Use when writing vanilla JavaScript or TypeScript that touches the DOM in an evergreen-2026 browser, picking an ES2024 grouping / deep-clone / promise / iterator API, typing an event handler, narrowing a `querySelector` or `getElementById` return, or breaking up a long task to protect INP. Prevents the most common modern JS/TS regressions in 2026 : `JSON.parse(JSON.stringify(x))` deep clones that drop `Date` and crash on cycles, hand-rolled "deferred" promise patterns that leak resolvers out of an executor closure, `reduce` accumulator boilerplate for grouping, blind `as HTMLInputElement` casts on `event.target` that lie under delegation, `requestAnimationFrame` misused as a generic yield that breaks the INP budget, Iterator helpers shipped without feature detection to older mobile WebViews, and the deprecated `import ... assert { type: "json" }` syntax replaced by `with`. Covers ES2024 DOM-relevant features (`Object.groupBy`, `Map.groupBy`, `Promise.withResolvers`, `structuredClone` with `transfer`, top-leve
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Use when writing vanilla JavaScript or TypeScript that touches the DOM in an evergreen-2026 browser, picking an ES2024 grouping / deep-clone / promise / iterator API, typing an event handler, narrowing a `querySelector` or `getElementById` return, or breaking up a long task to protect INP. Prevents the most common modern JS/TS regressions in 2026 : `JSON.parse(JSON.stringify(x))` deep clones that drop `Date` and crash on cycles, hand-rolled "deferred" promise patterns that leak resolvers out of an executor closure, `reduce` accumulator boilerplate for grouping, blind `as HTMLInputElement` casts on `event.target` that lie under delegation, `requestAnimationFrame` misused as a generic yield that breaks the INP budget, Iterator helpers shipped without feature detection to older mobile WebViews, and the deprecated `import ... assert { type: "json" }` syntax replaced by `with`. Covers ES2024 DOM-relevant features (`Object.groupBy`, `Map.groupBy`, `Promise.withResolvers`, `structuredClone` with `transfer`, top-level `await`, the regex `v` flag, `Array.findLast` / `findLastIndex`, well-formed string helpers), Iterator helpers on `Iterator.prototype` (`.map .filter .flatMap .take .drop .reduce .toArray`) with the lazy-vs-terminal distinction and feature detection, `scheduler.yield()` with `setTimeout(_, 0)` fallback and priority inheritance from `postTask`, TypeScript DOM strict-mode rules (the required tsconfig flags `strict` / `noUncheckedIndexedAccess` / `noImplicitOverride` / `exactOptionalPropertyTypes`), `instanceof` narrowing on `event.target`, `lib.dom.d.ts` quirks (`querySelector` overload set, `getElementById` returning `HTMLElement | null`, `dataset` typing, custom events via `HTMLElementEventMap` module augmentation), and Import Attributes (`import x from "./x.json" with { type: "json" }`). Keywords: Object groupBy, Map groupBy, Promise withResolvers, structuredClone, Iterator prototype, iterator helpers, scheduler yield, instanceof, HTMLElementEventMap, lib dom d ts, querySelector, EventTarget, ValidityState, strict mode, noUncheckedIndexedAccess, exactOptionalPropertyTypes, noImplicitOverride, import attributes, top-level await, regex v flag, findLast, findLastIndex, isWellFormed, deep clone broken, type cast lies, INP regression, long task, mobile WebView crash, Property does not exist on EventTarget, querySelector returns null, TypeScript yells at DOM, can not narrow event target, deferred promise pattern, how do I group an array in JavaScript, what is Promise withResolvers, how to deep clone in JS, TypeScript with DOM, how to narrow event target type, how to type DOM events, what is iterator helpers, how to break up a long task
license
MIT
compatibility
Designed for Claude Code. Requires Frontend Design evergreen-2026.
