如何在 typescript 中使用多种类型的数组调用 map function

Question

I'm developing a Typescript function which needs to iterate over an array of type Employee[] | Guest[] Employee[] | Guest[] . The problem is that TypeScript is emitting the error TS2349: This expression is not callable... has signatures but none of those are compatible with each other . I was expecting to be able to iterate the array and would just need to use a type guard before using the local variable, but that doesn't appear to be the case. Is there a way I can use the map function without having to create a custom type? The function will only ever need to iterate over the two types, so I was hoping to keep the parameter's type definition as Employee[] | Guest[] Employee[] | Guest[] . Below is a simple example where there's an error with people.map .

interface Employee {
  id: string;
  firstName: string;
}

interface Guest {
  firstName: string;
}

function getFirstNames(people: Employee[] | Guest[]): string[] {
  return people.map((x: Employee | Guest) => {
    return x.firstName;
  });
}

const employeeFirstNames = getFirstNames([
  {id: '1', firstName: 'Steve'},
  {id: '2', firstName: 'James'},
] as Employee[]);

const guestFirstNames = getFirstNames([
  {firstName: 'Mary'},
] as Guest[]);

Answer 1

Type the array as union of Employee and Guest .

function getFirstNames(people: (Employee | Guest)[]): string[] {
  return people.map((x: Employee | Guest) => x.firstName);
}

Answer 2

I think you can use overload in this case, if you want to pass array with fixed element type

function getFirstNames(people: Employee[]): string[];
function getFirstNames(people: Guest[]): string[];
function getFirstNames(people: (Employee | Guest)[]): string[] {
  return people.map((x: Employee | Guest) => {
    return x.firstName;
  });
}

In this case, you won't be worried about public interface, so something like the following with mixed type will give error

const guestFirstNames = getFirstNames([
   { id: '1', firstName: 'Steve' },
  { firstName: 'Mary' }
]); // <-- No overload matches this call. Overload 1 of 2, '(people: Employee[]): string[]', gave the following error. Property 'id' is missing in type '{ firstName: string; }' but required in type 'Employee'. Overload 2 of 2, '(people: Guest[]): string[]', gave the following error. Type '{ id: string; firstName: string; }' is not assignable to type 'Guest'. Object literal may only specify known properties, and 'id' does not exist in type 'Guest'.