fix: preserve subclasses in duplicate() (#2053)

[rit3sh-x]

Closes #2053.

Summary

Redis#duplicate() and Cluster#duplicate() now preserve the actual class of the instance, so subclasses are returned from duplicate() instead of plain Redis / Cluster.

Root cause

The applyMixin utility in lib/utils/applyMixin.ts copied every own property from the mixin prototype onto the derived class prototype — including the built-in constructor property. When applyMixin(Redis, EventEmitter) runs at module load, EventEmitter.prototype.constructor overwrites Redis.prototype.constructor, so any Redis instance had r.constructor === EventEmitter instead of Redis.

That is why a naive new this.constructor(...) fix for duplicate() previously broke Cluster (as noted in Pavel's comment on #2053) — the constructor was wrong at the root.

Fix

1. lib/utils/applyMixin.ts — skip the constructor key when copying descriptors, using the canonical Object.getOwnPropertyDescriptors + delete + Object.defineProperties pattern. Preserves getters/setters, non-enumerable props, and symbol keys.
2. lib/Redis.ts — duplicate() now uses this.constructor and returns this.
3. lib/cluster/index.ts — same treatment for Cluster.duplicate(), typed with ClusterNode[] / ClusterOptions for proper type safety.

Example

class InstrumentedRedis extends Redis {
  getTag() { return "instrumented"; }
}

const redis = new InstrumentedRedis(6379);
const dup = redis.duplicate();
// Before: dup instanceof InstrumentedRedis === false (plain Redis)
// After:  dup instanceof InstrumentedRedis === true
// After:  dup.getTag() === "instrumented"

Test plan

New unit test file test/unit/duplicate.ts with 15 tests covering:

• Redis.prototype.constructor correctly resolves to Redis (applyMixin fix)
• Cluster.prototype.constructor correctly resolves to Cluster
• redis.duplicate() returns a Redis instance on plain Redis
• redis.duplicate() returns a subclass instance when called on a subclass
• Subclass methods are preserved across duplicate()
• Subclass instance fields (set in constructor) are preserved
• Override options are applied to the duplicate
• Deeply-nested subclass hierarchies (A → B → C → Redis) work
• Same coverage for Cluster subclasses
• applyMixin still mixes in EventEmitter methods (on, emit, removeAllListeners) onto both Redis and Cluster

Verification

• npm run build — PASS
• npm run test:tsd — PASS
• Full unit + functional suite: 609 passing, 26 pending, only pre-existing flakes unrelated to this change
• Cluster integration tests (Docker): 17/17 passing
• New duplicate tests: 15/15 passing

Relation to #2073

Independent but complementary. #2073 proposes a redisClass option on Cluster to thread a custom class through all internal connections. This PR fixes the underlying inheritance bug that prevented new this.constructor(...) from working in the first place.

Either PR can land first — there are no file-level conflicts (different line ranges in cluster/index.ts).

---

Note

Low Risk
Targeted inheritance/mixin fix with broad unit tests; duplicate() is used internally (e.g. MONITOR, slot refresh) but behavior aligns with documented intent.

Overview
Fixes #2053 so duplicate() on custom Redis / Cluster subclasses returns the same class (instrumentation, extra methods, constructor fields) instead of a plain base instance.

Root cause: applyMixin copied EventEmitter.prototype.constructor onto Redis/Cluster, so instance.constructor was wrong and duplicate() could not safely use new this.constructor(...).

Changes: applyMixin now skips copying constructor when merging prototypes. Redis#duplicate() and Cluster#duplicate() instantiate via this.constructor and are typed to return this. New unit tests cover constructors, nested subclasses, overrides, and that EventEmitter APIs still work.

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[PavelPashov]

Thanks for the PR. I will review it and follow up once I have gone through it.

[rit3sh-x]

@PavelPashov any updates?