TypeScript Ecosystem

function isCat(pet: Cat | Dog): pet is Cat { return (pet as Cat).meow !== undefined; } Does TypeScript need help narrowing union types? Implement custom type guard with is predicate Use built-in typeof or instanceof guards

function createUserId(id: string): UserId { return id as UserId; } Prevent mixing similar primitive types

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

const successful = results .filter((r): r is PromiseFulfilledResult<User> => r.status === "fulfilled") .map((r) => r.value);

for await (const item of paginate(fetchUsers)) { console.log(item); }

// Default export export default class Service {}

// Re-exports export { util } from "./util.js"; export type { UtilOptions } from "./util.js";

export default tseslint.config( eslint.configs.recommended, ...tseslint.configs.strictTypeChecked, { languageOptions: { parserOptions: { projectService: true, tsconfigRootDir: import.meta.dirname, }, }, } ); import.meta.dirname requires Node.js 20.11+ or 21.2+ (not available in Node.js 18 LTS)

Prettier configuration for TypeScript { "semi": true, "singleQuote": false, "tabWidth": 2, "trailingComma": "es5", "printWidth": 100 }

export default defineConfig({ test: { globals: true, environment: "node", coverage: { provider: "v8", reporter: ["text", "json", "html"], }, }, });

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

export default config;

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

type Status = (typeof Status)[keyof typeof Status];