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+# Plan: Modernize `@tko/utils`
+
+**Risk class:** LOW–MEDIUM — each phase is independently shippable and touches a
+leaf package. Public API surface is preserved per phase.
+**Owner:** brianmhunt
+
+## Progress
+
+| Phase | Status | PR | Notes |
+|-------|--------|-----|-------|
+| A. Dead polyfill probes | Ready | TBD | symbol, function, string, css — drop legacy shims. Net -40 lines. Branch `modernize/utils-dead-polyfills` already built and green (2698 browser + 2671 happy-dom tests). |
+| B. tasks.ts microtask | Not started | — | Drop MutationObserver fallback; `queueMicrotask` is universally available. |
+| C. object.ts | Not started | — | Adopt `Object.hasOwn` (ES2022); delete `@deprecated clonePlainObjectDeep`. |
+| D. memoization.ts | Not started | — | Replace prototype-exposed `{}` store with `Map`. |
+| E. array.ts idioms | Not started | — | Kill `arguments.length > 2 ? bind(thisArg)` pattern; inline native methods; standardise `[]` over `new Array()`. Public export names preserved. |
+| F. ~~deferError bug~~ | Investigated — **not a bug** | — | Initial read claimed `deferError` routed through `options.onError` incorrectly. Existing spec `packages/utils/spec/onErrorBehaviors.ts` codifies this as intentional: when `onError` is installed, all TKO errors (deferred ones included) flow through it and do not re-escape to `window.onerror`. False positive. |
+
+## Context
+
+`@tko/utils` is the leaf dependency of almost every other TKO package. Parts of
+it pre-date ES2015 as a baseline:
+
+- `useSymbols = typeof Symbol === 'function'` (always `true` now)
+- `functionSupportsLengthOverwrite` runtime probe (ES6 made `Function.length`
+  configurable universally)
+- `stringTrim` IE `\xa0` regex fallback (`String.prototype.trim` is ES5)
+- `stringStartsWith` polyfill (ES6 native)
+- `toggleDomNodeCssClass` SVGAnimatedString and string-className fallbacks
+  (all `Element`s have `classList`)
+- `tasks.ts` MutationObserver fallback (`queueMicrotask` since Safari 12.1 / 2018)
+- `clonePlainObjectDeep` — flagged `@deprecated Function is unused`
+- Plain-object dictionary stores with string keys (prototype pollution surface)
+- `new Array()`, `substring`, `.apply(null, args || [])` — pre-spread idioms
+
+The AGENTS.md rule "TKO is perf-sensitive — keep function bodies lean for
+inlining" applies here; most modernization here reduces branching inside
+hot-path utilities.
+
+## Scope
+
+**In scope.** Everything listed in the phase table. Public function names,
+signatures, and documented behaviour are preserved. Types may be tightened
+(e.g. `stringTrim(value: unknown): string` over the inferred `any`).
+
+**Out of scope.**
+
+- The `@tko/lifecycle` refactor (issue #322 — composition over mixins,
+  `Symbol.dispose`). That is a separate, larger plan.
+- Any public API removal. If a consumer depends on a function name today, it
+  still works after this plan.
+- The `deferError` swallowing bug — fixed on its own branch to keep this plan's
+  scope surgical.
+
+## Phases
+
+### Phase A — Dead polyfill probes
+
+Files: `symbol.ts`, `function.ts`, `string.ts`, `css.ts`, and a follow-on
+null-default in `utils.parser/src/preparse.ts` that a tightened return type
+surfaces.
+
+- Remove `useSymbols` and `functionSupportsLengthOverwrite` internal probe
+  exports. Neither has external consumers in the monorepo.
+- `createSymbolOrString(id) -> Symbol(id)`.
+- `overwriteLengthPropertyIfSupported` always applies.
+- `stringTrim` delegates to `String.prototype.trim`.
+- `stringStartsWith` delegates to `String.prototype.startsWith`.
+- `toggleDomNodeCssClass` uses only `node.classList`.
+
+Already implemented and green on `modernize/utils-dead-polyfills` —
+branch: `f348e975 Drop dead polyfill probes from @tko/utils`.
+
+### Phase B — tasks.ts microtask scheduler
+
+File: `tasks.ts`.
+
+- `queueMicrotask` is available in every supported runtime; drop the
+  MutationObserver branch and the `setTimeout` final fallback.
+- Collapse the scheduler selection block.
+- Keep `schedule`, `cancel`, `runEarly`, `resetForTesting` exports intact.
+- Validate against the MutationObserver-specific tests (if any) — `processTasks`
+  recursion guard stays untouched.
+
+### Phase C — object.ts
+
+File: `object.ts`.
+
+- Swap `Object.prototype.hasOwnProperty.call(obj, prop)` for `Object.hasOwn`.
+  Public `hasOwnProperty` export keeps its name.
+- Delete `clonePlainObjectDeep` (already `@deprecated Function is unused`).
+  Verify no monorepo consumer first.
+- Consider whether `extend` should simply delegate to `Object.assign` (keeps
+  existing `hasOwn`-filtering semantics if any consumer relies on it — check
+  first).
+
+### Phase D — memoization.ts
+
+File: `memoization.ts`.
+
+- Replace `const memos = {}` dictionary with `new Map<string, Fn>()`.
+- Replace `callback.apply(null, params || [])` with `callback(...params ?? [])`.
+- Replace `new Array()` with `[]`.
+- `generateRandomId` could adopt `crypto.randomUUID().slice(0,16)` but that's a
+  values-change — skip for this plan.
+
+### Phase E — array.ts idioms
+
+File: `array.ts`.
+
+- Remove `arguments.length > 2 ? action.bind(owner) : action` pattern; native
+  `forEach`/`map`/`filter` accept `thisArg` directly. Public export signatures
+  preserved (drop the third "owner" parameter? — API question, discuss in PR).
+- Standardise `[]` over `new Array()`.
+- Keep `compareArrays` / `findMovesInArrayComparison` — Levenshtein hot path,
+  don't touch behaviour. Only cosmetic cleanup.
+
+## Done signal
+
+- All 2698 browser tests (chromium, firefox, webkit) pass.
+- All 2671 happy-dom tests pass.
+- `bunx tsc --noEmit` clean.
+- `bunx @biomejs/biome check .` clean.
+- Net line count decreases; public exports unchanged.
+
+## Non-goals
+
+- No changes to `dom/*` beyond what Phase A already covers in `css.ts`.
+- No changes to `options.ts` — already well-typed and self-contained.
+- No new utilities or abstractions. "Don't add features, refactor, or introduce
+  abstractions beyond what the task requires" (AGENTS.md).