typescript/
├── project-system/       # tsconfig.json, build tools, bundlers, compilation
├── package-management/   # npm, yarn, pnpm, bun, workspaces, publishing
├── cli/                  # Commander, yargs, oclif, ink, chalk, CLI tooling
└── packages/             # Popular libraries (Express, Next.js, Zod, Prisma, etc.)

// Union types — value can be one of several types
type Result = "success" | "error" | "pending";
type ID = string | number;

// Intersection types — combine multiple types
type Employee = Person & { employeeId: string };

// Generics — parameterized types
function first<T>(arr: T[]): T | undefined {
  return arr[0];
}

// Conditional types — type-level if/else
type IsString<T> = T extends string ? true : false;

// Mapped types — transform properties
type Readonly<T> = { readonly [K in keyof T]: T[K] };
type Optional<T> = { [K in keyof T]?: T[K] };

// Template literal types — string manipulation at the type level
type EventName<T extends string> = `on${Capitalize<T>}`;
type ClickEvent = EventName<"click">; // "onClick"

// satisfies operator — validate type without widening
const palette = {
  red: [255, 0, 0],
  green: "#00ff00",
} satisfies Record<string, string | number[]>;

// const assertions — narrow to literal types
const routes = ["home", "about", "contact"] as const;
type Route = (typeof routes)[number]; // "home" | "about" | "contact"

// Discriminated unions — tagged union pattern
type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "rectangle"; width: number; height: number };

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