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Programming by Stealth

A blog and podcast series by Bart Busschots & Allison Sheridan.

PBS 97 of X — Class Data Attributes & Functions

We’ll start this instalment by introducing the penultimate concept for this series-within-a-series on Object Oriented programming in JavaScript — class data attributes and class functions.

The theory is quite simple to lay out, but it doesn’t make much sense in the abstract, so we’ll spend most of this assignment making our way through a worked example.

We’ll also use the worked example as an opportunity to look at some techniques for creating linkages between instances of classes, and DOM objects within web pages. These techniques should prove helpful in solving the challenge set at the end of the previous instalment 😉

Instalment Resources

This instalment uses 4 example files:

Listen along to this instalment on episode 640 of the Chit Chat Across the Pond Podcast.

You can also Download the MP3

Quick Summary

Before we move forward, let’s take a moment to summarise where we’ve gotten to.

We have learned to use classes to allow us to construct concrete examples of abstract concepts. Each concrete example taking the form of an encapsulated object — that is to say, an object that contains both data, and, the functions to manipulate that data. We call these encapsulated objects instances of the class that constructed them. Because the data attributes and functions are encapsulated within each instance, we refer to them as instance data attributes, and instance functions.

Instance attributes and functions are accessed using the dot notation on instances of a class, so it’s someInstance.someAttribute or someInstance.someFunction().

Instance attributes are designed to hold information that is unique to each instance, and instance functions are designed to operate on a specific instance’s data. What if you have some data that is relevant to all instances of a class, but does not vary from instance to instance? Or what if you have functions that are related to the abstract concept a class represents, but not applicable to a single instance of the class. Where where should such data and functions go?

Class Data Attributes & Functions (in Any OO Language)

Well, if something is better associated with the abstract concept than with individual instances, they should be added to the class, not encapsulated into the instances. Rather unimaginatively, we refer to data attributes associated directly with a class as class data attributes, and functions associated directly with a class as class functions. Perhaps a little too imaginatively, many programmers also refer to class data attributes and class functions as static data attributes, and static functions, and many languages use the keyword static to mark an attribute or function as belonging to the class.

Class data attributes and class functions do not get encapsulated into instances of a class. This means they cannot be accessed via an instance of the class. Class data attributes and functions are instead accessed via the class, i.e. SomeClass.someData and SomeClass.someFunction().

Instance data attributes get initialised by a class’s constructor function(s). Many languages support so-called static initialiser functions to initialise class data attributes as the class gets loaded for the first time. However, this is not a universal concept, so in some languages you need to work around that limitation in less elegant ways (spoiler — JavaScript is one of those languages 🙁).

If this all sounds a little too abstract, let’s consider a class to represent circles again (like we did in the previous instalment). The value of Pi is clearly associated with the concept of a circle, but is it a property that varies from circle to circle? No it is not! It’s a single value that all circles have in common. This means it should be added as a class data attribute, not an instance data attribute (like we did in the previous instalment).

Similarly, a function for testing whether or not a given value is a reference to an instance of our circle class is clearly related to our circle class. However, it would make no sense to add it as an instance function, because it would not be encapsulated into anything that was not an instance of our class! Clearly, this should be a class function.

Class Data Attributes & Functions in JavaScript — The static Keyword

Let’s move from a generic description of the concept that applies to any OO language and focus in on how JavaScript implements class data attributes and functions. Remember that in this series-within-a-series we’re confining ourselves to the modern JavaScript OO syntax. Before ES6 things were done very differently, but we’re ignoring that history.

From our ES6 and beyond point of view, the correct way to add class data attributes is with getters and setters.

To mark a getter or setter as being for a class data attribute rather than an instance data attribute, simply pre-fix the declaration with the keyword static.

Similarly, to mark a function defined within a class as being a class function rather than an instance function, simply pre-fix the declaration with the keyword static.

Because the keyword static is used to mark attributes and functions as belonging to the class, many developers use static data attribute as a synonym for class data attribute, and static function as a synonym for class function.

