typescript-ecosystem

name	TypeScript Ecosystem
description	This skill should be used when the user asks to "write typescript", "typescript config", "tsconfig", "type definition", "generics", "utility types", or works with TypeScript language patterns and configuration. Provides comprehensive TypeScript ecosystem patterns and best practices.
version	2.0.0

Provide comprehensive patterns for TypeScript language, configuration, type system, and tooling integration. Read - Analyze tsconfig.json and TypeScript source files Edit - Modify TypeScript configurations and source code Bash - Run tsc, tsx, eslint, and build commands mcp__context7__get-library-docs - Fetch latest TypeScript documentation Enable all strict checking options (strict: true) for maximum type safety nodenext for ESM Node.js, bundler for Vite/esbuild/webpack projects Built-in generic types: Partial, Required, Pick, Omit, Record, Extract, Exclude, ReturnType Use type guards (typeof, instanceof, in, custom predicates) to safely narrow union types Node.js version-specific recommended configurations Active LTS - use ES2024 for stable features Current release - use ES2025 for latest features { "compilerOptions": { "target": "ES2024", "lib": ["ES2024"], "module": "nodenext", "moduleResolution": "nodenext", "strict": true, "verbatimModuleSyntax": true, "skipLibCheck": true, "declaration": true, "declarationMap": true, "sourceMap": true, "outDir": "./dist", "rootDir": "./src" }, "include": ["src"], "exclude": ["node_modules", "dist"] } Node.js 24 Active LTS - use ES2024 target/lib for stable features { "compilerOptions": { "target": "ES2025", "lib": ["ES2025"], "module": "nodenext", "moduleResolution": "nodenext", "strict": true, "verbatimModuleSyntax": true, "skipLibCheck": true, "declaration": true, "declarationMap": true, "sourceMap": true, "outDir": "./dist", "rootDir": "./src" }, "include": ["src"], "exclude": ["node_modules", "dist"] } Node.js 26 current release - use ES2025 target/lib for latest features

<strict_options> Enables all strict type-checking options null and undefined handled explicitly Stricter function type checking Check bind, call, apply methods Class properties must be initialized Error on implicit any types Error on missing return statements Error on implicit this Add undefined to index signatures Error on unused local variables Error on unused parameters </strict_options>

<module_resolution> Modern Node.js ESM resolution (recommended) { "compilerOptions": { "module": "nodenext", "moduleResolution": "nodenext" } } Requires "type": "module" in package.json

<pattern name="bundler">
  <description>For projects using bundlers (Vite, esbuild, webpack)</description>
  <example>
    {
    "compilerOptions": {
    "module": "esnext",
    "moduleResolution": "bundler"
    }
    }
  </example>
</pattern>

<pattern name="path_aliases">
  <description>Import path aliases</description>
  <example>
    {
    "compilerOptions": {
    "baseUrl": ".",
    "paths": {
    "@/*": ["src/*"],
    "@components/*": ["src/components/*"]
    }
    }
    }
  </example>
  <warning>baseUrl: deprecated in TS 6.x and expected to be removed in TS 7.x; prefer paths without baseUrl</warning>
  <warning>moduleResolution: "node" (alias "node10"): removed in TS 6.0; use "nodenext" or "bundler"</warning>
  <decision_tree name="when_to_use">
    <question>Are you using a bundler or working with Node.js modules?</question>
    <if_yes>Configure appropriate moduleResolution: bundler for bundlers, nodenext for Node.js</if_yes>
    <if_no>Stick with default module resolution for simple projects</if_no>
  </decision_tree>
</pattern>

</module_resolution>

<project_references> Monorepo and incremental builds { "compilerOptions": { "composite": true, "incremental": true, "tsBuildInfoFile": ".tsbuildinfo" }, "references": [ { "path": "../shared" }, { "path": "../core" } ] } Use tsc --build for incremental compilation </project_references>

