TypeScript ConstructorParameters<T> Utility Type Explained

Understanding ConstructorParameters

ConstructorParameters<T> constructs a tuple type from the parameter types of a constructor function type. It is the constructor equivalent of Parameters.

Syntax

type ConstructorParameters<T extends abstract new (...args: any) => any> =
  T extends abstract new (...args: infer P) => any ? P : never;

Basic Example

class ApiClient {
  constructor(
    public baseUrl: string,
    public timeout: number,
    public headers: Record<string, string>
  ) {}
}

type ClientArgs = ConstructorParameters<typeof ApiClient>;
// Result: [baseUrl: string, timeout: number, headers: Record<string, string>]

Practical Pattern: Factory with Defaults

function createWithDefaults<T extends new (...args: any[]) => any>(
  Ctor: T,
  defaults: Partial<Record<number, ConstructorParameters<T>[number]>>
): (...args: ConstructorParameters<T>) => InstanceType<T> {
  return (...args) => new Ctor(...args);
}

With Built-in Types

type DateArgs = ConstructorParameters<typeof Date>;
// Result: [value: string | number | Date] (one overload)

type ErrorArgs = ConstructorParameters<typeof Error>;
// Result: [message?: string, options?: ErrorOptions]

type RegExpArgs = ConstructorParameters<typeof RegExp>;
// Result: [pattern: string | RegExp, flags?: string]

Practical Pattern: Testing Helpers

class UserService {
  constructor(
    private db: Database,
    private mailer: Mailer,
    private logger: Logger
  ) {}
}

type ServiceDeps = ConstructorParameters<typeof UserService>;
// [db: Database, mailer: Mailer, logger: Logger]

function createTestService(
  ...overrides: Partial<ServiceDeps>
): UserService {
  const defaults: ServiceDeps = [mockDb, mockMailer, mockLogger];
  const args = defaults.map((d, i) => overrides[i] ?? d);
  return new UserService(...(args as ServiceDeps));
}