Initialising Class Data Attributes in JavaScript

As mentioned above, JavaScript does not support static initialisers. This gives us two choices for how we initialise class data attributes:

  1. Initialise the class variables after the class definition.
  2. Write your getters in such a way that they gracefully deal with the current value being undefined, and behave appropriately.

Let’s see what the first approach looks like in a very simplified example:

class A{
  static get b(){
    return this._b;
  }
  static set b(b){
    this._b = b;
  }
}
A._b = 0; // initialise after class

And the same simplified example using one possible variant of the second approach:

class A{
  static get b(){
     // conditionally initialise in getter
    if(typeof this._b === 'undefined') this._b = 0;
    return this._b;
  }
  static set b(b){
    this._b = b;
  }
}

And here is another:

class A{
  static get b(){
    // don't initialise, have a default value instead
    return typeof this._b === undefined ? 0 : this._b ;
  }
  static set b(b){
    this._b = b;
  }
}

The first approach separates the initialisation from the class definition, so is only viable if you always define your classes in separate files, adding the initialisation code at the bottom of the file (after you close the class statement). Using a separate file is the only way to reliably insure the initialisation code won’t get separated from the class definition when you share your class between projects or with others.

I prefer to keep all the code for my classes within the class statement, so I prefer (and recommend) some variant of the second approach. That’s what I’ll be doing in this series.

The Keyword this Within Class (AKA Static) Functions

We’ve already learned that within instance functions the keyword this is a reference to the instance object I belong to. Similarly, within class functions, the keyword this is a reference to the class I belong to. This means that class functions can access other class functions and class data attributes using this.

Accessing Class Functions from Within Instance Functions

In JavaScript, every object constructed by a constructor, i.e. every instance of any class, automatically gets an instance data attribute named constructor. This automatically created data attribute will be a reference to the class the object is an instance of. That means that within instance functions, the class they belong to can be accessed with this.constructor. Since class data attributes belong to the class, they can be accessed via this.constructor.someClassAttribute. Similarly, class functions can be accessed via this.constructor.someClassFunction().

A Worked Example — the Nerdtouche Class

This all sounds very abstract, so let’s create a worked example — a class to represent a modern-day nerd-equivalent of the cartouches ancient Egyptian pharos used. The names of pharos were recorded in inscriptions as groups of pictograms encapsulated in a pill-shaped grouping.

In my imagination, a modern nerd equivalent would be a grouping of three emoji a person uses to represent themselves — I’ve decided to call them Nerdtouches 🙂

Since my biggest nerd loves are science, computing, and photography, I’ve chosen (🔭🖥️📷) as mine.

Let’s build a class to represent this nutty invention of mine.

We’ll start with a basic implementation of the idea. You’ll find the full code for this first pass at the problem in the file Nerdtouche1.js, and you can interact with the class via the JavaScript console on pbs97a.html.

Note that this class assumes it will be used on pages where the following third-party libraries have been loaded:

  1. jQuery
  2. Bootstrap 4
  3. is.js
  4. grapheme-splitter (more on this in a moment)

A Class Function

Let’s start by laying a useful foundation — a function for testing if a given value is a single emoji. This functionality is clearly related to the set of all Nerdtouches, and not specific to any given Nerdtouche, so it should be added as a class function.

Emoji are not characters in the traditional sense, they are Unicode graphemes, and because Unicode is … well … Unicode, that means you can’t use string length to test for the presence of a single grapheme. You can prove this to yourself using the JavaScript console on any web page:

''.length // 1
'💩'.length // 2
'🤦🏼‍'.length // 5

It’s also really hard to tell which unicode grapheme is an emoji, and which is some other symbol, so I made two decisions:

  1. For the purposes of this class, any single grapheme will count as an emoji.
  2. I won’t even try to write my own code for counting graphemes in a string, I’ll use a third-party library.

