_$name() {
    super._$name();

    const type = this._node.declarations[0].id.type;
    switch (type) {
      case 'Identifier':
        this._value.name = this._node.declarations[0].id.name;
        break;
      case 'ObjectPattern':
        // TODO: optimize for multi variables.
        // e.g. export const {a, b} = obj
        this._value.name = this._node.declarations[0].id.properties[0].key.name;
        break;
      case 'ArrayPattern':
        // TODO: optimize for multi variables.
        // e.g. export cont [a, b] = arr
        this._value.name = this._node.declarations[0].id.elements.find(v => v).name;
        break;
      default:
        throw new Error(`unknown declarations type: ${type}`);
    }
  }
set name by using self node.