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name: typescript-ops description: "TypeScript type system, generics, utility types, strict mode, and ecosystem patterns. Use for: typescript, ts, type, generic, utility type, Partial, Pick, Omit, Record, Exclude, Extract, ReturnType, Parameters, keyof, typeof, infer, mapped type, conditional type, template literal type, discriminated union, type guard, type assertion, type narrowing, tsconfig, strict mode, declaration file, zod, valibot, typescript 7, tsgo, native compiler." license: MIT allowed-tools: "Read Write Bash" metadata: author: claude-mods related-skills: react-ops, testing-ops

TypeScript Operations

Comprehensive TypeScript skill covering the type system, generics, and production patterns.

Ecosystem facts verified as of 2026-08-08 (TypeScript 7, Zod 4, Valibot 1).

Staleness check: python scripts/check-typescript-facts.py --offline asserts the catalogued version-bearing facts (TypeScript major, zod, valibot) are still named in the prose and the dated currency note above is present; run --live to confirm each package's npm major still matches the documented major. Catalog: assets/typescript-facts.json.

Type Narrowing Decision Tree

How to narrow a type?
│
├─ Primitive type check
│  └─ typeof: typeof x === "string"
│
├─ Instance check
│  └─ instanceof: x instanceof Date
│
├─ Property existence
│  └─ in: "email" in user
│
├─ Discriminated union
│  └─ switch on literal field: switch (event.type)
│
├─ Null/undefined check
│  └─ Truthiness: if (x) or if (x != null)
│
├─ Custom logic
│  └─ Type predicate: function isUser(x: unknown): x is User
│
└─ Assertion (you know better than TS)
   └─ as: value as string (escape hatch, avoid when possible)

Type Guard Example

interface Dog { bark(): void; breed: string }
interface Cat { meow(): void; color: string }

function isDog(pet: Dog | Cat): pet is Dog {
    return "bark" in pet;
}

function handlePet(pet: Dog | Cat) {
    if (isDog(pet)) {
        pet.bark(); // TS knows it's Dog here
    } else {
        pet.meow(); // TS knows it's Cat here
    }
}

Discriminated Unions

type Result<T> =
    | { status: "success"; data: T }
    | { status: "error"; error: string }
    | { status: "loading" };

function handle<T>(result: Result<T>) {
    switch (result.status) {
        case "success": return result.data;     // data is available
        case "error":   throw new Error(result.error); // error is available
        case "loading": return null;
    }
    // Exhaustiveness check: result is `never` here
    const _exhaustive: never = result;
}

Utility Types Cheat Sheet

Utility	What It Does	Example
Partial<T>	All props optional	Partial<User> for update payloads
Required<T>	All props required	Required<Config> for validated config
Readonly<T>	All props readonly	Readonly<State> for immutable state
Pick<T, K>	Select specific props	Pick<User, "id" | "name">
Omit<T, K>	Remove specific props	Omit<User, "password">
Record<K, V>	Object with typed keys/values	Record<string, number>
Exclude<U, E>	Remove types from union	Exclude<Status, "deleted">
Extract<U, E>	Keep types from union	Extract<Event, { type: "click" }>
NonNullable<T>	Remove null/undefined	NonNullable<string | null>
ReturnType<F>	Function return type	ReturnType<typeof fetchUser>
Parameters<F>	Function params as tuple	Parameters<typeof createUser>
Awaited<T>	Unwrap Promise type	Awaited<Promise<User>> = User

Generic Patterns

Constrained Generics

// Basic constraint
function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
    return obj[key];
}

// Multiple constraints
function merge<T extends object, U extends object>(a: T, b: U): T & U {
    return { ...a, ...b };
}

// Default generic type
type ApiResponse<T = unknown> = {
    data: T;
    status: number;
};

Conditional Types

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

// infer keyword - extract inner type
type UnwrapPromise<T> = T extends Promise<infer U> ? U : T;
type UnwrapArray<T> = T extends (infer U)[] ? U : T;

// Distributive conditional (distributes over union)
type ToArray<T> = T extends any ? T[] : never;
// ToArray<string | number> = string[] | number[]