metadata
{"author":"OpenAEC-Foundation","version":"1.0"}
Frontend Syntax : ES2024 + TypeScript DOM
This skill is the operational reference for ES2024 features that matter at the DOM boundary and the TypeScript narrowing patterns that make lib.dom.d.ts ergonomic instead of hostile. It covers ES2024 grouping / deep-clone / promise / iterator surfaces, scheduler.yield() for INP, TypeScript strict-mode flags, instanceof narrowing on event.target, HTMLElementEventMap augmentation, and Import Attributes. The skill does NOT cover React / Vue / Solid / Svelte / Angular hook patterns, bundler config, Node.js APIs, or Web Components lifecycle (see [[frontend-impl-web-components]]).
Quick Reference
Floor rules
ALWAYS use structuredClone(x) for deep cloning data that includes Date, Map, Set, typed arrays, or cycles. NEVER use JSON.parse(JSON.stringify(x)) for the same task ; it drops types and crashes on cycles.
ALWAYS use Promise.withResolvers() when the resolver must be called from outside the promise body. NEVER hoist resolve / reject out of a new Promise(...) executor with a let declaration.
ALWAYS group with Object.groupBy(items, fn) for string / symbol keys, Map.groupBy(items, fn) for arbitrary object keys. NEVER hand-roll a reduce accumulator for the same task.
ALWAYS narrow event.target with instanceof HTMLInputElement (or the relevant element class) before reading .value, .checked, or .dataset. NEVER use as HTMLInputElement ; the cast lies under delegation and programmatic dispatch.
ALWAYS feature-detect scheduler.yield() with a setTimeout(_, 0) fallback. NEVER assume it is available ; as of 2026-05-19 only Chromium ships it in stable channels.
ALWAYS feature-detect Iterator helpers (typeof Iterator?.prototype?.map === "function") when targeting non-evergreen mobile WebViews. NEVER ship .map().filter().take(n).toArray() chains without a polyfill or detection.
ALWAYS narrow null returns from getElementById, querySelector, and with . NEVER use the non-null assertion ; the runtime still throws when the ID is missing.
closest
if (!el) return;
!
ALWAYS use import x from "./x.json" with { type: "json" } (Import Attributes). NEVER use the deprecated assert { type: "json" } syntax.
Decision tree 1 : Iterator helper vs Array method ?
Source is an Array AND you do not chain ?
-> array methods (.map, .filter, .reduce). No advantage to wrapping.
Source is iterable (generator, Set, NodeList, Map.entries(), custom iterator) ?
-> Iterator.from(src).filter(fn).map(fn).take(n).toArray()
Avoids the intermediate Array allocations that .filter().map() on arrays causes.
You want laziness : only iterate as many source elements as needed ?
-> Iterator helpers (.take is lazy ; consumes only what it needs).
array.filter().map().slice(0, n) eagerly iterates the entire array.
You target only evergreen-2026 desktop ?
-> Iterator helpers are practically Baseline. Ship without polyfill.
You ship to older mobile WebViews ?
-> Feature-detect : typeof Iterator?.prototype?.map === "function"
Polyfill via core-js or es-iterator-helpers when missing.
Decision tree 2 : scheduler.yield() vs requestAnimationFrame vs setTimeout ?
You need to break up a long task and stay responsive to input (INP target < 200 ms) ?
-> await scheduler.yield() with setTimeout(_, 0) fallback.
scheduler.yield() resumes at user-visible priority, boosted ahead of equal-priority postTask.
You need to read layout or paint after a DOM change ?
-> requestAnimationFrame(cb). Tied to the frame boundary. NOT a generic yield.
You need to defer work that may never need to run ?
-> requestIdleCallback(cb). Lowest priority ; may starve.
You only care about "give up the current task" and have no priority needs ?
-> setTimeout(fn, 0) or await new Promise(r => setTimeout(r, 0)). Macrotask tail.
Decision tree 3 : instanceof vs as cast on event.target ?
Always prefer instanceof. The single exception : you just called
document.createElement<'input'>("input") and you control the variable.
