Avoid confusing 2 different ID types

My customers have 2 IDs : userId and customerId, that are both strings (UUIDs) and refer to 2 different objects in the database.

For now, I can easily send a customerId to a function that expects a userId as they are both string and I am not satisfied with this situation as it is error prone.

I had the idea to create some type aliases like this :

export type CustomerId = string;
export type UserId = string;

so my Customer class would look like :

export class Customer {
  id: CustomerId;
  userId: UserId;
}

Unfortunately, I can still send a CustomerId to a function that expect a UserId as both types are considered compatible :

  foo() {
    const userId: UserId = "abc123";
    // This line will compile but I want the compiler to throw an error
    this.bar(userId)
  }

  bar(customerId: CustomerId) {}

Any idea ?

>Solution :

TS uses structural typing.

If you want to distinguish those types you’ll have to rely on branded types.

export type CustomerId = string & {__brand : 'customerId' };
export type UserId = string & {__brand : 'userId' };