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

A TypeScript enum not only contains the keys you define but also the numerical inverse, so for example:

Mammals.Humans === 0 && Mammals[0] === 'Humans'

Now, if you try to merge them -- for example with Object#assign -- you'd end up with two keys having the same numerical value:

const AnimalTypes = Object.assign({}, Mammals, Reptiles);
console.log(AnimalTypes.Humans === AnimalTypes.Snakes) // true

And I suppose that's not what you want.

One way to prevent this, is to manually assign the values to the enum and make sure that they are different:

enum Mammals {
    Humans = 0,
    Bats = 1,
    Dolphins = 2
}

enum Reptiles {
    Snakes = 3,
    Alligators = 4,
    Lizards = 5
}

or less explicit but otherwise equivalent:

enum Mammals {
    Humans,
    Bats,
    Dolphins
}

enum Reptiles {
    Snakes = 3,
    Alligators,
    Lizards
}

Anyway, as long as you make sure that the enums you merge have different key/value sets you can merge them with Object#assign.

Playground Demo