/ts-language-patterns

name: ts-language-patterns
description: "Use when writing or reviewing modern TS syntax — const type parameters, using/await using, standard decorators, satisfies, or LLM anti-patterns (legacy enum/namespace). Not for tsconfig (ts-config)."
versions:
  typescript: "7.0"
user-invocable: false
references: references/resource-management.md, references/generics-and-inference.md, references/decorators.md, references/llm-pitfalls.md, references/templates/modern-patterns.md, references/templates/resource-management.md
related-skills: ts-config, solid-generic

<objective> This skill covers TypeScript 7.0 language features and idioms (7.0 keeps 6.0's type-checking behavior): const type parameters (<const T>) for narrowest-literal inference without as const, using/await using for deterministic Symbol.dispose/asyncDispose cleanup, standard ECMAScript decorators (never experimentalDecorators), satisfies for narrow-type validation, the 6.0 inference change making method-syntax callbacks order-independent, and 7.0's Unicode-aware template-literal inference.

It also covers fixing LLM-authored anti-patterns: legacy enum (un-erasable by Node's type stripper, replace with const objects as const), namespace wrapping runtime code, and missing import type under verbatimModuleSyntax.

Out of scope: tsconfig/compiler flags belong to ts-config; SOLID structure and file-size rules belong to solid-generic; framework-specific APIs are not covered. </objective>

TypeScript Language Patterns (TS 7.0)

Agent Workflow (MANDATORY)

Before writing non-trivial TypeScript, spawn 3 agents in parallel, one `Agent` call each with a `name`:

1. **fuse-ai-pilot:explore-codebase** - Detect existing idioms, `verbatimModuleSyntax`, tsconfig 2. **fuse-ai-pilot:research-expert** - Verify current syntax on typescriptlang.org 3. **mcp__context7__query-docs** - `/microsoft/typescript` for exact API shapes

After writing, run **fuse-ai-pilot:sniper** for validation.

---

Overview

| Feature | What it gives you | |---------|-------------------| | **`const` type parameters** | `<const T>` infers the narrowest literal type without `as const` at call sites | | **`using` / `await using`** | Deterministic cleanup via `Symbol.dispose` / `Symbol.asyncDispose` | | **Standard decorators** | ECMAScript decorators — NOT `experimentalDecorators` | | **`satisfies`** | Validate a value against a type while keeping its narrow inferred type | | **6.0 inference** | Method-syntax callbacks are no longer contextually sensitive → order-independent inference | | **7.0 template literals** | Inference splits on Unicode code points, not UTF-16 surrogate halves |

---

Critical Rules

1. **`import type` is mandatory under `verbatimModuleSyntax`** - type-only imports must say `type`, or emit/stripping breaks. 2. **No legacy `enum`** - prefer `const` objects `as const`; `enum` is un-erasable by Node's type stripper. 3. **No `namespace` with runtime code** - use ESM modules; type-only `namespace` is acceptable. 4. **Use standard decorators** - never enable `experimentalDecorators` for new code. 5. **Never rely on implicit `any`** - `strict` is the default since 6.0 (kept in 7.0); annotate or infer explicitly.

---

Reference Guide

Concepts

| Topic | Reference | When to Consult | |-------|-----------|-----------------| | **Resource management** | [resource-management.md](references/resource-management.md) | Load when using `using` / `await using` / disposables | | **Generics & inference** | [generics-and-inference.md](references/generics-and-inference.md) | Load when using `const` type params, `satisfies`, or debugging inference | | **Decorators** | [decorators.md](references/decorators.md) | Load when adding decorators | | **LLM pitfalls** | [llm-pitfalls.md](references/llm-pitfalls.md) | Load when reviewing or migrating legacy/AI-authored TypeScript |

Templates

| Template | When to Use | |----------|-------------| | [modern-patterns.md](references/templates/modern-patterns.md) | Copy-paste `const`/`satisfies`/decorator examples | | [resource-management.md](references/templates/resource-management.md) | Copy-paste `using` / `DisposableStack` examples |

---

Quick Reference

`const` type parameter

function first<const T extends readonly unknown[]>(arr: T): T[0] {
  return arr[0];
}
const x = first(["a", "b"]); // x: "a"  (literal, no `as const` needed)

`using` for deterministic cleanup

function openFile(path: string) {
  const handle = acquire(path);
  return { handle, [Symbol.dispose]() { release(handle); } };
}
{
  using file = openFile("./data"); // released automatically at block end
}

`satisfies`

const config = {
  port: 3000,
  host: "localhost",
} satisfies Record<string, string | number>;
// config.port stays `number`, not widened to `string | number`

---

Best Practices

DO

• Replace `enum Color { Red }` with `const Color = { Red: "red" } as const`
• Reach for `using` over manual `try/finally` cleanup when a resource has a disposer
• Use `satisfies` instead of a type annotation when you want to keep the narrow inferred type

DON'T

• Write `import { SomeType }` for a type-only import — use `import type { SomeType }`
• Enable `experimentalDecorators` or `emitDecoratorMetadata` for new code
• Use `namespace` to organize runtime code — that is what ESM modules are for