests again
   • If you prefer, you can run OVERWRITE=true yarn jest babel-parser and they will be automatically updated.
10. If it is working, run make test to run all the tests
11. Run git push and open a PR!

If you need any help, feel free to reply here or write me on slack!

[JLHwung]

An edge case: Currently Babel throws

Invalid or unexpected token

for 1.e, which should have thrown Missing exponent after 'e' in floating-point number. Apparently I thought of Chromium console as Babel.

[nicolo-ribaudo]

It’s your! If you need any help feel free to ask.