function doCoolStuff(a: string, b: number) {

  // ...

}

​

type DoCoolStuffArgs = Parameters<typeof doCoolStuff> // [string, number]

​

type SecondArg = Parameters<typeof doCoolStuff>[1] // number

​

// demo: https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABAEzgYTnANgZSiYYACgEMAuRAZygCcYwBzAGkQCMKwQBbVgUxoCUiAN4AoRIgD0kxADp5ogL6jRUAJ4AHXogAi6TLnyEAgjQaVEAXkQAFEjRJdeUfpQA86rXGAp92PATAAHxSMgDa1HSMLJw8-AC6KtKIAB6IABYkFp7aEbT0zIixfDTxLPTA-DS8yIjANHBciFDp2jkW3s2tiBr2js6uiJ2oGP5GwKIQCNSpZHqjhoGm5laIYQDk6TDrLACMAAyJoslqGVnNmtrF-OVglTTVtfWNXW2XQz4t2gBErN89fScLhoH18CwChEm0ygiDUZDsDiBrg8l2GfkWhCCYV28VWBySMhgMIA7nAaABrCxYGDktpbCznYm8LBYMiqd44XhTMDIZarBH9YHuHJo8HjLE4qFgGYALzInO5vLMeP2oiAA

function createPerson(name: string, doAction: () => void): void {

  console.log(`Hi, my name is ${name}.`);

  doAction(); // doAction as a function parameter.

}

​

// Hi, my name is Bob.

// performs doAction which is waveHands function.

createPerson('Bob', waveHands()); 

//parameter

type Parameter<T extends (...args: any[]) => any> = T extends (...args: infer P) => any ? P : never;

type paraMeterCheck = Parameter<(a: string, b: string) => void>;

// non typed function

function add(x, y) {

  return x + y;

}

​

// typed function

function add(x: number, y: number): number {

  return x + y;

}

​

// types can be found here: https://www.typescriptlang.org/docs/handbook/basic-types.html