How to merge two enums in TypeScript

Question

Suppose I have two enums as described below in Typescript, then How do I merge them

enum Mammals {
    Humans,
    Bats,
    Dolphins
}

enum Reptiles {
             


        

Answer 1

Enums, interfaces and types - a working solution for merging enums

What's confusing here is types vs. values.

• If you define a value (let, const etc.) it will have a value plus some computed but not separately named type.
• If you define a type or interface, it will create a named type but that will not be outputted or considered in the final JS in any way. It only helps when writing your app.
• If you create an enum in Typescript, it creates a static type name that you can use plus a real object outputted to JS that you can use.

From the TS handbook:

Using an enum is simple: just access any member as a property off of the enum itself, and declare types using the name of the enum.

So, if you Object.assign() two enums, it will create a new, merged value (object), but not a new named type.

Since it's not an enum anymore, you lose the advantage of having a value and a named type, but you can still create a separate type name as a workaround.

Fortunately, you can have the same name for the value and the type, and TS will import both if you export them.

// This creates a merged enum, but not a type
const Animals = Object.assign({}, Mammals, Reptiles);

// Workaround: create a named type (typeof Animals won't work here!)
type Animals = Mammals | Reptiles;

TS playground link