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Discussion on: OOP vs functional programming

skyjur profile image
Ski

I have read JavaScript the Good-Parts. Haven't yet had chance to read the similarly named one on TypeScript.

On function returning objects through closures as means of OO there are benefits and weaknesses compared to class (old prototype based) approach.

function MyObj {
  var name = 'something'

  return {
    get name() { return name }
  }
}

I think it's not a good approach. I think Crockfords originally didn't take all important aspects into consideration.

It's useable in isolated case but I believe it was never practical to accept this as a standard way to move forward with OO in JS.

I also believe even before introduction of 'class' it was never mainstream approach. The approach that was always considered "correct" was prototypal OO (check examplesf influencial js libraries such as dojo, prototype, jQuery v1 - most common are custom class builders due to verbosity of direct usage of prototype, or use of prototype directly)

function MyObj{}
MyObj.prototype.name = function() { }

And due to multitude of different approaches towards doing OO in JS, there was a need to have a standard. Class was a straight forward thing that is widely understood and played perfectly well with JS prototype thing.

The approach of doing OO through means of closures comes with some benefits over prototypal/class based approach

  • You don't need 'this' when writing the code
  • Truly private scope
  • methods are indefinitely are bound to closure, thus there is no risk of screwing this up

This can be worked without much difficulty in 'class' and it was already proven.

Regarding the private attributes

  • ES7 supports private members throughh #privateMember
  • there is static code checking available for jsdoc tag /** @private */

Regarding the this bounding problem

  • static type checking can report this error
  • onClick=e => myObject.onClick(e) solves this

I consider passing method from object as a function to always be a weak practice. If one wants to pass function - this is excellent use case for arrow function. Always create a new arrow function, if wanting to pass a function. Arrow functions are the link between functional and OO approaches. Using method without object (as in passing it as function) doesn't check out with core OO idea of message passing to object.

Weak points of this approach to do OO I think were much harder to iron out. It simply didn't played nicely with underlying language concepts that existed from 1st days of JavaScript (prototypes). Problems of this approach:

  • same thing as with higher-order-function this pattern does not declare a type thus there is no means of referring to it - you'd need to invent something here to be able to refer it (something like x' in ReScript)
  • there was no means of introspection (which was understood as important goal of dynamic language at certain point, however now with transpilers commonly used this might not be a needed feature at all)
  • certain performance implications - objects created through this way are more memory heavy and slower to instantiate, might again not matter that much today but was a factor to chose prototype over this method
  • most importantly: there is no means of type checking const x = MyObject(); x instanceof MyObject, keyword instanceof just no longer makes any sense, it's hard to move forward with a technique that doesn't play well with certain 1st class language features already in existence

I understand the point of not using classes, in favor of doing pure functional development in JS. I completely miss the point of trying to use functions for OO where classes are available to implement OO.

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skyjur profile image
Ski

Here is a case from 2009 that argued against
functional pattern proposed by Douglas Crockford bolinfest.com/javascript/inheritan...

Many arguments are Closure Compiler related. Now of course if to be purists arguments about specific tool maybe would not matter. But consider that job of developers is to produce working software. If you had to write a bigger piece of code in js then Closure Compiler was the tool to go with, it would be either impossible, or you'd need to invent your own tools.

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peerreynders profile image
peerreynders

Haven't yet had chance to read the similarly named one on TypeScript.

There isn't one.

But when Microsoft states "so that the programmers at Microsoft could bring traditional object-oriented programs to the web" they are talking about C# style class-based object-orientation.

TypeScript was designed with a good class-based OO development experience in mind - sacrificing the ease of other approaches that may be equally valid under JavaScript (which claims to be multi-paradigm).

I also believe even before introduction of 'class' it was never mainstream approach.

You are correct. Because mainstream OO is "class-based". "OOP with closures" is only about objects, not classes.

The approach that was always considered "correct" was prototypal OO.

I would argue that the mainstream didn't even accept "prototypal OO" as correct given that there is no explicit class mechanism and the confusion that it caused up to and including ES5. But it certainly is possible to emulate class-based OO with prototypal OO. Strictly speaking membership to a class should last for the lifetime of the object. JavaScript objects can be augmented after construction - so "class membership" isn't fixed.

