Agent Skills: TypeScript Operations

TypeScript Operations

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

Ecosystem facts verified as of 2026-07-05 (TypeScript 6, 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 (Current Major)

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

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
        "baseUrl": ".",
        "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/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