You can either have a class that is separate to the Enum and use it to get things you want, or you can merge a namespace into the Enum and get it all in what looks like the same place.

Mode Utility Class

So this isn’t exactly what you are after, but this allows you to encapsulate the “Mode to string” behaviour using a static method.

class ModeUtil {
    public static toString(mode: Mode) {
        return Mode[mode];
    }
}

You can use it like this:

const mode = Mode.portrait;
const x = ModeUtil.toString(mode);
console.log(x);

Mode Enum/Namespace Merge

You can merge a namespace with the Enum in order to create what looks like an Enum with additional methods:

enum Mode {
    X,
    Y
}

namespace Mode {
    export function toString(mode: Mode): string {
        return Mode[mode];
    }

    export function parse(mode: string): Mode {
        return Mode[mode];
    }
}

const mode = Mode.X;

const str = Mode.toString(mode);
alert(str);

const m = Mode.parse(str);
alert(m);