After a little Googling I decided to use grapheme-splitter which provides exactly the function I need — .countGraphemes()!

class Nerdtouche{

  // …

  static isEmoji(val){
    if(is.not.string(val)){
      return false;
    }
    return (new GraphemeSplitter()).countGraphemes(val) === 1;
  }
	
  // …
}

Because we pre-fixed the function definition with the static keyword we access our function via the class, not via an instance, so this function is Nerdtouche.isEmoji(). We can see this function in action on the JavaScript console in pbs97a.html:

Nerdtouche.isEmoji('💩') // true
Nerdtouche.isEmoji('🐎💩') // false

A Read-only Class Data Attribute

Next let’s add a read-only class data attribute to provide access to the number of emoji that make up a Nerdtouche:

class Nerdtouche{

  // …

  static get length(){
    return 3;
  }

  static set length(l){
    throw new Error(`Nerdtouches will always be ${this.length} emoji long!`);
  }
	
  // …
}

The only difference between this read-only class data attribute and a read-only instance data attribute is that both the getter and setter are pre-fixed with the keyword static.

We can see this property in action in the console:

Nerdtouche.length // 3
Nerdtouche.length = 4 // throws Error

Read/Write Class Data Attributes

Next let’s add a pair of class data attributes to store editable default values for the user’s handle and the emoji.

class Nerdtouche{

  // …

  static get defaultHandle(){
    return this._defaultHandle || 'Some Nerd';
  }
  static set defaultHandle(h){
    if(is.not.string(h)) throw new TypeError('Default Handle must be a string');
    if(is.empty(h)) throw new RangeError("Default Handle can't be an empty string");
    this._defaultHandle = h;
  }

  static get defaultEmoji(){
    return this._defaultEmoji || '⁉️';
  }
  static set defaultEmoji(e){
    if(is.not.string(e)) throw new TypeError('Default Emoji must be a string');
    if(!this.isEmoji(e)) throw new RangeError('Default Emoji must be a single Unicode character');
    this._defaultEmoji = e;
  }
  
  // …
}

There are two things I want to draw your attention to in these short getters and setters. Firstly, both use the approach of having the getter return a default value to work around JavaScript’s lack of support for static initialisers. Secondly, the setter for the default emoji calls the class function isEmoji() using the this keyword.

Two Instance Data Attributes

With the class functions and data attributes taken care of, let’s move on to the instance data attributes — there are just two of them, one for the handle, and one for the set of emoji:

class Nerdtouche{

  // …

  get handle(){
    return this._handle;
  }
  set handle(h){
    if(is.not.string(h)) throw new TypeError('Handle must be a string');
    if(is.empty(h)) throw new RangeError('Handle cannot be an empty string');
    this._handle = h;
  }
	
  get emoji(){
    return [...this._emoji] // shallow clone with spread operator
  }
  set emoji(e){
    const errMsg = `emoji must be an array of ${this.constructor.length} single Unicode graphemes`;
    if(is.not.array(e) || !is.all.string(e)){
      throw new TypeError(errMsg);
    }
    for(const emoji of e){
      if(!this.constructor.isEmoji(emoji)){
        throw new RangeError('each emoji must be a single Unicode graphemes');
      }
    }
    if(e.length < this.constructor.length){
      throw new TypeError(errMsg);
    }
    this._emoji = e.slice(0, this.constructor.length);
  }
  
  // …
}

The getter and setter for the handle are very much by-the-book, but the setter for the emoji warrants a closer look.

The first thing to notice is that this instance getter access the class function isEmoji() and the class data attribute length using the this.constructor syntax.

Secondly, note that the function avoid hard-coding the number of emoji by making use of the length class data attribute. We know that the setter for this class data attribute prevents users of the class from altering this value, but that’s no reason to hard-code the value in this getter. By making use of the read-only length class attribute I’m making it easy for myself to change my mind on the length later. Should I decide some day that Nerdtouches should be 4 emoji long, I just have to edit a single line of code!

Finally, notice the use of the Array instance function .slice() to do a small amount of data coercion – the setter throws an error if passed too few emoji, but truncates any excess of emoji to just the required number.

The Constructor

Next, there is of course a constructor which we can use to build actual Nerdtouche instances:

class Nerdtouche{

  // …

  constructor(handle, ...emoji){
    // set defaults if needed
    if(is.undefined(handle)) handle = this.constructor.defaultHandle;
    while(emoji.length < this.constructor.length){
      emoji.push(this.constructor.defaultEmoji);
    }
		
    // store the instance data
    this.handle = handle; // could throw error
    this.emoji = emoji; // could throw error
  }
  
  // …
}

This is pretty much by-the-book, but again, notice the use of this.constructor to access class data attributes. Also notice the use of default values to make all arguments to the constructor optional, and the use of the spread operator to bundle all arguments after the first one into an array named emoji.

