TypeScript Utility Types - Parameters

| 2 min read
typescript

What is Parameters?

It is a built-in utility type that extracts the parameter types of a function. It accepts a function type as input and gives back a tuple type that lists the parameter types of the function.

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

How Does Parameters Work?

function myFunc(a: string, b: number, c: boolean) {
	//...
}

type myParams = Parameters<typeof myFunc>;
// [a: string, b: number, c: boolean]
  • Parameters Utility Type:
    • Parameters<Type> extracts parameter types from a function type Type.
    • Applied to myFunc, it gives [string, number, boolean].
  • typeof Operator:
    • typeof myFunc gets the type of the function.
  • Result:
    • myParams is a type representing myFunc’s parameter types.
    • It’s a tuple: [a: string, b: number, c: boolean].

You also can extract one item like you would with arrays.

type LastParam = Parameters<typeof myFunc>[2];
// boolean

The syntax Parameters<typeof myFunc>[2] gets the third element (a boolean) from the array of parameter types.

Recap

The Parameters utility type in TypeScript is a valuable tool, allowing developers to extract a function’s parameter types and gain insights into the anticipated arguments that the function is designed to handle.

Resources

TS Handbook - Parameters

Thank you!

Thank you for your time and for reading this!