function-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.

Definition

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

Usage patterns

Pattern	Purpose
Parameters<typeof fn>	Reuse a function's arg list from its value
Parameters<T>[0]	Get a single parameter by index
...args: Parameters<T>	Spread into a wrapper function signature
ConstructorParameters<T>	Class-constructor equivalent

Examples

function add(a: number, b: number) { return a + b; }
type A = Parameters<typeof add>;

Result: [a: number, b: number]

function wrap<T extends (...a: any) => any>(fn: T) {
  return (...args: Parameters<T>) => fn(...args);
}

Generic wrapper that preserves the original signature

type Second = Parameters<(x: string, y: boolean) => void>[1];

Result: boolean

type P = Parameters<() => void>;

Result: [] — empty tuple for zero-arg functions

Gotcha

Returns a tuple with parameter names as labels — those labels are cosmetic and can be dropped by later transforms. As with ReturnType, only the last overload signature is captured.

Related utility types

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.

ConstructorParameters<T>

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

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.

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.

OmitThisParameter<T>

Returns a function type identical to T but without its explicit `this` parameter. Commonly paired with `Function.prototype.bind` to model the bound function's signature.

← All TS utility types · TypeScript vs JavaScript · any vs unknown vs never