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# JavaScript Language Feature Checklist
So you're working on a new JavaScript feature in SpiderMonkey: Congratulations! Here's a set of checklists and guidelines to help you on your way.
## High Level Feature Ship Checklist.
(Note: Some of these pieces can happen in parallel, so it's not necessary to
work directly top-down)
- ☐ Send an Intent to Prototype email to `dev-platform`. This is part of the
historically haven't been amazing at sending intent-to-prototype emails, but
we can always get better.
- ☐ Create a shell option for the feature.
- ☐ Stage 2 or earlier proposals should be developed under compile time guards,
disabled by default.
- ☐ Create a browser preference for the feature.
- ☐ Implement the Feature.
- ☐ Land feature disabled by pref and shell-option.
- ☐ Import the test262 test cases for the feature, or enable them if they're
already imported. (See `js/src/test/Readme.txt` for guidance)
- ☐ Contact `fuzzing@mozilla.org` to arrange fuzzing for the feature.
- ☐ Add shell option to `js/src/shell/fuzz-flags.txt`. This signals to other
fuzzers as well that the feature is ready for fuzzing.
- ☐ Send an Intent to Ship Email to `dev-platform`. This is also part of the
- ☐ Ship the feature; default the preference to true and the command-line
option to true.
- ☐ Open a followup bug to later remove the preference and the command line
option.
## Supplemental Checklists
### Shipping Consideration Checklist
- ☐ If it seems possible that the feature will cause webcompat issues,
consider shipping `NIGHTLY_ONLY` for a cycle or two, to use nightly as an
attempt to shake out potential webcompat issues.
### Web Platform Integration Checklist
_Sometimes Complexity of the web-platform leaks into JS Feature works_
- ☐ Ensure the appropriate web-platform tests exist, and are being run.
- ☐ Is your feature correctly enabled inside of Workers? (They have different
option set than main thread, and it's easy to forget them!) You may want to
write a mochitest.
### Syntax Features Checklist
- ☐ Does `Reflect.parse` correctly parse and return results for your new syntax?
- `Reflect.parse` tests are interesting as well, because they can be written
for new syntax before bytecode emission is done.
- ☐ Are the locations correct for the new syntax entries in the parse tree?
- ☐ Are your errors emitted with sensible location info?
### Testing Consideration Checklist
_There's lots of complexity in SpiderMonkey that isn't always captured by the
specification, so the below is some useful guidance to behaviour to pay
attention to that may not be tested by a feature's test262 tests_
- ☐ How does your feature interact with multiple compartments? What happens if
references happen across compartments, or if `this` is a
`CrossCompartmentWrapper`?
- ☐ Are your error messages being emitted in the correct realm, with the
correct prototype?
- ☐ If async functions or promises are involved, are user-code objects
resolved? If so, does the feature correctly handle [the `.then` property
behaviour of promise
resolution?](https://www.stefanjudis.com/today-i-learned/promise-resolution-with-objects-including-a-then-property/)
- ☐ Have you written some OOM tests for your feature to ensure your OOM
handling is correct?
#### Web Platform Testing Considerations
- ☐ Does the feature have to handle exotic objects specially? Consider what
happens when your feature interacts with the very exotic objects on the web
platform, such as `WindowProxy`, `Location` (cross-origin objects).
- ☐ What happens when your feature interacts with
[X-rays](/dom/scriptSecurity/xray_vision.rst)?