Advanced TypeScript: Conditional Types, Mapped Types, and Template Literals

TypeScript’s type system is Turing complete. This means you can encode complex logic directly into your types.

Conditional Types

type IsString<T> = T extends string ? true : false
type Result = IsString<'hello'> // true
type Result2 = IsString<42>    // false

Mapped Types

Transform properties of an existing type:

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

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

Template Literal Types

type EventName = `on${Capitalize<string>}`
type ClickEvent = EventName & 'onClick' // 'onClick'

Conditional Inference

type GetReturnType<T> = T extends (...args: any[]) => infer R ? R : never
type FnType = GetReturnType<typeof someFunction>

Custom Utility Types

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

type NonEmptyArray<T> = [T, ...T[]]

Conclusion

Mastering these patterns reduces compile-time errors and eliminates entire categories of production bugs. An expressive type system is the best documentation your code can have.