typescript - pass type to function parameter

All you need to do is provide a generic parameter to your function like this:

interface ReturnObj<T> {
  returnObj: T | null
}

type AppError = {} | null
type Result<T extends unknown> = {
  isSuccessful: boolean,
  returnObj: ReturnObj<T> | null,
  errors: AppError,
  statusCode: number
}

const createResultObj = <T extends unknown>(
  isSuccessful: boolean = false,
  returnObj: ReturnObj<T> | null = null,
  errors: AppError = null,
  statusCode: number = 500
): Result<T> => {
  return {
    isSuccessful,
    returnObj,
    errors,
    statusCode
  }
}

type sth = { foo: string };
const s: sth = { foo: "bar" }
const returnS: ReturnObj<sth> = { returnObj: s }
const re = createResultObj(true, returnS, null, 200);
console.log(re);

playground link

Note: This<T extends unknown> is an obligation due to the use of arrow functions, see this