Today I Learned

2 posts by grzegorzzaręba

Offset pagination with subquery in Sequelize

When we are using offset pagination and try to use LIMIT in our query, we could encounter some problems due to Sequelize specification. There are two methods to select multiple records, findAll and findAndCountAll where it’s possible to define limit property. If our query contains some subqueries, then in Sequelize the limit could be applied to our subqueries rather than the main query.

After checking code on the GitHub repository, we can find additional property subQuery not posted in the official documentation. If we add it to findAll parameters with false value, the limit and offset are placed at the end of the main query and not evaluated to the subquery. Now should be received well-paginated results.

      attributes: ["id", "name"],
      order: [["id", "DESC"]],
      include: [
       /* Some associations */
      offset: 5,
      limit: 5,
      subQuery: false,

The downside of disabling subqueries optimisation is a worse performance because all queries are separated. In every single case, it is essential to evaluate usability and performance cost before using that approach.

Babel loader transpilation for Jest

Let’s imagine a following situation: we need to create a new npm package with create-react-app, in order to bundle some files in a package to have them available in a project (it’s possible to use create-component-lib for this task). The aforementioned sometimes requires from us to update our babel config.

In my latest task I had a situation where my babel config contained invalid (for a situation) presets:

module.exports =
  "presets": ["@babel/preset-env", ["react-app", { "absoluteRuntime": false }]]

In the above example, the build process creates a transpiled code, which contains babel runtime for minimalizing package size:

var _interopRequireWildcard = require("@babel/runtime/helpers/interopRequireWildcard");

var _interopRequireDefault = require("@babel/runtime/helpers/interopRequireDefault");

Object.defineProperty(exports, "__esModule", {
  value: true
exports.default = void 0;

var _defineProperty2 = _interopRequireDefault(require("@babel/runtime/helpers/esm/defineProperty"));

var _objectSpread4 = _interopRequireDefault(require("@babel/runtime/helpers/esm/objectSpread"));

var _classCallCheck2 = _interopRequireDefault(require("@babel/runtime/helpers/esm/classCallCheck"));

var _createClass2 = _interopRequireDefault(require("@babel/runtime/helpers/esm/createClass"));

var _possibleConstructorReturn2 = _interopRequireDefault(require("@babel/runtime/helpers/esm/possibleConstructorReturn"));

var _getPrototypeOf3 = _interopRequireDefault(require("@babel/runtime/helpers/esm/getPrototypeOf"));

var _inherits2 = _interopRequireDefault(require("@babel/runtime/helpers/esm/inherits"));

Now when we start our test which uses a transpiled component it’s possible we receive error code like this:

Jest encountered an unexpected token

    By default, if Jest sees a Babel config, it will use that to transform your files, ignoring "node_modules".

    Here's what you can do:
     • To have some of your "node_modules" files transformed, you can specify a custom "transformIgnorePatterns" in your config.
     • If you need a custom transformation to specify a "transform" option in your config.
     • If you simply want to mock your non-JS modules (e.g. binary assets) you can stub them out with the "moduleNameMapper" config option.

    You'll find more details and examples of these config options in the docs:


    ({"Object.<anonymous>":function(module,exports,require,__dirname,__filename,global,jest){export default function _defineProperty(obj, key, value) {

    SyntaxError: Unexpected token export

      10 | exports.default = void 0;
      11 |
    > 12 | var _defineProperty2 = _interopRequireDefault(require("@babel/runtime/helpers/esm/defineProperty"));
         |                                               ^

      at ScriptTransformer._transformAndBuildScript (node_modules/jest-runtime/build/script_transformer.js:403:17)
      at Object.require (dist/FormGenerator.js:12:47)

This happens because by default jest ignores transformations for everything in node_modules directory.

Now we have two possibilities to resolve this problem. In the first scenario we can add transformIgnorePatterns for our jest config to transpile babel runtime module:

transformIgnorePatterns: [

In the above example transpiled code still will be smaller in application runtime but it doesn’t crash our tests.

The second option is using another babel preset for proper transpilation:

module.exports = {
  plugins: [
  presets: [

Now after build, we get transpiled code working in a test environment, however, our code will be bloated with helpers required to mimic transpiled features (for example class inheritance):

Object.defineProperty(exports, "__esModule", {
  value: true
exports.default = void 0;

var _react = _interopRequireWildcard(require("react"));

var _propTypes = _interopRequireDefault(require("prop-types"));

var _formik = require("formik");

var _yup = require("yup");

var _classnames = _interopRequireDefault(require("classnames"));

var _moment = _interopRequireDefault(require("moment"));

var _BooleanField = _interopRequireDefault(require("./FieldTypes/BooleanField"));

var _EnumField = _interopRequireDefault(require("./FieldTypes/EnumField"));

var _MoneyField = _interopRequireDefault(require("./FieldTypes/MoneyField"));

var _TextField = _interopRequireDefault(require("./FieldTypes/TextField"));

var _QuarterDateField = _interopRequireDefault(require("./FieldTypes/QuarterDateField"));