Typescript has unions, so are enums redundant?

typescript enums unions
prmph picture prmph · Oct 27, 2016 · Viewed 31.1k times · Source

Ever since TypeScript introduced unions types, I wonder if there is any reason to declare an enum type. Consider the following enum type declaration:

enum X { A, B, C }
var x:X = X.A;

and a similar union type declaration:

type X: "A" | "B" | "C"
var x:X = "A";

If they basically serve the same purpose, and unions are more powerful and expressive, then why are enums necessary?

Answer

Amid picture Amid · Oct 27, 2016

As far as I see they are not redundant, due to the very simple reason that union types are purely a compile time concept whereas enums are actually transpiled and end up in the resulting javascript (sample).

This allows you to do some things with enums, that are otherwise impossible with union types (like enumerating the possible enum values)

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