rxjs-core-internals.mdc

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description: Non-operator RxJS internals — observables, subjects, schedulers, ajax, util
globs: packages/rxjs/src/internal/*.ts,packages/rxjs/src/internal/observable/**,packages/rxjs/src/internal/scheduler/**,packages/rxjs/src/internal/ajax/**,packages/rxjs/src/internal/util/**,packages/rxjs/src/internal/scheduled/**,packages/rxjs/src/internal/testing/**,packages/rxjs/src/ajax/**,packages/rxjs/src/fetch/**,packages/rxjs/src/testing/**,packages/rxjs/src/webSocket/**
alwaysApply: false
---

# RxJS Core Internals

Not pipeable operators. For operator patterns, see `rxjs-operators`.

## Categories

| Area | Path | Examples |
|------|------|----------|
| Creation functions | `internal/observable/` | `of`, `from`, `defer`, `timer` |
| Subjects | `internal/*Subject.ts` | `BehaviorSubject`, `ReplaySubject` |
| Schedulers | `internal/scheduler/` | `async`, `queue`, `animationFrame` |
| Ajax / WebSocket | `internal/ajax/`, `webSocket/` | HTTP helpers, socket wrapper |
| Utilities | `internal/util/` | errors, type guards, helpers |
| Testing | `internal/testing/`, `src/testing/` | `TestScheduler`, test doubles |

## Sibling references

Before inventing patterns, read the nearest existing implementation and spec pair:

| Area | Implementation | Spec |
|------|----------------|------|
| Creation | `internal/observable/defer.ts`, `timer.ts` | `spec/observables/defer-spec.ts` |
| Schedulers | `internal/scheduler/AsyncScheduler.ts`, `async.ts`, `AsyncAction.ts` | `spec/schedulers/QueueScheduler-spec.ts` |
| Timer limits | `internal/util/maxTimerDelay.ts` (when present), `AsyncAction.schedule` | `spec/observables/timer-spec.ts`, `spec/operators/delay-spec.ts` |
| Subjects | `internal/BehaviorSubject.ts` | `spec/subjects/BehaviorSubject-spec.ts` |
| Ajax | `internal/ajax/ajax.ts` | `spec/ajax/index-spec.ts` |

Real timer validation for `asyncScheduler` belongs in **`AsyncAction.schedule`** (the `setInterval` choke point), not only in individual operators.

## Implementation

- Creation functions return `Observable` directly (not `OperatorFunction`)
- Subjects extend `Subject`; schedulers implement `Scheduler` / `SchedulerAction`
- Use `@rxjs/observable` primitives; keep operator logic out of this layer

## Exports

- Main API: `packages/rxjs/src/index.ts`
- Subpath barrels: `src/ajax/`, `src/fetch/`, `src/testing/`, `src/webSocket/`

## Tests

- `spec/observables/`, `spec/subjects/`, `spec/schedulers/`, `spec/ajax/`, `spec/websocket/`
- Marble tests where timing matters; synchronous Chai tests where appropriate
- Not the operator-only canonical-case checklist — match patterns in sibling specs

## JSDoc

Public API still needs JSDoc with `@example`. Marble PNG diagrams when the API is stream-shaped.