Description

When running harness tests on React Native apps using JSC (JavaScriptCore) runtime, the runtime crashes immediately with:

Object.hasOwn is not a function. (In 'Object.hasOwn(globalThis, MATCHERS_OBJECT)', 'Object.hasOwn' is undefined)

Environment

• react-native-harness: 1.0.0-alpha.20
• React Native: 0.76.x
• JS Engine: JSC (JavaScriptCore)
• Platform: iOS / Android with Hermes disabled

Root Cause

@vitest/expect v4.x uses Object.hasOwn (ES2022) which JSC doesn't support:

// @vitest/expect/dist/index.js:674
if (!Object.hasOwn(globalThis, MATCHERS_OBJECT)) {

Note: This was previously misattributed to Hermes in #51, but Hermes has supported Object.hasOwn since recent versions. The issue specifically affects JSC runtime.

Proposed Solution

Add an Object.hasOwn polyfill to the runtime package that loads before @vitest/expect:

if (typeof Object.hasOwn !== 'function') {
  Object.hasOwn = (obj: object, prop: PropertyKey): boolean =>
    Object.prototype.hasOwnProperty.call(obj, prop);
}

Workaround

Consumers can add the polyfill themselves via Metro's getPolyfills, but this should be handled in the harness since it targets React Native environments that may use JSC.