<native_typescript> Node.js 24+ runs TypeScript natively without flags or external runners. Node.js 22.6+ required --experimental-strip-types. Run .ts files directly with Node.js 24+ (no tsx/ts-node needed) node src/index.ts Node.js strips type annotations at runtime. Code must be "erasable syntax only" — no enums, namespaces, or parameter properties with runtime semantics. Enable erasableSyntaxOnly in tsconfig to ensure compatibility with Node.js native execution { "compilerOptions": { "erasableSyntaxOnly": true } } Errors on constructs that Node.js cannot strip (const enums, namespaces with runtime code, legacy parameter properties). </native_typescript>

<type_patterns> <utility_types> Make all properties optional Make all properties required Make all properties readonly Object type with key K and value V Select specific properties from T Exclude specific properties from T Exclude types from union Extract types from union Remove null and undefined Get function return type Get function parameter types as tuple Unwrap Promise type </utility_types>

Basic generic function function identity<T>(arg: T): T { return arg; }

<pattern name="constraints">
  <description>Generic with type constraints</description>
  <example>
    function getProperty&lt;T, K extends keyof T&gt;(obj: T, key: K): T[K] {
    return obj[key];
    }
  </example>
</pattern>

<pattern name="default_type">
  <description>Generic with default type parameter</description>
  <example>
    interface Container&lt;T = string&gt; {
    value: T;
    }
  </example>
</pattern>

<pattern name="multiple_constraints">
  <description>Multiple generic parameters with constraints</description>
  <example>
    function merge&lt;T extends object, U extends object&gt;(a: T, b: U): T &amp; U {
    return { ...a, ...b };
    }
  </example>
</pattern>

<conditional_types> Basic conditional type type IsString<T> = T extends string ? true : false;

<pattern name="infer">
  <description>Extract types within conditional</description>
  <example>
    type UnwrapPromise&lt;T&gt; = T extends Promise&lt;infer U&gt; ? U : T;
    type ArrayElement&lt;T&gt; = T extends (infer E)[] ? E : never;
  </example>
</pattern>

<pattern name="distributive">
  <description>Distributes over union types</description>
  <example>
    type ToArray&lt;T&gt; = T extends any ? T[] : never;
    // ToArray&lt;string | number&gt; = string[] | number[]
  </example>
</pattern>

</conditional_types>

<mapped_types> Basic mapped type type Readonly<T> = { readonly [P in keyof T]: T[P]; };

<pattern name="key_remapping">
  <description>Map keys with renaming</description>
  <example>
    type Getters&lt;T&gt; = {
    [K in keyof T as `get${Capitalize&lt;string &amp; K&gt;}`]: () =&gt; T[K];
    };
  </example>
</pattern>

<pattern name="filtering">
  <description>Filter properties by type</description>
  <example>
    type OnlyStrings&lt;T&gt; = {
    [K in keyof T as T[K] extends string ? K : never]: T[K];
    };
  </example>
</pattern>

</mapped_types>

<template_literal_types> Template literal type construction type EventName = on${Capitalize<string>}; type Locale = ${Language}-${Country};

<pattern name="inference">
  <description>Extract parameters from template literals</description>
  <example>
    type ExtractRouteParams&lt;T&gt; = T extends `${string}:${infer Param}/${infer Rest}`
    ? Param | ExtractRouteParams&lt;Rest&gt;
    : T extends `${string}:${infer Param}`
    ? Param
    : never;
  </example>
</pattern>

</template_literal_types>

<type_guards> Built-in typeof type guard function process(value: string | number) { if (typeof value === "string") { return value.toUpperCase(); } return value.toFixed(2); }

<pattern name="instanceof">
  <description>Built-in instanceof type guard</description>
  <example>
    function handle(error: Error | string) {
    if (error instanceof Error) {
    return error.message;
    }
    return error;
    }
  </example>
</pattern>

<pattern name="custom">
  <description>Custom type guard function</description>
  <example>
    interface Cat { meow(): void; }
    interface Dog { bark(): void; }

    function isCat(pet: Cat | Dog): pet is Cat {
    return (pet as Cat).meow !== undefined;
    }
  </example>
  <decision_tree name="when_to_use">
    <question>Does TypeScript need help narrowing union types?</question>
    <if_yes>Implement custom type guard with is predicate</if_yes>
    <if_no>Use built-in typeof or instanceof guards</if_no>
  </decision_tree>
</pattern>

