Method then

src/openseadragon.js:2999–3009

(handler)

        }

        then(handler) {
            if (!this._error) {
                try {
                    this._value = handler(this._value);
                } catch (e) {
                    this._value = e;
                    this._error = true;
                }
            }
            return this;
        }

        catch(handler) {
            if (this._error) {