Core implementation and migration map

ArtPlayer keeps its existing constructor, player mixins, plugins and DOM/CSS hooks. The production entry is still src/index.js; this document marks actual migrated boundaries rather than describing the entire core as TypeScript.

Current TypeScript boundary: utilities

Module	Responsibility and constraints
`src/utils/index.ts`	Existing export barrel; no added runtime names or wrapper methods
`src/utils/format.ts`	Clamp, capitalization, clock formatting and HTML entity conversion; reusable private lookup maps
`src/utils/property.ts`	Native defineProperty alias, own-property inspection and recursive merge; keys are written as own data properties
`src/utils/time.ts`	Sleep, trailing debounce and leading throttle; infer argument tuple/receiver, preserve scheduling and synchronous return behavior
`src/utils/error.ts`	ArtPlayerError, truthiness guard and internal rejection handling; public media promises are handled elsewhere
`src/utils/file.ts`	Historical extension parsing and transient download anchor lifecycle
`src/utils/subtitle.ts`	Existing SRT/ASS-to-VTT text conversions and VTT Blob creation

These modules do not import the player, UI components or each other except for the barrel. Existing DOM and browser capability utilities remain JS and are re-exported unchanged. Emitter is now typed as described below; Component remains a later task. Imports from the barrel must not silently add runtime fields to Artplayer.utils.

Typed Emitter

src/utils/emitter.ts keeps the original on/once/emit/off class and a lazy ordinary object registry. declare e emits no instance field; a fresh emitter still has no own keys. There are no additional prototype methods or mandatory constructor options.

The generic event map associates event names with payload tuples, including readonly tuples, symbols and optional arguments. An open Record default preserves arbitrary JS event channels; internal typed consumers can use a closed map or intersect their known events with an open map. Receiver inference is checked on registration and erased inside each stored registration, where the original callback and ctx remain paired. Current JS player consumers remain incremental; public declaration expansion is CORE-07.

Dispatch takes a shallow registration-array snapshot. Normal listeners removed during dispatch still run if already captured, additions wait for a later dispatch, and a throw stops that dispatch and propagates unchanged. Off accepts either a once wrapper or its original callback; duplicate registrations of a callback are removed together. The once wrapper unsubscribes before invoking user code and now records consumption, so a nested dispatch cannot execute a captured copy twice. This fixes a demonstrated old defect without changing ordinary snapshot behavior. Event names such as proto and toString use own properties rather than inherited Object.prototype values.

Shared contracts in test/contracts/emitter.js run against published, workspace and installed versions. Candidate corrections have separate published-defect observations in test/public-behavior.test.js and test/browser/emitter.spec.js. The source type fixture checks event name/payload correlation, receiver types, custom channels and chain typing. Emitter owns registrations only; instance teardown must remove the relevant callbacks, not clear unrelated subscribers before destroy dispatch. Resource scopes follow in CORE-03.

Observable utility behavior

def is the native Object.defineProperty function. Property keys include symbols; descriptors, return identity and own-property behavior remain native.
mergeDeep iterates own enumerable string keys. Existing nested objects are recursively merged, a previously unseen value retains its reference, and the historical previous.concat(...incoming) behavior flattens incoming nested arrays by one level. It returns a new outer object and does not mutate inputs. __proto__ now becomes an own data key instead of invoking an inherited setter or replacing the result prototype.
Debounce discards earlier pending calls and uses the final arguments/receiver. Throttle runs immediately, ignores calls in the wait window and does not schedule a trailing call. Its wait flag is set after invoking the callback: synchronous reentry and retry after a thrown callback retain historical behavior. Both wrappers return undefined. No new public cancel/flush methods were introduced.
silencePromise handles only values with a callable catch and preserves other values. It is for internal event handlers; do not wrap public play/toggle promises to hide their rejections. The property-access assertion is local to that duck-typed boundary, and stack capture is guarded for engines that do not implement it.
Subtitle conversion preserves the existing limited formats, whitespace and time normalization; it is not a complete ASS parser. The caller owns and must revoke vttToBlob's returned URL. Download owns its temporary anchor and removes it in finally, including click failure. It does not revoke caller-owned URLs.

Types and remaining work

Migrated sources compile with strict/noUncheckedIndexedAccess and browser-only ambient types. The merge accumulator is a dynamic string-key boundary with one documented cast to the pre-existing generic return contract. No file-wide any or type-check suppression was added. The main public declarations remain a separate compatibility surface; see types/README.md.

Existing declaration discrepancies are still tracked for CORE-07: Utils lacks some runtime exports, def is declared void, debounce/throttle declare the callback return instead of void, and debounce's old optional context argument is ignored at runtime. Sleep's declaration also requires a delay while runtime defaults to zero. These were not silently tightened during source migration; use the actual source types internally. Lifecycle-owned timer cancellation is part of CORE-04/17 and the existing BASE-PERF-01 finding, not a claim that a standalone debounce can know when its owner is destroyed.

Verification and maintenance

Run yarn test:unit for shared published/workspace utility contracts and controlled timers. test/utils.test.js also accepts ARTPLAYER_TEST_CORE to test an actual UMD, legacy or ESM file. test/types/utils-source.ts checks source inference, receiver types and invalid arguments. Test fixtures supply timers explicitly to isolated published UMD contexts; production code is not patched for tests.

Run yarn typecheck, yarn build artplayer, then yarn build:i18n (core build clears dist first). yarn test:package:release rebuilds and installs a candidate outside the workspace. Set ARTPLAYER_BROWSER_ARTIFACTS to its mapping and run yarn test:browser for real playback, chapter integration, downloads and Blob URLs. Preserve generated core files in docs/compiled when committing a shippable core change.

For new utility behavior, extend the same old/new contract tests. Clearly separate intentional defect corrections from preserved behavior. For timers or Blob URLs, also identify the owner and verify cleanup in the consuming module. Remaining constructor, playback, UI, Component and capability migrations are recorded in refactor/tasks.json and should extend this map as they land.

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