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.
<strict_options> <option name="strict">Enables all strict type-checking options</option> <option name="strictNullChecks">null and undefined handled explicitly</option> <option name="strictFunctionTypes">Stricter function type checking</option> <option name="strictBindCallApply">Check bind, call, apply methods</option> <option name="strictPropertyInitialization">Class properties must be initialized</option> <option name="noImplicitAny">Error on implicit any types</option> <option name="noImplicitReturns">Error on missing return statements</option> <option name="noImplicitThis">Error on implicit this</option> <option name="noUncheckedIndexedAccess">Add undefined to index signatures</option> <option name="noUnusedLocals">Error on unused local variables</option> <option name="noUnusedParameters">Error on unused parameters</option> </strict_options>
<module_resolution> <pattern name="nodenext"> <description>Modern Node.js ESM resolution (recommended)</description> <example> { "compilerOptions": { "module": "nodenext", "moduleResolution": "nodenext" } } </example> <note>Requires "type": "module" in package.json</note> </pattern>
<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 6.0, removed in TS 7.0; prefer paths without baseUrl</warning>
<warning>moduleResolution: "node" (alias "node10"): deprecated in TS 5.x; 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> <description>Monorepo and incremental builds</description> <example> { "compilerOptions": { "composite": true, "incremental": true, "tsBuildInfoFile": ".tsbuildinfo" }, "references": [ { "path": "../shared" }, { "path": "../core" } ] } </example> <note>Use tsc --build for incremental compilation</note> </project_references> </tsconfig>
<type_patterns> <utility_types> <type name="Partial<T>">Make all properties optional</type> <type name="Required<T>">Make all properties required</type> <type name="Readonly<T>">Make all properties readonly</type> <type name="Record<K,V>">Object type with key K and value V</type> <type name="Pick<T,K>">Select specific properties from T</type> <type name="Omit<T,K>">Exclude specific properties from T</type> <type name="Exclude<T,U>">Exclude types from union</type> <type name="Extract<T,U>">Extract types from union</type> <type name="NonNullable<T>">Remove null and undefined</type> <type name="ReturnType<T>">Get function return type</type> <type name="Parameters<T>">Get function parameter types as tuple</type> <type name="Awaited<T>">Unwrap Promise type</type> </utility_types>
<pattern name="constraints">
<description>Generic with type constraints</description>
<example>
function getProperty<T, K extends keyof T>(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<T = string> {
value: T;
}
</example>
</pattern>
<pattern name="multiple_constraints">
<description>Multiple generic parameters with constraints</description>
<example>
function merge<T extends object, U extends object>(a: T, b: U): T & U {
return { ...a, ...b };
}
</example>
</pattern>
<conditional_types> <pattern name="basic"> <description>Basic conditional type</description> <example> type IsString<T> = T extends string ? true : false; </example> </pattern>
<pattern name="infer">
<description>Extract types within conditional</description>
<example>
type UnwrapPromise<T> = T extends Promise<infer U> ? U : T;
type ArrayElement<T> = T extends (infer E)[] ? E : never;
</example>
</pattern>
<pattern name="distributive">
<description>Distributes over union types</description>
<example>
type ToArray<T> = T extends any ? T[] : never;
// ToArray<string | number> = string[] | number[]
</example>
</pattern>
</conditional_types>
<mapped_types> <pattern name="basic"> <description>Basic mapped type</description> <example> type Readonly<T> = { readonly [P in keyof T]: T[P]; }; </example> </pattern>
<pattern name="key_remapping">
<description>Map keys with renaming</description>
<example>
type Getters<T> = {
[K in keyof T as `get${Capitalize<string & K>}`]: () => T[K];
};
</example>
</pattern>
<pattern name="filtering">
<description>Filter properties by type</description>
<example>
type OnlyStrings<T> = {
[K in keyof T as T[K] extends string ? K : never]: T[K];
};
</example>
</pattern>
</mapped_types>
<template_literal_types>
<pattern name="basic">
<description>Template literal type construction</description>
<example>
type EventName = on${Capitalize<string>};
type Locale = ${Language}-${Country};
</example>
</pattern>
<pattern name="inference">
<description>Extract parameters from template literals</description>
<example>
type ExtractRouteParams<T> = T extends `${string}:${infer Param}/${infer Rest}`
? Param | ExtractRouteParams<Rest>
: T extends `${string}:${infer Param}`
? Param
: never;
</example>
</pattern>
</template_literal_types>
<type_guards> <pattern name="typeof"> <description>Built-in typeof type guard</description> <example> function process(value: string | number) { if (typeof value === "string") { return value.toUpperCase(); } return value.toFixed(2); } </example> </pattern>
