// -- ORM lib code --

​

function ORM() {

  // We will maintain a special property on each instance to keep track of

  // its dirty properties.  The property holds an object where the keys are

  // the dirty property keys.

  const dirtyPropsSymbol = Symbol('Prop holding obj where keys are dirty props');

  const resetDirtyProps  = (instance) =>       instance[dirtyPropsSymbol] = {};

  const markDirtyProp    = (instance, prop) => instance[dirtyPropsSymbol][prop] = true;

  const getDirtyProps    = (instance) =>       Object.keys(instance[dirtyPropsSymbol]);

  return {

    // Get an instance of `Fn` with the id of `id`

    getById: (Fn, id) => {

      // In real version we would perhaps hit an API or DB, but here we'll

      // just `new` an instance

      const instance = new Fn();

      // Wrap the instance in a proxy that tracks dirty props

      resetDirtyProps(instance);

      return new Proxy(instance, {

        set: (target, property, value) => {

          markDirtyProp(instance, property);

          // Perform the actual property assignment

          target[property] = value;

          return true;

        },

      });

    },

    // Persist and changes made on `instance`

    update: (instance) => {

      // In real version we'd perform the necessary updates to

      // persist any "dirty" properties, here we just log them

      const dirtyPropertyNames = getDirtyProps(instance);

      dirtyPropertyNames.length === 0 ?

        console.log('Nothing to update') :

        console.log('Dirty properties:', dirtyPropertyNames);

​

      // All properties are now persisted, so clear the list

      resetDirtyProps(instance);

    },

  };

}

​

// --- App code --

​

// Initialise the orm

const orm = new ORM();

​

// Define our constructor function

function Person(name, dob) {

  this.name = name;

  this.dob = dob;

}

​

// Get an instance

const person1 = orm.getById(Person, 1);

// no dirty properties so `update` will do nothing

orm.update(person1);

​

// Get an instance

const person2 = orm.getById(Person, 2);

// Mutate some state

person2.name = 'new name';

// `update` will persist any dirty properties

orm.update(person2);

​