git:20260617.c5d2c7b to git:20260617.fbe923d

---

name: sota-javascript-typescript

description: State-of-the-art JavaScript and TypeScript engineering (2026) for both writing and auditing code. Covers strict TypeScript configuration and type design, language idioms and pitfalls, async patterns, Node.js backends, JS/TS-specific security (XSS, prototype pollution, supply chain, injection), frontend/React and Node performance, and testing/tooling. Use whenever building, reviewing, refactoring, or security-auditing code involving JavaScript, TypeScript, Node, npm, React, frontend code, tsconfig, package.json, vitest, or any .ts/.tsx/.js/.mjs files.

---

# SOTA JavaScript / TypeScript Engineering

## Purpose

This skill encodes 2026 state-of-the-art for JS/TS so generated code is strict, secure, and fast by default — and so audits of existing code find the bug classes that actually bite: untyped boundaries, floating promises, XSS sinks, prototype pollution, supply-chain gaps, event-loop blocking, and leak-prone listeners. It has two operating modes; pick one explicitly at the start of a task.

Baseline assumptions (mid-2026): TypeScript ≥5.9 strict (6.0 is current and the last JS-based compiler; TS 7 "tsgo" native preview available), ESM-first, Node LTS ≥22 (24 = active LTS; Node 26 ships Temporal by default), ES2024+ available, React 19.2-era with Server Components and React Compiler 1.0 where relevant, vitest 4 + flat-config ESLint (v9/v10).

## BUILD mode (writing or modifying code)

1. **Read the relevant rules files first** (index below) for the area you're touching. Don't generate from memory what a rules file specifies.

2. **Defaults unless the codebase dictates otherwise**: strict tsconfig (rules/01), ESM, `unknown` over `any`, discriminated unions for state, zod/valibot parse at every untrusted boundary, `??`/`?.` discipline, AbortController on cancellable ops, pino logging in services, Web APIs over deps.

3. **Match the host codebase** for style, framework, and structure — but do not replicate its security bugs or `any`-sprawl into new code. New code meets the bar even in old repos.

4. **Boundary rule**: every input from outside the type system (HTTP, env, JSON.parse, storage, postMessage, DB without typed client) is parsed with a schema before use. No `as T` on external data.

5. **Finish the job**: new code compiles under `tsc --noEmit`, passes lint, and ships with behavior-level tests (rules/07). Handle the error path of every async call — no floating promises.

6. When a requirement conflicts with a rule (e.g., legacy CJS, jest), follow the codebase and note the deviation; don't silently half-apply both.

## AUDIT mode (reviewing existing code)

Scope first (frontend? Node service? library?), then read the matching rules files and run their audit checklists — each ends with grep/eslint hunt patterns. Validate findings: confirm attacker-controlled data actually reaches the sink, confirm the perf issue is on a hot path. No speculative findings.

**Severity conventions:**

- **CRITICAL** — remotely exploitable now: untrusted input reaching eval/innerHTML/exec/SQL, auth bypass, secrets exfiltratable via XSS.

- **HIGH** — exploitable with conditions, or guaranteed-corruption bug class: missing boundary validation, prototype-pollution-prone merge of request data, floating promises dropping errors, tokens in localStorage, unbounded request bodies, float money math, sync crypto blocking the loop.

- **MEDIUM** — weakened posture or latent defect: missing strict tsconfig flags, no graceful shutdown, missing CSP/timeouts, `||` vs `??` on falsy-valid values, index keys on mutable lists, unbounded caches, missing supply-chain controls.

- **LOW** — hygiene/debt: dead deps, `hasOwnProperty`, console.log in services, snapshot sprawl, missing memoization on profiled-hot paths.

**Finding format:**

```

[SEVERITY] Title (CWE-xxx if security)

File: path/to/file.ts:42

Issue: what is wrong, in one or two sentences

Evidence: the offending snippet

Impact: what an attacker/user/operator experiences

Fix: concrete change (code if short)

```

Order the report CRITICAL→LOW, deduplicate repeated patterns into one finding with a file list, and end with the top 3 systemic recommendations (e.g., "enable noUncheckedIndexedAccess", "adopt MSW", "add zod to route boundaries").

## Rules index

| File | Read this when... |

|---|---|

| [rules/01-typescript-config-and-types.md](rules/01-typescript-config-and-types.md) | touching tsconfig; designing types/interfaces; seeing `any`/casts; modeling state; validating input shape; setting up a library or monorepo; deciding zod-vs-types questions |

| [rules/02-language-idioms.md](rules/02-language-idioms.md) | writing any JS/TS logic: equality, `??`/`?.`, array methods, immutability, Map/Set, error classes and Result types, generators, dates (Temporal), money/number precision |

| [rules/03-async-patterns.md](rules/03-async-patterns.md) | anything with promises/async: combinator choice, floating promises, AbortController/timeouts, event-loop ordering, top-level await, workers, streams, async race conditions |

| [rules/04-node-backend.md](rules/04-node-backend.md) | building/auditing Node services: runtime choice, dropping deps for built-ins, env config, HTTP hardening (timeouts/body limits), graceful shutdown, process error policy, pino |

| [rules/05-security.md](rules/05-security.md) | any security-relevant code or audit: XSS sinks, CSP, prototype pollution, npm supply chain, ReDoS, token storage/JWT, postMessage, child_process injection, SSRF |

| [rules/06-performance.md](rules/06-performance.md) | bundle size, React re-renders/keys/RSC, virtualization, debounce/throttle, memory leaks, Node event-loop blocking, profiling before optimizing |

For a full audit, read 01→07 in order; security-focused audits prioritize 05, 01, 03, 04.

## Top 10 non-negotiables

1. **`strict: true` + `noUncheckedIndexedAccess`** in every tsconfig; no `any`, no `@ts-ignore` — `unknown` + narrowing, `@ts-expect-error` with reason.

2. **Parse, don't cast, at boundaries**: zod/valibot on every HTTP body/query, env var, JSON.parse, webhook, postMessage payload. Types inside, schemas at the edge.

3. **No floating promises**: every promise awaited or explicitly `.catch`-handled; `@typescript-eslint/no-floating-promises` as error. `allSettled` for independent work; bounded concurrency.

4. **Discriminated unions for state**, exhaustive `switch` with `never` default — no boolean-soup interfaces with optional data/error pairs.

5. **`===` always; `??`/`?.` over `||`/`&&`** for null-handling; immutable updates (`toSorted`, `structuredClone`, spread) on shared data.

6. **Errors are `Error` subclasses with `cause`**; never throw strings; never swallow with empty catch; Node policy = log fatally then exit on uncaught.

7. **XSS sinks are forbidden by default**: no `innerHTML`/`dangerouslySetInnerHTML` with non-constant input unless DOMPurify-sanitized at render; no eval family ever; CSP on HTML responses.

8. **No secrets in localStorage; no shell interpolation**: httpOnly cookies for tokens; `execFile`/`spawn` array-args, never `exec` with template strings; parameterized SQL.

9. **Supply chain controlled**: committed lockfile + `npm ci`, install scripts disabled/allowlisted, update cooldown, minimal deps — prefer platform built-ins (fetch, node:test, crypto.randomUUID).

10. **AbortController + timeouts on all I/O**; never block the event loop (>50ms CPU → worker; no `*Sync` in request paths); listeners and intervals always cleaned up.