fusengine/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