<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> <pattern name="branded_primitives"> <description>Nominal typing via branding</description> <example> 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> <pattern name="type_checking_without_widening"> <description>Type checking without widening</description> <example> const config = { endpoint: "/api", timeout: 3000, } satisfies Record<string, string | number>; // config.endpoint is inferred as "/api" (literal), not string </example> </pattern> </satisfies_operator> </type_patterns>
<runtime_patterns> <error_handling> <pattern name="result_type"> <description>Rust-inspired Result type for error handling</description> <example> 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>
<async_patterns> <pattern name="promise_all"> <description>Parallel promise execution</description> <example> const [users, posts] = await Promise.all([ fetchUsers(), fetchPosts(), ]); </example> </pattern>
<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<User> => r.status === "fulfilled")
.map((r) => r.value);
</example>
</pattern>
<pattern name="async_iterator">
<description>Async generator for pagination</description>
<example>
async function* paginate<T>(fetchPage: (page: number) => Promise<T[]>) {
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> <pattern name="esm_exports"> <description>ES module export patterns</description> <example> // 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>
export default tseslint.config(
eslint.configs.recommended,
...tseslint.configs.strictTypeChecked,
{
languageOptions: {
parserOptions: {
projectService: true,
tsconfigRootDir: import.meta.dirname,
},
},
}
);
</example>
<note>import.meta.dirname requires Node.js 20.11+ or 21.2+ (not available in Node.js 18 LTS)</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>
export default defineConfig({
test: {
globals: true,
environment: "node",
coverage: {
provider: "v8",
reporter: ["text", "json", "html"],
},
},
});
</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: ["<rootDir>/src"],
moduleNameMapper: {
"^@/(.\*)$": "<rootDir>/src/$1",
},
};
export default config;
</example>
</pattern>
</jest>
<build_tools> <tsc> <tool name="tsc"> <description>TypeScript compiler commands</description> <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> </tool> </tsc>
<tsx>
<tool name="tsx">
<description>TypeScript execution with esbuild</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> </tooling>
<context7_integration> <library_id>/microsoft/typescript</library_id> <trust_score>9.9</trust_score> <snippets>16397</snippets>
<usage_pattern> <step>Resolve library ID if needed (already known: /microsoft/typescript)</step> <step>Fetch documentation with specific topic</step> <examples> <example topic="tsconfig">Configuration options and patterns</example> <example topic="generics">Generic type patterns</example> <example topic="utility types">Built-in utility types</example> <example topic="module resolution">Module resolution strategies</example> </examples> </usage_pattern>
<common_queries> <query topic="strict mode">Strict compiler options</query> <query topic="path mapping">Path aliases configuration</query> <query topic="declaration files">Type declaration generation</query> <query topic="nodenext">ESM support in Node.js</query> </common_queries> </context7_integration>
<anti_patterns> <avoid name="any_abuse"> <description>Overusing 'any' defeats type safety</description> <instead>Use 'unknown' and narrow with type guards</instead> </avoid>
type Status = (typeof Status)[keyof typeof Status];
</example>
<best_practices> <practice priority="critical">Enable strict mode in all projects</practice> <practice priority="critical">Use noUncheckedIndexedAccess for safer array/object access</practice> <practice priority="high">Prefer 'unknown' over 'any' for unknown types</practice> <practice priority="high">Use 'satisfies' to check types without widening</practice> <practice priority="high">Create branded types for domain primitives</practice> <practice priority="medium">Use Result types for error handling over exceptions</practice> <practice priority="medium">Keep type definitions close to usage</practice> <practice priority="medium">Export types separately with 'export type'</practice> <practice priority="medium">Use 'const' assertions for literal types</practice> <practice priority="medium">Prefer interfaces for public APIs, types for unions/utilities</practice> </best_practices>
<error_escalation> <level severity="low"> <example>Minor type inference issue</example> <action>Add explicit type annotation</action> </level> <level severity="medium"> <example>Type error in implementation</example> <action>Fix type, verify with tsc</action> </level> <level severity="high"> <example>Breaking type change in public API</example> <action>Stop, present migration options to user</action> </level> <level severity="critical"> <example>Type safety bypass with any or type assertion</example> <action>Block operation, require proper typing</action> </level> </error_escalation>
<related_skills> <skill name="serena-usage">Symbol-level navigation for type definitions and interfaces</skill> <skill name="context7-usage">Fetch latest TypeScript compiler and tooling documentation</skill> <skill name="investigation-patterns">Debug type errors and investigate compilation issues</skill> </related_skills>