TypeScript InstanceType<T> Utility Type Explained

Understanding InstanceType

InstanceType<T> constructs a type consisting of the instance type of a constructor function in T. Given a class constructor (the typeof ClassName), it returns what new ClassName() would produce.

Syntax

type InstanceType<T extends abstract new (...args: any) => any> =
  T extends abstract new (...args: any) => infer R ? R : any;

Basic Example

class Logger {
  level: "info" | "warn" | "error" = "info";
  log(msg: string): void {
    console.log(`[${this.level}] ${msg}`);
  }
}

type LoggerInstance = InstanceType<typeof Logger>;
// Result: Logger (the instance type, with level and log)

Practical Pattern: Generic Factory

function createInstance<T extends new (...args: any[]) => any>(
  Ctor: T,
  ...args: ConstructorParameters<T>
): InstanceType<T> {
  return new Ctor(...args);
}

class Database {
  constructor(public url: string, public port: number) {}
}

const db = createInstance(Database, "localhost", 5432);
// Type: Database (not any)

With Abstract Classes

abstract class Shape {
  abstract area(): number;
}

class Circle extends Shape {
  constructor(public radius: number) { super(); }
  area() { return Math.PI * this.radius ** 2; }
}

type ShapeInstance = InstanceType<typeof Circle>;
// Result: Circle

Practical Pattern: Plugin Registry

const plugins = {
  auth: AuthPlugin,
  cache: CachePlugin,
  logger: LoggerPlugin,
};

type PluginInstances = {
  [K in keyof typeof plugins]: InstanceType<typeof plugins[K]>;
};