// Prevent distribution with wrapping
type ToArrayNonDist<T> = [T] extends [any] ? T[] : never;
// ToArrayNonDist<string | number> = (string | number)[]

Mapped Types

// Make all properties optional and nullable
type Nullable<T> = { [K in keyof T]: T[K] | null };

// Add prefix to keys
type Prefixed<T, P extends string> = {
    [K in keyof T as `${P}${Capitalize<string & K>}`]: T[K];
};
// Prefixed<{ name: string }, "get"> = { getName: string }

// Filter keys by value type
type StringKeys<T> = {
    [K in keyof T as T[K] extends string ? K : never]: T[K];
};

Deep dive: Load ./references/generics-patterns.md for advanced type-level programming, recursive types, template literal types.

Modern Language Features (TypeScript 5.x → 6.0)

Feature	Since	What It Gives You
satisfies operator	4.9	Check a value against a type without widening it
Standard (TC39) decorators	5.0	@decorator on classes/methods without experimentalDecorators
const type parameters	5.0	function f<const T>(x: T) infers literal types without as const at call sites
using declarations	5.2	Explicit resource management (Symbol.dispose), auto-cleanup at scope exit
Inferred type predicates	5.5	arr.filter(x => x !== null) narrows without a hand-written x is T guard
verbatimModuleSyntax	5.0	Enforces import type for type-only imports — replaces importsNotUsedAsValues

// const type parameters (5.0) - literal inference without as const
function routes<const T extends readonly string[]>(paths: T): T { return paths; }
const r = routes(["/home", "/about"]); // readonly ["/home", "/about"], not string[]

// using declarations (5.2) - deterministic cleanup
function readConfig() {
    using file = openFile("config.json"); // file[Symbol.dispose]() runs at scope exit
    return parse(file.contents);
}

// Inferred type predicates (5.5) - no manual guard needed
const names = ["a", null, "b"].filter(x => x !== null); // string[], not (string | null)[]

TypeScript 6.0 (The Bridge Release)

TS 6.0 is the last release on the JavaScript-based compiler — it exists to bridge to the native (Go) compiler in TS 7, so its headline is stricter, modernised defaults:

• strict: true is the default — a tsconfig that never set it now gets full strict checks
• Defaults modernised: module: esnext, target: es2025; es2025 lib ships types for Temporal, Map.getOrInsert, RegExp.escape
• Legacy options removed: moduleResolution: classic; module: amd/umd/system/none; minimum target is now ES2015 (es5 deprecated)
• Interop always on: esModuleInterop / allowSyntheticDefaultImports can no longer be disabled
• New --stableTypeOrdering flag eases 6.0 → 7.0 migration diffing

TypeScript 7 (Current Major — Native Compiler)

TS 7 is the Go-native rewrite (formerly tsgo), stable on npm since 2026-07-08. Same checking semantics, ~8–16× faster typechecks — but the package ships only a bin/tsc shim, no JavaScript compiler API: require('typescript') throws MODULE_NOT_FOUND on 7.0.x (the API returns in 7.1+). Consequences that gate adoption:

• Repo tooling using the TS programmatic API (ts.createSourceFile, custom lint scripts, codemods, typescript-eslint) must go AST-free, switch parser, or pin an alias
• Tools typechecking embedded languages (vue-tsc, svelte-check, Astro, MDX) are pinned to TS 6 until they port to the 7.1+ API — a real stack-selection input
• A tsconfig already on TS 6 defaults adopts directly (the 6.0 bridge is skippable); baseUrl is a hard error (TS5102)
• Running a typescript5 fallback alias alongside 7 makes bare npx tsc ambiguous — scripts must use explicit compiler paths during the soak

Deep dive: Load ./references/ts7-native-compiler.md for the no-JS-API workarounds, dual-install bin ambiguity, ecosystem lockout table, measured adoption benchmarks (12.1×), and the go/no-go checklist.

tsconfig Quick Reference

{
    "compilerOptions": {
        // Strict mode (default in TS 6; state it explicitly anyway)
        "strict": true,               // Enables all strict checks
        "noUncheckedIndexedAccess": true,  // arr[0] is T | undefined