Class was a straight forward thing that is widely understood and played perfectly well with JS prototype thing.

The important aspect was that it aligned with the mainstream mindset of class-based object-orientation. However the code before ES2015 wasn't straightforward.

some benefits

You missed:

  • Elimination of inheritance. Commonalities have to be managed entirely through composition.
  • Don't need new to instantiate an object (useful if instances are cached);

ES7 supports private members through #privateMember

They landed in Chrome 74 but as such the proposal has been stuck at stage 3 since 2017 - it didn't get into ES2020; maybe it will be part of ES2021 (ES2016 is ECMA-262 7ᵗʰ Edition).

Regarding the this bounding problem

The problem with this has more to do with developers from other languages not understanding how it works - that it is a deliberate decision not to bind the function to the object.

(something like x' in ReScript)

'x is a simply type variable just like T is a type variable in Op2Args<T>. And the object returned by the factory still has a structural type.

keyword instanceof just no longer makes any sense,

instanceof can be a great escape hatch but the whole point of polymorphism is that the object should know what to do without the type needing to be known first (Replace Conditional with Polymorphism).

I completely miss the point of trying to use functions for OO where classes are available to implement OO.

There isn't just one kind of object-orientation. You are correct that the mainstream assumes "class-based object-orientation" when OO is mentioned; really COP - class-oriented programming would have been a better name (the code is declaring classes, not assembling objects).

"OOP with closures" doesn't seek to emulate "class-based object-orientation". Without inheritance, composition is the only option which leads to a simpler style of object-orientation. Also the notion isn't that "closures are like classes" but that "closures are like objects".

Once "closures are like objects" sinks in, it should become apparent that there are situations where closures can be more succinct than objects (created by a class).

Also consider that in 2008 ES5 wasn't even finalized yet - class wasn't official until 2015.

Which one is easier to understand

// closures as objects
function phone(phoneNumber) {
  return {
    getPhoneNumber: getPhoneNumber,
    getDescription: getDescription
  };

  function getPhoneNumber() {
    return phoneNumber;
  }

  function getDescription() {
    return 'This is a phone that can make calls.';
  }
}

function smartPhone(phoneNumber, signature) {
  var core = phone(phoneNumber);
  signature = signature || 'sent from ' + core.getPhoneNumber();

  return {
    getPhoneNumber: core.getPhoneNumber,
    getDescription: getDescription,
    sendEmail: sendEmail
  };

  function getDescription() {
    return core.getDescription() + ' It can also send email messages';
  }

  function sendEmail(emailAddress, message) {
    console.log('To: ' + emailAddress + '\n' + message + '\n' + signature);
  }
}

Or this one?

// Combination inheritance
// prototype chaining + constructor stealing
//
function Phone(phoneNumber) {
  this.phoneNumber = phoneNumber;
}

function getPhoneNumber() {
  return this.phoneNumber;
}

function getDescription() {
  return 'This is a phone that can make calls.';
}

Phone.prototype.getPhoneNumber = getPhoneNumber;
Phone.prototype.getDescription = getDescription;


function SmartPhone(phoneNumber, signature) {
  Phone.call(this, phoneNumber); // inherit properties

  this.signature = signature || 'sent from ' + this.getPhoneNumber();
}
SmartPhone.prototype = new Phone(); // inherit methods

function sendEmail(emailAddress, message) {
  console.log('To: ' + emailAddress + '\n' + message + '\n' + this.signature);
}

function getDescriptionSmart() {
  var description = Phone.prototype.getDescription.call(this);
  return description + ' It can also send email messages';
}

SmartPhone.prototype.sendEmail = sendEmail;
SmartPhone.prototype.getDescription = getDescriptionSmart;

If you had to write a bigger piece of code in js then Closure Compiler was the tool to go with, it would be either impossible, or you'd need to invent your own tools.

Not everybody has Google size problems - and the tradeoffs of the closure-based approach are known.

Clearly in a project using the Closure compiler one would stick to the recommended coding practices. But when in the past I had a look at the Closure library it struck me that it was organized to appeal to Java programmers - so it's not that surprising that the compiler would favour the pseudo-classical approach (apart from being more optimizable).

In any case I'm not recommending ignoring class - just to be familiar with the closure based approach; it does exist in the wild and in some situations it could come in handy.