Typescript Best Practices

What it does

When your agent uses it

• Writing TypeScript/JavaScript files
• Tasks that involve React components

Example prompts

• “Use the typescript-best-practices skill to provide TypeScript patterns for type-first development, making illegal states unrepresentable, exhaustive…”
• “/typescript-best-practices”

Requirements

• A credential in API_KEY

Workflow steps

4 steps, taken from the first numbered list in SKILL.md.

1. 1Define the data model - types, interfaces, and schemas first
2. 2Define function signatures - input/output types before logic
3. 3Implement to satisfy types - let the compiler guide completeness
4. 4Validate at boundaries - runtime checks where data enters the system

What it can do on your machine

Read from SKILL.md and the folder at commit 31732e8. It shows what the files ask for, not the result of running them.

• Tool permissions

  Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

  From allowed-tools in the SKILL.md frontmatter.

• Runs code

  No scripts in the folder and no shell commands in SKILL.md (its code samples are typescript).

  From the folder's file list and the shell code blocks in SKILL.md.

• Network

  Links to these hosts (documentation or services it may open):

  • github.com

  From URLs in SKILL.md, links to its own repository left out.

• Credentials

  Names these keys or tokens, usually read from environment variables:

  • API_KEY

  From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Typescript Best Practices loads about 2.2k tokens when it runs. Until then it costs about 57 tokens; SKILL.md has 540 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it

~57

When it runs · the whole SKILL.md, loaded when a task matches

~2.2k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from bretzel-app/crumbs at commit 31732e8, republished under its MIT licence (© bretzel-app). 540 words, ~2,176 tokens.

name

typescript-best-practices

description

Provides TypeScript patterns for type-first development, making illegal states unrepresentable, exhaustive handling, and runtime validation. Must use when reading or writing TypeScript/JavaScript files.

// Good: only valid combinations possible
type RequestState<T> =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: T }
  | { status: 'error'; error: Error };

// Bad: allows invalid combinations like { loading: true, error: Error }
type RequestState<T> = {
  loading: boolean;
  data?: T;
  error?: Error;
};

type UserId = string & { readonly __brand: 'UserId' };
type OrderId = string & { readonly __brand: 'OrderId' };

// Compiler prevents passing OrderId where UserId expected
function getUser(id: UserId): Promise<User> { /* ... */ }

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

const ROLES = ['admin', 'user', 'guest'] as const;
type Role = typeof ROLES[number]; // 'admin' | 'user' | 'guest'

// Array and type stay in sync automatically
function isValidRole(role: string): role is Role {
  return ROLES.includes(role as Role);
}

// Creation: some fields required
type CreateUser = {
  email: string;
  name: string;
};

// Update: all fields optional
type UpdateUser = Partial<CreateUser>;

// Database row: all fields present
type User = CreateUser & {
  id: UserId;
  createdAt: Date;
};

export function buildWidget(widgetType: string): never {
  throw new Error(`buildWidget not implemented for type: ${widgetType}`);
}

type Status = "active" | "inactive";

export function processStatus(status: Status): string {
  switch (status) {
    case "active":
      return "processing";
    case "inactive":
      return "skipped";
    default: {
      const _exhaustive: never = status;
      throw new Error(`unhandled status: ${_exhaustive}`);
    }
  }
}

export async function fetchWidget(id: string): Promise<Widget> {
  const response = await fetch(`/api/widgets/${id}`);
  if (!response.ok) {
    throw new Error(`fetch widget ${id} failed: ${response.status}`);
  }
  return response.json();
}

import debug from "debug";

const log = debug("myapp:widgets");

export function createWidget(name: string): Widget {
  log("creating widget: %s", name);
  const widget = { id: crypto.randomUUID(), name };
  log("created widget: %s", widget.id);
  return widget;
}

import { z } from "zod";

const UserSchema = z.object({
  id: z.string().uuid(),
  email: z.string().email(),
  name: z.string().min(1),
  createdAt: z.string().transform((s) => new Date(s)),
});

type User = z.infer<typeof UserSchema>;

import { z, SafeParseReturnType } from "zod";

export function parseUserInput(raw: unknown): SafeParseReturnType<unknown, User> {
  return UserSchema.safeParse(raw);
}

// Caller handles both success and error:
const result = parseUserInput(formData);
if (!result.success) {
  setErrors(result.error.flatten().fieldErrors);
  return;
}
await submitUser(result.data);

export async function fetchUser(id: string): Promise<User> {
  const response = await fetch(`/api/users/${id}`);
  if (!response.ok) {
    throw new Error(`fetch user ${id} failed: ${response.status}`);
  }
  const data = await response.json();
  return UserSchema.parse(data); // throws if API contract violated
}

const CreateUserSchema = UserSchema.omit({ id: true, createdAt: true });
const UpdateUserSchema = CreateUserSchema.partial();
const UserWithPostsSchema = UserSchema.extend({
  posts: z.array(PostSchema),
});

import { z } from "zod";

const ConfigSchema = z.object({
  PORT: z.coerce.number().default(3000),
  DATABASE_URL: z.string().url(),
  API_KEY: z.string().min(1),
  NODE_ENV: z.enum(["development", "production", "test"]).default("development"),
});

export const config = ConfigSchema.parse(process.env);

import { config } from "./config";

const server = app.listen(config.PORT);
const db = connect(config.DATABASE_URL);

import type { Opaque, PartialDeep, SetRequired } from 'type-fest';

// Branded type (cleaner than manual approach)
type UserId = Opaque<string, 'UserId'>;

// Deep partial for patch operations
type UserPatch = PartialDeep<User>;

// Make specific fields required
type UserWithEmail = SetRequired<Partial<User>, 'email'>;

© bretzel-app, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

Just SKILL.md in .agents/skills/typescript-best-practices of bretzel-app/crumbs.

Open the folder on GitHubat commit 31732e8

Used in 1 other repository

We found 1 copy of this SKILL.md (exact, near-identical or edited) in other folders, from 1 other GitHub owner. This page covers the copy in bretzel-app/crumbs, which our catalogue first saw on October 7, 2026.

• NeverSight/learn-skills.devdata/skills-md/0xbigboss/claude-code/typescript-best-practices