function exportIndex(callback, _field, _index_doc = 0, _index_obj = 0) {

    let key, chunk;

    switch (_index_obj) {

        case 0:

            key = "reg";
            chunk = reg_to_json(this.reg);
            break;

        case 1:

            key = "cfg";
            chunk = null;
            break;

        case 2:

            key = "map";
            chunk = map_to_json(this.map, this.reg.size);
            break;

        case 3:

            key = "ctx";
            chunk = ctx_to_json(this.ctx, this.reg.size);
            break;

        default:

            return;
    }

    return save.call(this, callback, _field, key, chunk, _index_doc, _index_obj);
}
@param {function(string,string):Promise|void} callback
@param {!string|null=} _field
@param {number=} _index_doc
@param {number=} _index_obj
@this {Index}