ts-language-patterns

TypeScript Language Patterns (TS 6.0)

Agent Workflow (MANDATORY)

Before writing non-trivial TypeScript, use TeamCreate to spawn 3 agents:

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.

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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

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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 6.0 default; annotate or infer explicitly.

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Reference Guide

Concepts

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

Templates

Template	When to Use
modern-patterns.md	Copy-paste const/satisfies/decorator examples
resource-management.md	Copy-paste using / DisposableStack examples

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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`

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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