How to map an enum to another enum in typescript?

Question

I want to map one object's property that has an enum type to another object's property that is other enum type.

I've tried with type1.a as Enum2 or Enum2[type1.a] with no success.

Here is my simplified code problem:

enum Enum1 {
  N = 0,
  A = 1,
  B = 2
}

enum Enum2 {
  A = 1,
  B = 2
}

interface Type1 {
  a: Enum1;
}

interface Type2 {
  a: Enum2;
}

const type1: Type1 = {
  a: Enum1.A
};

const type2: Type2 = {
  a: type1.a
};

try it

Throws the error:

Type 'Enum1' is not assignable to type 'Enum2'.
(property) Type2.a: Enum2

Answer 1

You need to cast the type1.a prop to Enum2. To do this you need to use a "Type Guard".

Here is an example with your code:

enum Enum1 {
  N = 0,
  A = 1,
  B = 2
}

enum Enum2 {
  A = 1,
  B = 2
}

interface Type1 {
  a: Enum1;
}

interface Type2 {
  a: Enum2;
}

const type1: Type1 = {
  a: Enum1.N
};

function isEnum2(value: any): value is Enum2 {
  let isEnum2 = false;

  for (let key in Enum2) {
    if (Enum2[key] === value) {
      isEnum2 = true;
    }
  }

  return isEnum2;
}

if (isEnum2(type1.a)) {
  const type2: Type2 = {
    a: type1.a
  };
}

try it

When the isEnum2 function is used in the if block the prop type1.a becomes of type Enum2 inside that if block only.

Note: you could replace the content in the isEnum2 using for..in loop if you are using ES2017 or higher using Object.values:

return Object.values(Enum2).indexOf(value) > -1;

try it (Doesn't work in www.typescriptlang.org)

More info about Type Guards