babel: Better parser error when using jsx

This bug needs different steps in order to be fixed, but they can be done in the same PR.

1. First, we should make @babel/parser throw a developer-friendly error. Consider these examples:

   require("@babel/parser").parse("<div></div>", {});
   require("@babel/parser").parse("a |> b", {});
   

   The first one throws SyntaxError: Unexpected token (1:0), while the second one throws a more actionable SyntaxError: This experimental syntax requires enabling the parser plugin: 'pipelineOperator' (1:2). Why this difference? If you search for the "pipelineOperator" string in packages/babel-parser/src, you’ll see that (as @hzoo mentioned above) we are calling the this.expectPlugin("pipelineOperator") function when we find the “pipeline” (|>) operator.

   We can do something similar for JSX, but it’s harder. The main problem is that checks for ECMAScript proposals (such as |>) are inlined in the parser code, while checks for JSX code are only enabled when the jsx plugin is enabled.

   However, we know that JSX elements are _atomic expressions (parsed by the parseExprAtom method) and that no other expression can start with <. That method contains a big switch statement, and (at the end) we throw this.unexpected() if we don’t find any valid token. We could add a new case and, if the token is <, call this.expectPlugin("jsx").

   Here are a few possible tests:

   • When the jsx plugin is not enabled, this code should throw the “nice” error:

     <div /></div>
     

   • This is valid JS code, and it should never throw:

     "use strict"
     <div
     

   • When the typescript plugin is enabled, this code shouldn’t throw because it’s a type parameter:

     <div>() => {}
     

2. Then, we should make @babel/core handle the parser error and suggest the correct @babel/... plugin/preset. You can add the syntax plugin and the react preset at packages/babel-core/src/parser/util/missing-plugin-helper.js.

---

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 some tests: packages/babel-parser/test/fixtures/jsx/errors/_no-plugin, packages/babel-parser/test/fixtures/jsx/errors/_no-plugin-no-jsx and packages/babel-parser/test/fixtures/jsx/errors/_no-plugin-ts-type-param. You can see an example at packages/babel-parser/test/fixtures/experimental/_no-plugin/import-meta.
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.js 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!

@karansapolia it’s yours! let us know (here or on slack) if you have any questions!

Awesome! Please feel free to reach out if you have any questions.

i want to try this.