Why do we not need checks with Union Types when using a variable but it's needed inside a function?

I'm doing some Typescript challenges and came across this union type example.

We are saying getIstanbulPostalCode returns a string or a number:

function getIstanbulPostalCode(): string | number {
  return 34116;
}
let postalCode: number = getIstanbulPostalCode();
postalCode;

Result: type error: Type 'string | number' is not assignable to type 'number'.

The variable postalCode references getIstanbulPostalCode and returns a number which doesn't match the function.

The function needs a string or a number so it results in a type error.
Type 'string' is not assignable to type 'number'.

function istanbul(): number { return 34116; }
let istanbul2: string | number = istanbul();
istanbul2;

Result: 34116

My confusion is in this second example for function istanbul, this function returns a number which makes sense.

But why is it when we create istanbul2 which can be either string or number we don't get any type errors?

I ask this because inside a function if you provide a union type as a parameter you have to check if it's a string or a number otherwise it complains, but here using a variable we can provide either with no checks?

Thanks

The postalCode variable expects a value which is number, but function getIstanbulPostalCode can return either a number or a string. Given that typescript works at compile time, and you are telling it that the function return either number or string, it believes it and you get the error since it has no way of knowing which one of the two union type is returned at runtime.

If you were to rewrite your function as following, typescript will infer it's return type to be number and will not throw error.

function getIstanbulPostalCode() {
  return 34116;
}
let postalCode: number = getIstanbulPostalCode(); // no error
postalCode;

For your second case, you are saying that istanbul2 can be either a string or a number, and the function istanbul returns a number, which satisfies the constrain of it belonging to either number or string, and hence you do not see any errors.

> But why is it when we create istanbul2 which can be either string or number we don't get any type errors?

Well, you do not get any errors because number is a subtype of string | number. Read the pipe as or and you get the (valid) statement "number is (string or number)" therefore the assignment is alllowed.

Using that variable, you are however restricted to the intersection of methods on these types.
While console.log will take pretty much anything, the only methods you can use now (without checking) are toLocaleString(), toString() and valueOf()
(You can not even call the + operator with itself)

console.log is typed in TS to take any[], so it is very generous. But if you wanted to to something useful with the variable, you likely need to check it before using.