Conditional Types in TypeScript

flowchart TB
    CT["type IsString T = T extends string ? true : false"] --> CD{T extends string?}
    CD -->|Yes| True[type Result = true]
    CD -->|No| False[type Result = false]

    RT["type ReturnType T =<br/>T extends (...args: any) => infer R ? R : never"] --> FN["Fn: (x: number) => string"]
    FN --> EX[infer R = string 🎯]

    style CT fill:#7c3aed,color:#fff
    style CD fill:#f59e0b,color:#fff
    style True fill:#3b82f6,color:#fff
    style False fill:#ef4444,color:#fff
    style RT fill:#059669,color:#fff
    style EX fill:#10b981,color:#fff

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

type A = IsString<"hello">;  // true
type B = IsString<42>;       // false
type C = IsString<string>;   // true
type D = IsString<number>;   // false

// Extract only string types from a union
type ExtractString<T> = T extends string ? T : never;

type StringsOnly = ExtractString<string | number | boolean | "hello">;
// Result: string | "hello"

// Remove null/undefined
type NonNullable<T> = T extends null | undefined ? never : T;

// Extract the return type of a function
type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never;

type Fn = (x: number) => string;
type Result = ReturnType<Fn>;  // string

// Extract array element type
type ElementType<T> = T extends (infer U)[] ? U : never;

type Items = ElementType<string[]>;  // string

// Extract API response data types
type ApiResponse<T> =
  T extends { data: infer D } ? D :
  T extends { error: infer E } ? E :
  never;

type SuccessResponse = { data: { id: number; name: string } };
type UserData = ApiResponse<SuccessResponse>;  // { id: number; name: string }

// Type-safe event payload extraction
type EventPayload<T> =
  T extends { type: "user_created"; payload: infer P } ? P :
  T extends { type: "user_deleted"; payload: infer P } ? P :
  never;