Using an enum as a dictionary key

You can do it as follows:

type EnumDictionary<T extends string | symbol | number, U> = {
    [K in T]: U;
};

enum Direction {
    Up,
    Down,
}

const a: EnumDictionary<Direction, number> = {
    [Direction.Up]: 1,
    [Direction.Down]: -1
};

I found it surprising until I realised that enums can be thought of as a specialised union type.

The other change is that enum types themselves effectively become a
union of each enum member. While we haven’t discussed union types yet,
all that you need to know is that with union enums, the type system is
able to leverage the fact that it knows the exact set of values that
exist in the enum itself.

The EnumDictionary defined this way is basically the built in Record type:

type Record<K extends string, T> = {
    [P in K]: T;
}