chapter2.md

Best practice in inheritance

In this section we will try to combine the 3 ways of making inheritance in JS. So we will mix:

1. Prototype Delegation:

   • best for shared methods.
   • best in memory consumption.

2. Concatenative inheritance(mixins):

   • best for saving states using Object.assign.

3. Functional Inheritance:

   • best for encapsulating private data using closures.

Example:

const animal = {
  hello() {
    return `Hello, my name is ${ this.info.name }`;
  }
};

const mouseFactory = function mouseFactory(info, secret) {
  // closure to handle private data
  const privateData = secret;
  return Object.assign(Object.create(animal),
    // here we can store states with safety as the instances and the 
    // "source" 
    // object and the instances among them do not retain a reference.
    // So, changing user.surname will affect only the current instance, and 
    // not the other ones.
    {
      info: {
        name: info.name,
        surname: info.surname,
      },
      getPrivateData() {
        return privateData;
      }
    }
  )
};

const mouse = mouseFactory({ name: 'micky', surname: 'mouse'}, 'disney');
const cat = mouseFactory({ name: 'perseas', surname: 'nasikas'}, 'pet');

console.log('mouse', mouse);
console.log('cat', cat);
console.log(mouse.info); // -> { name: 'micky', surname: 'mouse'}
console.log(cat.info); // -> {name: "perseas", surname: "nasikas"}
console.log(mouse.hello()); // -> Hello, my name is micky
console.log(cat.hello()) // -> Hello, my name is perseas

Now we will mutate the object state (info) in an instance to see if this change will affect the other instance.

mouse.info.name = 'mutated';
console.log(mouse.info); // -> {name: "mutated", surname: "mouse"}
console.log(cat.info); // -> {name: "perseas", surname: "nasikas"}

Lets check if we can access the private data.

console.log(mouse.privateData); // -> undefined
console.log(mouse.secret); // -> undefined
console.log(mouse.getPrivateData()); // -> disney
mouse.secret = 'mutated'
console.log(mouse.getPrivateData()); // -> disney

Conclusions:

1. Mutating any member of the object or array does NOT affect the other instances, because we use Object.assign() to save states, so it copis all of the enumerable own properties by assignment from the source objects to the destination objects with last in priority and the the instances do not retain a reference.

2. Using function factory with closure we can encapsulate private data(secret). The produced instance has not any direct access to this private data. It can only access it using the method getPrivateData(). Also, it is not possible to change the private data directly (for example using: mouse.secret=...). In order to be able to change the private data, we have to define a new method for example: setPrivateData(newSecret) { privateData = newSecret; return this; } and the call mouse.settPrivateData('changed').

3. We use a prototype object only for shaving public/shared methods.