Finally, we have some instance functions for rendering our Nerdtouche in various formats — plain text, as an HTML string, and as a jQuery object — and a function to append a Nerdtouche into a desired part of the document:

class Nerdtouche{

  // …

  asString(){
   return `(${this.emoji.join('')})`;
  }
	
  as$(){
    const $nerdtouche = $('<span>').html(this.emoji.join('<br>'));
    $nerdtouche.attr('title', this.handle);
    $nerdtouche.addClass('nerdtouche badge badge-secondary badge-pill p-1 m-1 align-middle');
    $nerdtouche.css({
      fontSize: '0.5em',
      lineHeight: 1.5
    });
    return $nerdtouche;
  }
	
  asHTML(){
    return this.as$()[0].outerHTML;
  }
	
  appendTo($container){
    if(is.not.object($container) || !$container.jquery){
      throw new TypeError('the container must be a jQuery object');
    }
    return $container.append(this.as$());
  }
  
  // …
}

This is all very much by-the-book, making use of various standard JavaScript and jQuery functions we’ve seen many times throughout this series. The only new addition is the use of the fact that all jQuery objects have an instance data attribute named jquery to quickly and easily test whether or not an object is a jQuery object. This is in keeping with jQuery’s developer documentation.

We can see our class in action using the JavaScript console on pbs97a.html:

Let’s start by creating some Nerdtouches that show the construtor’s defaulting in action and adding them into the DOM:

// everything defaulted
const genericNerd = new Nerdtouche();
genericNerd.appendTo($OUT_HTML);

// all but handle defaulted
const boringNerd = new Nerdtouche('unimaginator');
boringNerd.appendTo($OUT_HTML);

// just one emoji defaulted
const indecisiveNerd = new Nerdtouche('indecisivor', '🖥', '🤷‍♂️');
indecisiveNerd.appendTo($OUT_HTML);

// nothing defaulted
const trueNerd = new Nerdtouche('nerdificent', '🖥', '🕹', '🎮');
trueNerd.appendTo($OUT_HTML);

Hover over each Nerdtouche to see the handles.

Connecting Objects to the DOM Elements

In the web environment it’s quite normal for instances of your classes to map to one or more elements in the DOM. If you’re writing a class to represent a world clock, you’ll need to display that clock on the page! Similarly, the whole point of a class to represent Nerdtouches is to embed them into web pages.

It can often be very useful to create a two-way connection between our instance objects and the DOM objects representing them to the user. There is no single correct approach to this, and many possible approaches you could take.

At the moment our Nerdtouche class provides no linkage at all between instances of the class and the DOM objects built by those instances. Let’s add such linkages as a means of illustrating some useful concepts and approaches.

With something like a world clock you would have a clear one-to-one mapping between instance objects and DOM objects, but Nerdtouches are different. There’s no one-to-one mapping because each Nerdtouche can be included into the document arbitrarily many times.

The final code for our updated Nerdtouche class can be found in the file Nerdtouche2.js, and you can interact with this class via the JavaScript console on the file pbs97b.html.

Unique IDs or Classes

When you have a one-to-one mapping between instance objects and DOM objects you can use unique IDs to allow each instance to find it’s one matching DOM element.

When you have a many-to-one mapping you can’t use IDs, but you can uses classes, which is what we’ll do in our worked example.

However, regardless of whether you use a unique ID or class, you’re left with the same fundamental problem — how do you reliably generate unique identifiers?

My preferred solution is to add a counter to the class as a class data attribute. You can safely allow public read access to this counter, but you should prevent users from changing the value of the counter. Instead, the counter should get updated only by the constructor.

