TypeScript Types & Interfaces Cheat Sheet: Unions, Aliases & Extends

Interfaces describe object contracts; type aliases name any type, including unions and intersections. Use extends (or &) to compose, optional ? for fields that may be missing, and readonly for props you do not want assigned. Prefer a discriminated union over a pile of optional fields.

This TypeScript cheat sheet is written for people searching for a fast, accurate reference: students, junior developers, and teams shipping production software. Pin it, copy the snippets, and come back when syntax slips.

Quick reference

• Optional: email?: string may be undefined. Combine with | null if the API sends null.
• readonly: readonly id: number — assignment is a type error.
• extends: interface Admin extends User { role: "admin" }.
• Intersection: type A = B & C must satisfy both.
• Union: type Status = "draft" | "live" — use instead of magic booleans.
• Index: Record<string, number> or { [slug: string]: Post }.
• Merge: Two interface User in the same scope merge. Type aliases do not.
• typeof / keyof: keyof User is "id" | "email". typeof config captures a value’s type.

Copy-paste examples

Compose and discriminate

A tag field (kind) lets switch narrow the rest of the object.

interface Timestamps {
  createdAt: string;
  updatedAt: string;
}
export interface Post extends Timestamps {
  id: number;
  title: string;
  published?: boolean;
}
export type Nav =
  | { kind: 'link'; href: string; label: string }
  | { kind: 'group'; label: string; items: Nav[] };

export function hrefOf(item: Nav): string | null {
  return item.kind === 'link' ? item.href : null;
}

Common mistakes

• Optional everything (user?: { email?: string }) so you never know what exists.
• Extending interfaces in a .d.ts you do not own and breaking upgrades (unintentional merging).
• type X = {} which is surprisingly wide (almost everything except null/undefined in some configs).
• Mixing null and undefined without documenting which the API uses.

FAQ

1. When must I use a type alias? Unions, tuples, mapped types, and typeof results cannot be expressed as an interface body alone.

1. Can interfaces express unions? Not as the alias itself. An interface property can be a union. The whole type as A | B is a type alias.

1. readonly array? readonly string[] or ReadonlyArray<string>. as const on literals is even narrower.

Related TypeScript cheat sheets

• Typescript Cheat Sheet 2026
• Typescript Generics Cheat Sheet
• Typescript For React Cheat Sheet

Build with this stack

When a cheat sheet is not enough — you need a production app, a student FYP, or a custom dashboard — ArpaNeuro builds custom web development and also sells ready-made source code. Request a quote and tell us the stack.

Browse software development services or the source code marketplace if you want a working codebase instead of starting from a blank file.