TypeScript 5.0 - Enums are now union enums

In a previous blog post I complained about how terrible TypeScript enums were, why to avoid them and better alternatives. Since that post TypeScript 5.0 has been released, which has changed enums for the better and is now worth using again.

TypeScript enums are union enums

TypeScript has come a long way since the introduction of enums. Initially, enums were just a set of numeric constants with the same type.

Although enum members were assignable to anything expecting the type, they were essentially just numbers. However, with the introduction of enum literal types in TypeScript 2.0, each enum member was given its own type, and the enum itself was turned into a union of each member type. This allowed developers to refer to only a subset of the types of an enum and to narrow away those types, which made it easier to catch bugs.

However, one issue with giving each enum member its own type was that those types were associated with the actual value of the member. In cases where the value of the member could not be computed, TypeScript would revert to the old enum strategy, which meant giving up all the advantages of unions and literal types.

Fortunately, TypeScript 5.0 has solved this issue by creating a unique type for each computed member. This means that all enums are now union enums, and developers can narrow them and reference their members as types. This is a significant improvement that will make working with enums in TypeScript much easier and more powerful.