        // Module system (TS 6 defaults to module: esnext; interop is always on)
        "module": "esnext",           // or "nodenext" for Node
        "moduleResolution": "bundler", // or "nodenext"

        // Output (TS 6 defaults target to es2025; min supported is es2015)
        "target": "es2022",
        "outDir": "dist",
        "declaration": true,          // Generate .d.ts
        "sourceMap": true,

        // Paths — tsconfig-relative; don't add baseUrl (TS 7 hard-errors on it, TS5102)
        "paths": { "@/*": ["./src/*"] },

        // Strictness extras
        "noUnusedLocals": true,
        "noUnusedParameters": true,
        "noFallthroughCasesInSwitch": true,
        "forceConsistentCasingInFileNames": true
    },
    "include": ["src"],
    "exclude": ["node_modules", "dist"]
}

Deep dive: Load ./references/config-strict.md for strict mode migration, monorepo config, project references.

Common Gotchas

Gotcha	Why	Fix
any leaks	any disables type checking for everything it touches	Use unknown + narrowing instead
as assertions hide bugs	Assertion doesn't check at runtime	Use type guards or validation (Zod)
enum quirks	Numeric enums are not type-safe, reverse mappings confuse	Use as const objects or string literal unions
object vs Record vs {}	{} matches any non-null value, object is non-primitive	Use Record<string, unknown> for "any object"
Array index access	arr[999] returns T not T | undefined by default	Enable noUncheckedIndexedAccess
Optional vs undefined	{ x?: string } allows missing key, { x: string | undefined } requires key	Be explicit about which you mean
! non-null assertion	Silences null checks, no runtime effect	Use ?? defaultValue or proper null check
Structural typing surprise	{ a: 1, b: 2 } assignable to { a: number }	Use branded types for nominal typing

Branded / Nominal Types

// Prevent accidentally mixing types that are structurally identical
type UserId = string & { readonly __brand: "UserId" };
type OrderId = string & { readonly __brand: "OrderId" };

function createUserId(id: string): UserId { return id as UserId; }

function getUser(id: UserId) { /* ... */ }

const userId = createUserId("u-123");
const orderId = "o-456" as OrderId;

getUser(userId);   // OK
getUser(orderId);  // Error: OrderId not assignable to UserId

Runtime Validation (Zod 4)

import { z } from "zod";

// Define schema (Zod 4: string formats are top-level - z.email(), not z.string().email())
const UserSchema = z.object({
    id: z.number(),
    name: z.string().min(1),
    email: z.email(),
    role: z.enum(["admin", "user"]),
    settings: z.object({
        theme: z.enum(["light", "dark"]).default("light"),
    }).optional(),
});

// Infer type from schema
type User = z.infer<typeof UserSchema>;

// Validate
const user = UserSchema.parse(untrustedData);       // throws on invalid
const result = UserSchema.safeParse(untrustedData);  // returns { success, data/error }

Zod 4 changes to know (if you learned Zod 3): string formats moved to the top level (z.email(), z.uuid(), z.url() — the z.string().email() method form is deprecated); error customisation unified under a single error param (invalid_type_error / required_error dropped); much faster parsing and a tree-shakeable zod/mini entry point.

Reference Files

Load these for deep-dive topics. Each is self-contained.

Reference	When to Load
./references/type-system.md	Advanced types, branded types, type-level programming, satisfies operator
./references/generics-patterns.md	Generic constraints, conditional types, mapped types, template literals, recursive types
./references/utility-types.md	All built-in utility types with examples, custom utility types
./references/config-strict.md	tsconfig deep dive, strict mode migration, project references, monorepo setup
./references/ts7-native-compiler.md	Adopting the TS 7 native (Go) compiler: no JS API, bin ambiguity, ecosystem lockout, benchmarked go/no-go
./references/ecosystem.md	Zod/Valibot, type-safe API clients, ORM types, testing with Vitest

See Also

• testing-ops - Cross-language testing strategies
• ci-cd-ops - TypeScript CI pipelines, type checking in CI