Typed enum instance in TypeScript

Question

I have the following method which transforms a string instance of an enum to the corresponding enum member on an object:

function transformEnum<TBase>(base: TBase, member: keyof TBase, enumInstance: any) {
    base[member] = enumInstance[base[member]];
}

It's called like this:

transformEnum(result, "day", DayOfWeek);

Is there any way to type the enumInstance variable? The typing doesn't have to be perfect, but constraining it at least somewhat would be nice.

Alternatively I've tried

function transform<T>(get: () => T, set: (x: T) => void, enumInstance: T) {
    set((enumInstance as any)[get()]);
}

But when I call this like so:

transform<DayOfWeek>(() => result.day, (x) => { result.day = x; }, DayOfWeek);

I get

[ts] Argument of type 'typeof DayOfWeek' is not assignable to parameter of type 'DayOfWeek'.

Please check if this works for you : stackoverflow.com/questions/17380845/… — Balaji V
– Balaji V, Commented Mar 27, 2017 at 9:33
@BalajiV if you read my code sample you'll see that that's what I'm doing. The question isn't how to do the conversion but how to type the function parameter. — Levi Botelho
– Levi Botelho, Commented Mar 27, 2017 at 9:35

aboersch · Accepted Answer · 2017-03-27 16:54:24Z

3

function transform<T, TKey extends keyof T>(get: () => TKey, set: (x: T[TKey]) => void, enumInstance: T) {
  set(enumInstance[get()]);
}

Example:

enum DayOfWeek {
    Monday, Tuesday, Wednesday, ...
}

type DayOfWeekKey = keyof typeof DayOfWeek;

class ClassWithDay {
  day: DayOfWeekKey | DayOfWeek;
}

let c = new ClassWithDay();
c.day = "Monday";
transform(() => c.day as DayOfWeekKey, v => {c.day = v;}, DayOfWeek);
// c.day is now 0

answered Mar 27, 2017 at 16:54

aboersch

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