Is it possible to add functions to an Enum type in TypeScript?

for example:

enum Mode {
    landscape,
    portrait,

    // the dream...
    toString() { console.log(this); } 
}

Or:

class ModeExtension {
    public toString = () => console.log(this);
}

enum Mode extends ModeExtension {
    landscape,
    portrait,
}

Of course the toString() function would contain something like a switch But a use-case would flow along the lines of:

class Device {
    constructor(public mode:Mode) {
        console.log(this.mode.toString());
    }
}

I understand why extending an enum might be a strange thing, just wondering if it is possible.

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);