function-types

`ConstructorParameters<T>`

Extracts the parameter tuple of a class constructor T. Mirrors Parameters but operates on `new (...)` signatures instead of call signatures.

Definition

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

Usage patterns

Pattern	Purpose
ConstructorParameters<typeof Cls>	Reuse a class's constructor arguments
abstract new (...args)	The signature form it destructures
InstanceType<typeof Cls>	Companion for extracting the instance type
...args: ConstructorParameters<T>	Spread into a factory or wrapper

Examples

class Point { constructor(public x: number, public y: number) {} }
type PArgs = ConstructorParameters<typeof Point>;

Result: [x: number, y: number]

function factory<T extends new (...a: any) => any>(Cls: T, ...args: ConstructorParameters<T>) {
  return new Cls(...args);
}

Type-safe generic factory function

type EArgs = ConstructorParameters<ErrorConstructor>;

With modern lib (ES2022+), Error's second overload adds `options`, so `EArgs = [message?: string, options?: ErrorOptions]`. Last overload wins.

type Bad = ConstructorParameters<() => void>; // Error

T must have a construct signature — plain functions error out

Gotcha

Applies to `typeof ClassName` (the constructor), not the instance type. Abstract classes are supported since TS 4.2 thanks to the `abstract new` signature.

Related utility types

Parameters<T>

Extracts the parameters of a function type T as a tuple. Enables reusing a function's argument list for wrappers, currying, or higher-order utilities.

InstanceType<T>

Given a constructor type T (typically obtained via `typeof ClassName`), returns the type of an instance produced by `new T()`. Useful for factory functions and generic wrappers over classes.

ReturnType<T>

Infers the return type of a function type T using `infer R`. Widely used to derive types from existing functions without redeclaring them.

infer R

A keyword usable only inside the extends clause of a conditional type that introduces a fresh type variable inferred from the matched shape. It is the primitive that powers ReturnType, Parameters, InstanceType, Awaited, and countless custom utilities.

ThisParameterType<T>

Extracts the explicit `this` parameter type from a function T, or `unknown` if none is declared. Useful for reflecting on method-style callbacks that require a specific caller.

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