Xanthan: A cute, declarative way to graft methods to specific Sequelize instances behind feature flags

xanthan.js

/**
* Xanthan is a cute declarative way to compose different sets of traits onto Sequelize models
* 
* It is distributed under the terms of the WTFPL (http://www.wtfpl.net/about/) 
*
* Usage:
* const {Sequelize, DataTypes: { TEXT, INTEGER } } = require('sequelize');
* const { getFoo, setFoo } = require('./expandos/foo');
* require('./xanthan')(Sequelize)({
*  isFoo: { // this means only models that pass an `isFoo: true` option will get the methods below grafted to them
*    getFoo,
*    setFoo
*  }
* });
* const connection = new Sequelize("sqlite::memory:");

* const Foos = connection.define("foos", {
*   id: {
*     type: INTEGER,
*     primaryKey: true,
*     autoIncrement: true
*   },
*   content: {
*     type: TEXT,
*     allowNull: false
*   }
* }, { isFoo: true} );
* // Foos now has getFoo and setFoo instance methods. 
*/
module.exports = function xanthan(Sequelize) {
  return function (features) {
    Sequelize.addHook("afterInit", function (connection) {
      connection.addHook("afterDefine", function (model) {
        for (let feature in features) {
          const expandos = features[feature];
          const hasFeature = feature in model.options;
          const isFeatureEnabled =
            hasFeature && model.options[feature] === true;
          if (isFeatureEnabled) {
            for (let expando in expandos) {
              model.prototype[expando] = expandos[expando];
            }
          }
        }
      });
    });
  };
};

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