Handler bound to a single known element ?
-> Prefer e.currentTarget over e.target ; the handler signature can be
typed once with a type assertion on the receiver instead of casting per event.
Handler delegated from a parent (e.g. one click listener for a list) ?
-> if (!(e.target instanceof HTMLButtonElement)) return;
Inside the if, e.target is HTMLButtonElement with no cast.
Custom event with typed detail ?
-> Augment HTMLElementEventMap so addEventListener types the handler param.
Decision tree 4 : Deep clone strategy ?
Plain data : numbers, strings, booleans, plain objects, arrays ?
-> structuredClone(x). Works ; no surprises.
Includes Date, Map, Set, typed arrays, ArrayBuffer, Blob, File, RegExp, Error ?
-> structuredClone(x). Preserves type. JSON round-trip drops these.
Contains cycles (a.b.a === a) ?
-> structuredClone(x). JSON round-trip throws TypeError: cyclic object.
Contains functions, getters, setters, class instances with private fields ?
-> structuredClone throws DataCloneError. Either restructure the data, or
write a manual clone for the specific shape. A library is overkill.
You want to TRANSFER ownership of an ArrayBuffer / OffscreenCanvas / MessagePort ?
-> structuredClone(value, { transfer: [buf] }) ; source buffer is detached.
When the group key is an arbitrary object reference (two distinct objects with identical shape must produce two distinct groups), use Map.groupBy(items, fn) and read with .get(key).
Pattern : Promise.withResolvers for event-driven flows
.take(10) is lazy : iteration stops after 10 elements survive both filters, even if the NodeList has 10 000 items. The same chain with array methods ([...nodes].filter(...).map(...).filter(...).slice(0, 10)) materializes three intermediate arrays of size ~10 000.
Pattern : scheduler.yield with fallback
asyncfunctionyieldToMain() {
if ("scheduler"in globalThis && "yield"in globalThis.scheduler) {
return globalThis.scheduler.yield();
}
returnnewPromise((r) =>setTimeout(r, 0));
}
asyncfunctionprocessChunks(items) {
for (const item of items) {
work(item);
if (navigator.scheduling?.isInputPending?.()) {
awaityieldToMain();
}
}
}
Yields only when input is actually pending. The yielded continuation resumes at the same priority as the surrounding postTask call, or user-visible by default.
Pattern : TypeScript narrowing on event.target
const form = document.querySelector("form");
if (!form) thrownewError("form missing");
form.addEventListener("input", (e) => {
if (!(e.targetinstanceofHTMLInputElement)) return;
// e.target is HTMLInputElement here. .value is string. No cast.console.log(e.target.name, e.target.value);
});
The instanceof check survives event delegation, programmatic dispatch, and shadow-DOM retargeting. An as HTMLInputElement cast does not.
Pattern : Typed custom events via module augmentation
interfaceOpenModalDetail {
id: string;
}
declareglobal {
interfaceHTMLElementEventMap {
"app:open-modal": CustomEvent<OpenModalDetail>;
}
}
document.addEventListener("app:open-modal", (e) => {
// e is CustomEvent<OpenModalDetail>. e.detail.id is string.loadModal(e.detail.id);
});
Augmentation must live in a .d.ts or .ts file that is part of the compilation. After augmentation, no cast is needed at any call site.
Pattern : querySelector with generic vs literal tag
The literal-tag overload only matches tag-name selectors. For class / id / attribute selectors, supply the generic to recover the type. Never write (document.querySelector(".submit") as HTMLButtonElement).
The attribute forces the host to refuse the module if the response is anything other than JSON. Replaces the deprecated assert { type: "json" } syntax.
strict bundles strictNullChecks and friends. noUncheckedIndexedAccess adds | undefined to arr[i] and dataset.foo. noImplicitOverride catches typos like disconnectCallback versus the spec-defined disconnectedCallback. exactOptionalPropertyTypes distinguishes "property missing" from "property set to undefined".
