Enum inside class (TypeScript definition file)

I think the following is an improvement on KoenT’s solution:

export class Image
{
    constructor ()
    {
        this.state = Image.State.Idle;
    }

    state: Image.State;
}

export namespace Image
{
    export enum State
    {
        Idle,
        Loading,
        Ready,
        Error
    }
}

The advantage being that you can leverage named imports:

import {Image} from './image';
let img = new Image()
img.state = Image.State.Error