fix: make Event phase constants writable/configurable to prevent TypeError

[IsaacIsrael]

Summary

Fixes #54732

The Event class in src/private/webapis/dom/events/Event.js defines NONE, CAPTURING_PHASE, AT_TARGET, and BUBBLING_PHASE as both readonly instance fields and via Object.defineProperty without writable/configurable flags. When Hermes compiles these class fields, they become non-writable/non-configurable properties, causing a TypeError at instantiation time:

TypeError: Cannot assign to read-only property 'NONE'
    at Event (Event.js:53:4)
    at anonymous (WebSocket.js:258:63)
    at apply (native)
    at emit (EventEmitter.js:130:7)

This crashes whenever a WebSocket event fires (Metro dev connection, push notifications), and the unhandled TypeError propagates into any concurrent Promise chain — breaking fetch() uploads and other network operations.

Changes

1. Remove readonly from instance-level field declarations — the static properties are inherited via the prototype chain automatically, so explicit readonly instance declarations are unnecessary and cause the Hermes conflict.

2. Add writable: true, configurable: true to all 8 Object.defineProperty calls — allows event-target-shim (used by abort-controller / fetch()) to redefine these properties when needed.

3. Replace Event.NONE initializer with literal 0 — the [EVENT_PHASE_KEY] field initializer was referencing Event.NONE before Object.defineProperty defines it at the bottom of the file.

Changelog:

[GENERAL] [FIXED] - Fix TypeError "Cannot assign to read-only property 'NONE'" in Event class when using New Architecture with Hermes

Test Plan

1. Create a React Native 0.85+ app with New Architecture enabled
2. Make any fetch() request or open a WebSocket connection
3. Verify no TypeError: Cannot assign to read-only property 'NONE' is thrown
4. Verify event.NONE === 0, event.CAPTURING_PHASE === 1, event.AT_TARGET === 2, event.BUBBLING_PHASE === 3 still hold on Event instances

[fabriziocucci]

Thanks for digging into this, the crash in #54732 is real and the stack trace points right at it.

I traced the fix and I think it clears the crash but changes the value of the instance constants in a way that might not be intended. Sharing what I found in case it is useful.

RN compiles class fields in loose mode, so the instance declaration readonly NONE: 0; becomes this.NONE = void 0 in the constructor. Today that assignment throws because the prototype NONE is non-writable, which is the crash you hit. Adding writable: true makes the assignment succeed, but it then creates an own NONE property set to undefined that shadows the 0 on the prototype. So after the change new Event().NONE is undefined instead of 0 (same for CAPTURING_PHASE, AT_TARGET and BUBBLING_PHASE).

I checked this against the transpiled output with a small repro:

• before: new Event() throws Cannot assign to read only property 'NONE'
• with writable: true: no throw, but event.NONE === undefined
• with the instance fields as declare: no throw, and event.NONE === 0

The existing tests would not catch this since they assert the static Event.CAPTURING_PHASE and the eventPhase getter, not the instance constants.

One option that fixes the crash and keeps event.NONE === 0 is to make the four instance declarations type-only with declare:

declare readonly NONE: 0;
declare readonly CAPTURING_PHASE: 1;
declare readonly AT_TARGET: 2;
declare readonly BUBBLING_PHASE: 3;

A declare field emits no runtime assignment, so there is no this.NONE = to fail and instances keep inheriting the values from the prototype. With that you also would not need the writable: true / configurable: true additions, so the constants stay read-only like the DOM spec.

I might be missing context on why the readonly instance fields were kept, so let me know if there is a Flow reason declare does not work here.