conditional

`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.

Definition

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

Usage patterns

Pattern	Purpose
InstanceType<typeof Cls>	Get the instance type from a class constructor
ConstructorParameters<typeof Cls>	Companion for the constructor's argument tuple
abstract new (...args: any)	Supported signature form (TS 4.2+)
InstanceType<T> & { extra: X }	Intersect an instance with extra runtime-added props

Examples

class Foo { hello() { return 'hi'; } }
type F = InstanceType<typeof Foo>;

Result: Foo

function make<T extends new (...a: any) => any>(C: T): InstanceType<T> {
  return new C();
}

Generic factory returning the correct instance type

type ErrInstance = InstanceType<ErrorConstructor>;

Result: Error — reuses built-in constructor

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

T must have a construct signature — plain functions error out

Gotcha

Takes the CONSTRUCTOR type, not the class body itself — always pair with `typeof`. If a class has multiple construct signatures (rare), only the last is used.

Related utility types

ConstructorParameters<T>

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

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.

ThisType<T>

A marker interface with no members that, when used in a contextual type, sets the `this` type inside method-like properties of an object literal. Requires `noImplicitThis` to be genuinely useful and is a compile-time only hint.

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.

Awaited<T>

Models the type produced by awaiting T, recursively unwrapping nested Promises (and other thenables). Introduced in TS 4.5 and used internally by the `await` expression's type.

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