Fix Event constructor self-conflict on Hermes

[mgjohnston]

Summary:

Removes the readonly NONE/CAPTURING_PHASE/AT_TARGET/BUBBLING_PHASE declarations from Event.js.

These declarations were intended as type-only annotations, but Babel emits them as runtime class fields, generating constructor assignments such as:

this.NONE = void 0;

That assignment conflicts with the non-writable Event.prototype.NONE property defined later in the same file via Object.defineProperty, causing Hermes to throw:

TypeError: Cannot assign to read-only property 'NONE'

This affected new Event(...) construction and broke APIs relying on Event creation, including fetch, XHR, and AbortController in RN 0.81.x / Expo SDK 54 Hermes environments.

The runtime constants themselves are already defined via Object.defineProperty below, so removing these declarations preserves the public API and DOM constant semantics while preventing the conflicting class field emission.

Related:

• Event.NONE/CAPTURING_PHASE/AT_TARGET/BUBBLING_PHASE properties missing configurable: true breaks event-target-shim #54732
• fix: add configurable:true to Event phase constant property descriptors #55002

Changelog:

[GENERAL] [FIXED] - Prevent Hermes crash when constructing Event instances caused by emitted class fields in Event.js

Test Plan:

• Verified the emitted constructor no longer contains:

this.NONE = void 0;

• Reproduced before fix in Expo SDK 54 / RN 0.81.5 Hermes app using:

new Event('foo')

and:

fetch('https://example.com')

Both previously threw:

TypeError: Cannot assign to read-only property 'NONE'

• After patch:

  • new Event('foo') succeeds
  • fetch(...) succeeds
  • Event.NONE === 0
  • Event.CAPTURING_PHASE === 1
  • existing constant definitions remain unchanged

[meta-cla]

Hi @mgjohnston!

Thank you for your pull request and welcome to our community.

Action Required

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at cla@meta.com. Thanks!

[meta-cla]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!