Let’s add such a counter to our class:

class Nerdtouche{

  // …

  static get count(){
    return this._count || 0;
  }
  static set count(c){
    throw new Error('Only the constructor may update the counter!');
  }
  
  constructor(handle, ...emoji){
    // …
    
    // increment the instance counter and store the sequence number
    this.constructor._count = this.constructor.count + 1;
    this._sequenceNumber = this.constructor.count;
  }
  
  // …
}

So, we have a class data attribute named count that gets incremented each time a new instance is created, and, each instance stores the value of the count when it was created as the private instance data attribute _sequenceNumber.

We can now use this to generate unique class names for each instance by adding an instance data attribute named uniqueClass, and injecting it into the jQuery objects built by the .as$() instance function:

class Nerdtouche{

  // …

  get uniqueClass(){
    return `nerdtouche-${this._sequenceNumber}`;
  }
	
  set uniqueClass(uc){
    throw new Error("Nerdtouche's unique classes can't be changed");
  }
  
  as$(){
    // …
	
	$nerdtouche.addClass(this.uniqueClass);
	
	// …
  }
  
  // …
}

We can now see that the HTML or each Nerdtouche contains a different class:

const bart = new Nerdtouche('bartificer', '🔭', '🖥', '📷');
const allison = new Nerdtouche('podfeet', '🐕', '🖥', '🚘');
console.log(bart.asHTML());
console.log(allison.asHTML());

DOM-Storing Data Attributes

Particularly in cases where there ‘s a one-to-one mapping between instances and DOM objects, it often makes sense to store a reference to the matching DOM object in an instance data attribute. That doesn’t apply here, so there’s no example in the Nerdtouche class, but it’s an approach you should bear in mind because if often does make sense.

DOM-Searching Class & Instance Functions

If you have unique IDs or classes, it can make sense to add class and/or instance functions for searching some of all of the DOM for matching DOM elements.

Since every Nerdtouche copy created by any Nerdtouche instance has the class .nerdtouche, we can easily write a class function that finds all copies of Nerdtouches. To make the function more powerful I chose to add an optional argument that can be used to limit the search to a sub-set of the document (like you can do with jQuery’s $() function ):

class Nerdtouche{

  // …

  static $find($container){
    if(is.not.undefined($container)){
      if(is.not.object($container) || !$container.jquery){
        throw new TypeError('If passed, the container must be a jQuery object');
      }
    }else{
      $container = $(document);
    }
    return $('.nerdtouche', $container);
  }
  
  // …
}

Because each instance’s Nerdtouche copies have a unique ID, we can extend the concept to a similar instance function that will find all copies of a specific Nerdtouche:

class Nerdtouche{

  // …

  $find($container){
    if(is.not.undefined($container)){
      if(is.not.object($container) || !$container.jquery){
        throw new TypeError('If passed, the container must be a jQuery object');
      }
    }else{
      $container = $(document);
    }
    return $(`.${this.uniqueClass}`, $container);
  }
  
  // …
}

Notice that the class and instance functions are different, but, they share a name. This might seem confusing, but there will never be any doubt as to which function is being executed because one only exists in the class, and the other has been encapsulated into each instance.

We can see this difference in action in the JavaScript console on pbs97b.html (refresh the page before executing):

// create two Nerdtouche instances
const bart = new Nerdtouche('bartificer', '🔭', '🖥', '📷');
const allison = new Nerdtouche('podfeet', '🐕', '🖥', '🚘');

// Add two copies of one and one of the other into the DOM:
bart.appendTo($OUT_HTML);
allison.appendTo($OUT_HTML);
bart.appendTo($OUT_HTML);

// use the class function to find all copies
Nerdtouche.$find();
// finds 3

// use the class function to find only copies of allison
allison.$find();
// finds 1

// use the class function find only copies of bart
bart.$find();
// finds 2

Data Attributes

So far all the techniques we’ve looked at connect instances to DOM objects, can we go the other way? Can we teach DOM objects which instance they belong to?

As it happens, we can — we can add a data attribute with a known name and a reference to the instance to the generated DOM object. We can do this by adding a single line to the .as$() instance function:

class Nerdtouche{

  // …

  as$(){
    // …
		
    // add a data attribute linking back to the instance object
    $nerdtouche.data('nerdtouche-object', this);
		
    // …
  }
  
  // …
}

In this case I chose to name the data attribute ‘nerdtouche-object’.

