/sota-javascript-typescript

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 = last JS-based compiler; 7.0 Go-native stable since July 2026, shipped as the regular `typescript` package — frameworks embedding the compiler API via Volar, e.g. Vue/Angular/Astro/Svelte, stay on 6.0 until a stable plugin API lands), 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, **in-band sentinels (absence encoded as `-1`/`0`/`""`)** — **`-1` lies where `NaN` poisons**, 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 | | [rules/07-testing-and-tooling.md](r