Typescript iterate object with enum keys

Question

I would like to iterate over an object with enum keys. Problem is that the types are always interfering and I would prefer to not cast everything.

enum MyEnum { A, B, C }
type MyMap = { [key in MyEnum]: string };
const myMap: MyMap = {
    [MyEnum.A]: 'a?',
    [MyEnum.B]: 'b',
    [MyEnum.C]: 'c...',
};

If I try Object.keys or for..in it always shows errors.

Object.keys(myMap).forEach(key => {
    const data = myMap[key]; // <= Error TS7017: Element implicitly has an 'any' type because type 'MyMap' has no index signature.
});
// same with: for (const key in myMap) {

Object.keys(myMap).forEach((key: MyEnum) => { // Error TS2345: Argument of ... Type 'string' is not assignable to type 'MyEnum'.
});

I could cast the key but can't even do it directly...

Object.keys(myMap).forEach(key => {
    const data = myMap[key as MyEnum]; // Error TS2352: Conversion of type 'string' to type 'MyEnum' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.

    // have to do this but would prefer not to
    const data2 = myMap[key as unknown as MyEnum];
});

There probably is a solution somewhere but I didn't use the right keywords and didn't find it :( (edit: or maybe not)

Nguyen Phong Thien · Accepted Answer · 2019-02-21 14:33:36Z

2

If you don't want to use any or unknown

    const data = myMap[MyEnum[key as keyof typeof MyEnum]];

answered Feb 21, 2019 at 14:33

Nguyen Phong Thien

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Comments

Jack · Accepted Answer · 2020-08-21 10:19:58Z

The solution I'm most happy with now is adding a helper function that does the casting of the keys. This is also easily reusable for all kinds of similar objects.

export function objectKeys<Obj extends object>(obj: Obj): (keyof Obj)[] {
    return Object.keys(obj) as (keyof Obj)[];
}

// with example from above
// no specific type for the map needed
const myMap = {
    [MyEnum.A]: 'a?',
    [MyEnum.B]: 'b',
    [MyEnum.C]: 'c...',
};
objectKeys(myMap).forEach(key => {
  // key: MyEnum
  // if the enum has more values that are not part of the map (D..)
  // the types are still correct => key: MyEnum.A | MyEnum.B | MyEnum.C
});

Note: It's not perfect. e.g. with arrays

objectKeys([]).forEach(key => {
  // key: number | 'length' | ...
  // would be perfect if it returned only `number` or even `never` (empty array)
});

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