To see this functionality in action, refresh pbs97b.html and then run the following in the console:

// create a Nerdtouche and insert a copy into the document
const dorothy = new Nerdtouche('maclurker', '🖥', '⛵️', '🏰');
dorothy.appendTo($OUT_HTML);

// get a reference to a jQuery object representing all Nerdtouches
// since there's only one, that will be the one for dorothy
const $dorothy = $('.nerdtouche');

// access the instance via the data attribute
$OUT_TEXT.append(`the only Nerdtouche on the page is for ${$dorothy.data('nerdtouche-object').handle}`);

This is a very contrived example, so let’s make it a little more concrete. One of the biggest advantages having a reference to the instance embedded in the DOM is the ability to write one event handler than can correctly handle events on any relevant DOM object.

Let’s add the same click handler to ever Nerdtouche on a page and have that handler correctly interact with the appropriate instance object in every case.

To really prove that we are using the data attributes, we’ll add the Nerdtouches without ever saving references to the instance objects!

Please start by refreshing pbs97b.html, then enter the following in the JavaScript Console:

// create three Nerdtouches without saving them into named variables
// anonymous Nerdtouches if you will 🙂
(new Nerdtouche('bartificer', '🔭', '🖥', '📷')).appendTo($OUT_HTML);
(new Nerdtouche('maclurker', '🖥', '⛵️', '🏰')).appendTo($OUT_HTML);
(new Nerdtouche('podfeet', '🐕', '🖥', '🚘')).appendTo($OUT_HTML);

// add a click handler to all Nerdtouches
Nerdtouche.$find().click(function(){
  // get the instance object from the data attribute
  // this is the DOM element that was clicked on
  const nerdtouche = $(this).data('nerdtouche-object');
  
  // write a message to the plain text area
  $OUT_TEXT.append(`you clicked on ${nerdtouche.handle}'s Nerdtouche!\n`);
});

Now, click on any Nerdtouche and watch how the same event handler always know which anonymous instance object to use!

An important caveat to bear in mind is that data attributes linking to objects rather than primitive values can’t be expressed in HTML, so to use data attributes you need to work exclusively with native DOM objects or jQuery objects.

To illustrate this point, refresh pbs97b.html, and execute the following in the JavaScript console:

// add one Nerdtouche as a jQuery object
const bart = new Nerdtouche('bartificer', '🔭', '🖥', '📷');
$OUT_HTML.append( bart.as$() );

// add another as an HTML string
const allison = new Nerdtouche('podfeet', '🐕', '🖥', '🚘'); 
$OUT_HTML.append( allison.asHTML() );

// add a click handler to all Nerdtouches and check for the data attribute
Nerdtouche.$find().click(function(){
  $nerdtoucheDOM = $(this);
  nerdtoucheInstance = $nerdtoucheDOM.data('nerdtouche-object');
  if(nerdtoucheInstance){
    $OUT_TEXT.append(`🙂 found an instance object for ${nerdtoucheInstance.handle}\n`);
  }else{
    $OUT_TEXT.append(`🙁 no instance object found in DOM object with classes: ${$nerdtoucheDOM.attr('class')}\n`);
  }
});

And then click on each Nerdtouche in turn.

Final Thoughts

We’ve now almost completed our third attempt at describing Object Oriented programming in JavaScript. We have just one more concept left to explore, and it’s a big one!

In the real world, concepts tend to be hierarchical. The abstract concept of a manager is a sub-set of the abstract concept of an employee, is a sub-set of the abstract concept of a person. All people have names, but only employees have employment contracts, and only managers have other employees reporting to them. As things stand, if we were to try write three classes to represent these three abstract concept we would find ourselves with a lot of code duplication. All instance data attributes and functions that apply to people apply to employees and managers too, so would be triplicated! Similarly, all instance data attributes and functions that apply to employees apply to managers too, so that would all be duplicated.

Surely there must be a better way? Of course there is! If the real world can gave concept hierarchies, surely our classes should be able to have matching hierarchies? Well, they can, through the mechanism of inheritance (often referred to by the highfalutin term polymorphism).

This simple but powerful concept will be the focus of the next instalment.

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