ReturnType<T> and Parameters<T>

Syntax

ReturnType<typeof fn>
Parameters<typeof fn>

Examples

function createUser(name: string, age: number) {
  return { id: Date.now(), name, age, createdAt: new Date() };
}

type User = ReturnType<typeof createUser>;
// User is inferred as: { id: number; name: string; age: number; createdAt: Date }
// No need to manually write out this shape - it stays in sync with the function automatically

function greet(name: string, greeting: string = "Hello") {
  return `${greeting}, ${name}!`;
}

type GreetParams = Parameters<typeof greet>;
// GreetParams is: [name: string, greeting?: string]

function logCall(...args: Parameters<typeof greet>) {
  console.log("Calling greet with:", args);
}

Best practices

• Use ReturnType<typeof fn> to derive a type from a function's actual implementation, rather than manually duplicating and maintaining a matching interface
• Always use typeof when passing a function to these utilities (ReturnType<typeof myFunc>), since they operate on types, not runtime values
• Use Parameters<T> when writing a wrapper or decorator function that needs to accept the exact same arguments as another function
• These are especially valuable for third-party library functions whose exact return shape you do not want to manually re-type and keep in sync

At a glance

Purpose

Static types for JavaScript

File extension

.ts · .tsx

Runs in

Compiled to JavaScript

Usually used with

JavaScript and its ecosystem

Specifications & further reading

• TypeScript Handbook

Related TypeScript documentation