Why doesn't TS use Symbols for enums

Question

ES6 symbols is a great fit for enums as they avoid collision. I thouhgt that TS enum type used Symbols for enums if target:'es6', but it doesn't:

enum Role {Employee, Manager, Admin}
let role: Role = Role.Employee;

transpiles to

var Role;
(function (Role) {
    Role[Role["Employee"] = 0] = "Employee";
    Role[Role["Manager"] = 1] = "Manager";
    Role[Role["Admin"] = 2] = "Admin";
})(Role || (Role = {}));
let role = Role.Employee;

any ideas why? With such approach Role.Employee has the value of 0, and any other enum with the value of 0 can be used instead of Role.Employee in runtime?

Answer 1

ES Symbol is about uniqueness whereas TypeScript enum is about named constants. Being unique and being constant are two different things.

We can serialize enum structure as it is essentially just an object. But Symbol is a different story. You cannot serialize it.

So, when we say constant, we actually mean to use that value as it is which exactly what enum provides. But Symbol is not a value that you want to use. It is more like an identity that we wish to make that nobody should be able to clone/replicate.

A constant is something that stays constant. So some code that can run in two different environments, like Node.js and Browser, both will understand the constant value to be same (in simple words, the developer is creating a value for constant, ex. string or number). But for Symbol, imagine it to not have any notion of value. If we attempt to use it as value, then it means it is two separate things (because the runtime is creating those Symbol values, not developer).

In the same light, if you have used redux, we prefer string over Symbol to represent our action.

Further, in case of TypeScript, enum can also be used as ambient or substitution value. In this case, it is the only compiler for whom the enum value matters. During compilation, type erasure may remove enum reference altogether. Now, this is something you cannot do with Symbol. Creation of Symbol is runtime only activity. So if we use Symbol for enum, then we lose to the ability of enum to be compile-time only feature.

Thus not using Symbol for enum seems a sensible choice.