<pattern name="in_operator">
  <description>Property existence type guard</description>
  <example>
    function move(animal: Fish | Bird) {
    if ("swim" in animal) {
    animal.swim();
    } else {
    animal.fly();
    }
    }
  </example>
</pattern>

</type_guards>

<branded_types> Nominal typing via branding type UserId = string & { readonly **brand: unique symbol }; type OrderId = string & { readonly **brand: unique symbol };

    function createUserId(id: string): UserId {
    return id as UserId;
    }
  </example>
  <note>Prevent mixing similar primitive types</note>
</pattern>

</branded_types>

<satisfies_operator> Type checking without widening const config = { endpoint: "/api", timeout: 3000, } satisfies Record<string, string | number>; // config.endpoint is inferred as "/api" (literal), not string </satisfies_operator> </type_patterns>

<runtime_patterns> <error_handling> Rust-inspired Result type for error handling type Result<T, E = Error> = | { success: true; data: T } | { success: false; error: E };

    function parseJson<T>(json: string): Result<T> {
    try {
    return { success: true, data: JSON.parse(json) };
    } catch (e) {
    return { success: false, error: e as Error };
    }
    }
  </example>
  <decision_tree name="when_to_use">
    <question>Do you need explicit error handling without exceptions?</question>
    <if_yes>Use Result type for functional error handling</if_yes>
    <if_no>Use try-catch for traditional exception handling</if_no>
  </decision_tree>
</pattern>

<pattern name="custom_errors">
  <description>Custom error classes with additional context</description>
  <example>
    class ValidationError extends Error {
    constructor(
    message: string,
    public readonly field: string,
    public readonly code: string
    ) {
    super(message);
    this.name = "ValidationError";
    }
    }
  </example>
</pattern>

<pattern name="error_cause">
  <description>Error chaining with cause (ES2022+)</description>
  <example>
    try {
    await fetchData();
    } catch (e) {
    throw new Error("Failed to fetch data", { cause: e });
    }
  </example>
</pattern>

</error_handling>

<resource_management> Explicit Resource Management with using/await using (TC39 Stage 3, TS 5.2+) function readFile(path: string) { using file = openFile(path); return file.read(); // fileSymbol.dispose called automatically }

    async function withConnection() {
    await using conn = await getConnection();
    return conn.query("SELECT 1");
    // conn[Symbol.asyncDispose]() called automatically
    }
  </example>
  <note>Requires "lib": ["ESNext.Disposable"] or target ES2024+. Use DisposableStack for managing multiple resources.</note>
</pattern>

</resource_management>

<async_patterns> Parallel promise execution const [users, posts] = await Promise.all([ fetchUsers(), fetchPosts(), ]);

<pattern name="promise_allSettled">
  <description>Handle mixed success/failure</description>
  <example>
    const results = await Promise.allSettled([
    fetchUser(1),
    fetchUser(2),
    fetchUser(3),
    ]);

    const successful = results
    .filter((r): r is PromiseFulfilledResult&lt;User&gt; =&gt; r.status === "fulfilled")
    .map((r) =&gt; r.value);
  </example>
</pattern>

<pattern name="async_iterator">
  <description>Async generator for pagination</description>
  <example>
    async function* paginate&lt;T&gt;(fetchPage: (page: number) =&gt; Promise&lt;T[]&gt;) {
    let page = 0;
    while (true) {
    const items = await fetchPage(page++);
    if (items.length === 0) break;
    yield* items;
    }
    }

    for await (const item of paginate(fetchUsers)) {
    console.log(item);
    }
  </example>
</pattern>

</async_patterns>

<module_patterns> ES module export patterns // Named exports export const helper = () => {}; export type Config = { /* ... _/ };

    // Default export
    export default class Service {}

    // Re-exports
    export { util } from "./util.js";
    export type { UtilOptions } from "./util.js";
  </example>
</pattern>

<pattern name="barrel_exports">
  <description>index.ts for clean imports</description>
  <example>
    // src/components/index.ts
    export { Button } from "./Button.js";
    export { Input } from "./Input.js";
    export type { ButtonProps, InputProps } from "./types.js";
  </example>
  <warning>Can impact tree-shaking; use sparingly</warning>
</pattern>

