function groupBy<T, K extends string | number>( items: T[], keyFn: (item: T) => K ): Record<K, T[]> { return items.reduce((acc, item) => { const key = keyFn(item); (acc[key] ??= []).push(item); return acc; }, {} as Record<K, T[]>); }

type ApiResponse = { data: T; meta: { page: number; total: number }; };

async function fetchApi(url: string): Promise<ApiResponse> { const res = await fetch(url); return res.json(); }


## Conditional Types

```typescript
type IsString<T> = T extends string ? true : false;

type Flatten<T> = T extends Array<infer U> ? U : T;

type UnwrapPromise<T> = T extends Promise<infer U> ? UnwrapPromise<U> : T;

type FunctionReturn<T> = T extends (...args: any[]) => infer R ? R : never;

type ExtractRouteParams<T extends string> =
  T extends `${string}:${infer Param}/${infer Rest}`
    ? Param | ExtractRouteParams<Rest>
    : T extends `${string}:${infer Param}`
      ? Param
      : never;

type Params = ExtractRouteParams<"/users/:userId/posts/:postId">;

Mapped Types

type Readonly<T> = { readonly [K in keyof T]: T[K] };

type Optional<T> = { [K in keyof T]?: T[K] };

type Nullable<T> = { [K in keyof T]: T[K] | null };

type PickByType<T, V> = {
  [K in keyof T as T[K] extends V ? K : never]: T[K];
};

interface User {
  id: string;
  name: string;
  age: number;
  active: boolean;
}

type StringFields = PickByType<User, string>;

type EventMap<T> = {
  [K in keyof T as `on${Capitalize<string & K>}`]: (value: T[K]) => void;
};

type UserEvents = EventMap<{ login: User; logout: string }>;

Discriminated Unions

type Result<T, E = Error> =
  | { ok: true; value: T }
  | { ok: false; error: E };

function divide(a: number, b: number): Result<number, string> {
  if (b === 0) return { ok: false, error: "Division by zero" };
  return { ok: true, value: a / b };
}

const result = divide(10, 2);
if (result.ok) {
  console.log(result.value);
} else {
  console.error(result.error);
}

type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "rect"; width: number; height: number }
  | { kind: "triangle"; base: number; height: number };

function area(shape: Shape): number {
  switch (shape.kind) {
    case "circle": return Math.PI * shape.radius ** 2;
    case "rect": return shape.width * shape.height;
    case "triangle": return 0.5 * shape.base * shape.height;
  }
}

Type Guards

function isNonNull<T>(value: T | null | undefined): value is T {
  return value != null;
}

function hasProperty<K extends string>(
  obj: unknown,
  key: K
): obj is Record<K, unknown> {
  return typeof obj === "object" && obj !== null && key in obj;
}

const values = [1, null, 2, undefined, 3].filter(isNonNull);

function assertNever(value: never): never {
  throw new Error(`Unexpected value: ${value}`);
}

Utility Type Combinations

type DeepPartial<T> = {
  [K in keyof T]?: T[K] extends object ? DeepPartial<T[K]> : T[K];
};

type StrictOmit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>;

type RequireAtLeastOne<T, Keys extends keyof T = keyof T> = Pick<T, Exclude<keyof T, Keys>> &
  { [K in Keys]-?: Required<Pick<T, K>> & Partial<Pick<T, Exclude<Keys, K>>> }[Keys];

type UpdateUserInput = RequireAtLeastOne<{ name: string; email: string; age: number }>;

Anti-Patterns

Using any instead of unknown for values of uncertain type
Type assertions (as) when a type guard would be safer
Overly complex generic signatures that reduce readability
Not using discriminated unions for state machines or result types
Using enum when a union of string literals suffices
Ignoring strictNullChecks in tsconfig

Checklist

strict: true enabled in tsconfig
unknown used instead of any for external data
Type guards validate runtime types safely
Discriminated unions model state with exhaustive switches
Generic constraints (extends) prevent misuse
Mapped types used to derive related types from a source
Utility types (Pick, Omit, Partial) preferred over manual retyping
Template literal types used for string pattern enforcement

typescript-advanced

TypeScript Advanced

Generics with Constraints