Advanced Type System

Advanced Types

• Conditional Types: T extends string ? StringHandler : NumberHandler — type branching based on conditions. infer keyword extracts types: type ReturnType<T> = T extends (...args: any) => infer R ? R : never
• Mapped Types: { [K in keyof T]: T[K] | null } — transform each property of a type. Built-in: Partial<T>, Required<T>, Readonly<T>, Pick<T, K>, Omit<T, K>, Record<K, V>
• Template Literal Types: type EventName = `on${Capitalize<string>}` — create string literal types from combinations. type CSSProperty = `${string}-${string}` for type-safe CSS property names
• Discriminated Unions: type Shape = { kind: 'circle'; radius: number } | { kind: 'square'; side: number } — switch on 'kind' with exhaustive checking. Never type catches unhandled cases
• Type Guards: function isString(x: unknown): x is string { return typeof x === 'string' } — narrow types in conditional blocks. Custom type guards for complex objects
• Recursive Types: type DeepPartial<T> = { [K in keyof T]?: T[K] extends object ? DeepPartial<T[K]> : T[K] } — types that reference themselves. Useful for nested data structures and JSON parsing
• Branded Types: type USD = number & { readonly _brand: 'USD' } — prevent mixing semantically different values. Can't pass USD where EUR is expected even though both are numbers