<pattern name="dynamic_import">
  <description>Code splitting with dynamic imports</description>
  <example>
    const module = await import("./heavy-module.js");
    module.doSomething();
  </example>
</pattern>

</module_patterns> </runtime_patterns>

ESLint 10 with TypeScript (flat config only; eslintrc fully removed in v10) // eslint.config.js import eslint from "@eslint/js"; import tseslint from "typescript-eslint"; import { defineConfig, globalIgnores } from "eslint/config";

    export default defineConfig([
    globalIgnores(["dist/", "node_modules/"]),
    eslint.configs.recommended,
    ...tseslint.configs.strictTypeChecked,
    {
    languageOptions: {
    parserOptions: {
    projectService: true,
    tsconfigRootDir: import.meta.dirname,
    },
    },
    },
    ]);
  </example>
  <note>ESLint 10 locates eslint.config.* from each linted file's directory, not cwd. defineConfig() and globalIgnores() provide type-safe config composition.</note>
</pattern>

<key_rules>
  <rule name="@typescript-eslint/no-explicit-any">Prefer unknown over any</rule>
  <rule name="@typescript-eslint/no-unused-vars">Detect unused variables</rule>
  <rule name="@typescript-eslint/strict-boolean-expressions">Require explicit boolean conditions</rule>
  <rule name="@typescript-eslint/no-floating-promises">Require awaiting promises</rule>
  <rule name="@typescript-eslint/prefer-nullish-coalescing">Use ?? over ||</rule>
</key_rules>

Prettier configuration for TypeScript { "semi": true, "singleQuote": false, "tabWidth": 2, "trailingComma": "es5", "printWidth": 100 } Biome — unified fast linter and formatter (alternative to ESLint+Prettier) // biome.json { "$schema": "https://biomejs.dev/schemas/2.0/schema.json", "linter": { "enabled": true, "rules": { "recommended": true } }, "formatter": { "enabled": true, "indentStyle": "space", "indentWidth": 2 } } Biome is significantly faster than ESLint+Prettier. Supports JS, TS, JSX, TSX, JSON, CSS. Use when unified tooling and speed are priorities. Vitest 4.x - fast test runner with stable Browser Mode (requires Vite >= 6.0, Node.js >= 20) // vitest.config.ts import { defineConfig } from "vitest/config";

      export default defineConfig({
      test: {
      globals: true,
      environment: "node",
      coverage: {
      provider: "v8",
      reporter: ["text", "json", "html"],
      },
      },
      });
    </example>
    <note>Vitest 4 removed the experimental tag from Browser Mode. Visual regression testing available via @vitest/browser-playwright.</note>
  </pattern>
  <pattern name="workspace">
    <description>Vitest workspace for monorepo testing</description>
    <example>
      // vitest.workspace.ts
      import { defineWorkspace } from "vitest/config";

      export default defineWorkspace([
      "packages/*/vitest.config.ts",
      ]);
    </example>
  </pattern>
</vitest>

<jest>
  <pattern name="config">
    <description>Jest configuration for TypeScript</description>
    <example>
      // jest.config.ts
      import type { Config } from "jest";

      const config: Config = {
      preset: "ts-jest",
      testEnvironment: "node",
      roots: ["&lt;rootDir&gt;/src"],
      moduleNameMapper: {
      "^@/(.\*)$": "&lt;rootDir&gt;/src/$1",
      },
      };

      export default config;
    </example>
  </pattern>
</jest>

<build_tools> TypeScript compiler commands <use_case name="compile">tsc - Compile TypeScript</use_case> <use_case name="build">tsc --build - Incremental build (monorepo)</use_case> <use_case name="check">tsc --noEmit - Type check only</use_case> <use_case name="watch">tsc --watch - Watch mode</use_case>

<tsx>
  <tool name="tsx">
    <description>TypeScript execution with esbuild (fallback for Node.js &lt; 24 or code using non-erasable syntax)</description>
    <use_case name="run">tsx src/index.ts - Run TypeScript directly</use_case>
    <use_case name="watch">tsx watch src/index.ts - Watch mode</use_case>
  </tool>
</tsx>

<tsup>
  <pattern name="config">
    <description>Bundle TypeScript libraries</description>
    <example>
      // tsup.config.ts
      import { defineConfig } from "tsup";

      export default defineConfig({
      entry: ["src/index.ts"],
      format: ["cjs", "esm"],
      dts: true,
      clean: true,
      sourcemap: true,
      });
    </example>
  </pattern>
</tsup>

</build_tools>

<typescript_versions> <current_stable> TypeScript 6.0 Final JavaScript-based compiler release. Bridge release between TS 5.x and 7.0. No 6.1 planned. npm install -D typescript </current_stable> TypeScript 7.0 Go-native compiler rewrite with up to 10x performance improvement. New language service with ~8x project load time reduction. npm install -D @typescript/native-preview Near completion; nightly preview available. TS 7.0 is the future of TypeScript. </typescript_versions>

<context7_integration> <library_id>/microsoft/typescript</library_id> <trust_score>9.9</trust_score> 16397

<usage_pattern> Resolve library ID if needed (already known: /microsoft/typescript) Workflow guidance Step completed Fetch documentation with specific topic Workflow guidance Step completed Configuration options and patterns Generic type patterns Built-in utility types Module resolution strategies Go-native compiler migration </usage_pattern>

<common_queries> Strict compiler options Path aliases configuration Type declaration generation ESM support in Node.js </common_queries> </context7_integration>

<best_practices> Enable strict mode in all projects Use noUncheckedIndexedAccess for safer array/object access Prefer 'unknown' over 'any' for unknown types Use 'satisfies' to check types without widening Create branded types for domain primitives Use TS 6.x as the current baseline in this repository and prepare for TS 7.0 migration Use Result types for error handling over exceptions Keep type definitions close to usage Export types separately with 'export type' Use 'const' assertions for literal types Prefer interfaces for public APIs, types for unions/utilities </best_practices>

<anti_patterns> Overusing 'any' defeats type safety Use 'unknown' and narrow with type guards

Excessive 'as' casts bypass type checking Use type guards or proper typing Missing type annotations with noImplicitAny disabled Enable strict mode, add explicit types Barrel files can hurt tree-shaking Use direct imports for large modules Enums have runtime overhead and quirks Use const objects with 'as const' const Status = { Active: "active", Inactive: "inactive", } as const;

  type Status = (typeof Status)[keyof typeof Status];
</example>

Namespaces are legacy, use ES modules Use regular imports/exports Decorators add complexity; use judiciously even though TC39 decorators are now stable (since TS 5.0) Prefer composition and higher-order functions for simpler cases eslintrc configuration format is fully removed in ESLint 10 Use flat config with eslint.config.js and defineConfig() Enable strict mode in all TypeScript projects Never use any without documented justification; prefer unknown Run tsc --noEmit before committing to catch type errors Use noUncheckedIndexedAccess for safer array/object access Export types separately with 'export type' for clarity Define types before implementation for better design Use satisfies operator to check types without widening Understand TypeScript code requirements 1. Check tsconfig.json for project settings Workflow guidance Step completed 2. Review existing type patterns in project Workflow guidance Step completed 3. Identify type dependencies and imports Workflow guidance Step completed Write type-safe TypeScript code 1. Define types before implementation Workflow guidance Step completed 2. Use strict type checking features Workflow guidance Step completed 3. Follow project naming conventions Workflow guidance Step completed Verify TypeScript correctness 1. Run tsc --noEmit for type checking Workflow guidance Step completed 2. Check with ESLint for style issues Workflow guidance Step completed 3. Verify tests pass Workflow guidance Step completed

<error_escalation> Minor type inference issue Add explicit type annotation Type error in implementation Fix type, verify with tsc Breaking type change in public API Stop, present migration options to user Type safety bypass with any or type assertion Block operation, require proper typing </error_escalation>

Enable strict mode in tsconfig.json Define explicit types for public APIs Use type guards for runtime type checking Using any type without justification Type assertions without validation Ignoring TypeScript errors with ts-ignore

<related_skills> Symbol-level navigation for type definitions and interfaces Fetch latest TypeScript compiler and tooling documentation Debug type errors and investigate compilation issues </related_skills>