For a long time Adapt Framework ECMAScript support has been limited by our minimum supported browser, most recently IE11, however it is now possible to use ES6, ES7 and ES8 in large parts of Adapt Framework whilst retaining IE11 support.

We still use requirejs to bundle our modules but rather than using UglifyJS for minification, which supports up to ES5, we now use Babel for minification and for transpilation and we transpile from ES6-8 to IE11 supported ES5.

Babel supports ECMAScript beyond ES8 but our module bundler requirejs uses esprima to parse ECMAScript and esprima only supports up to ES8. Introducing ES6-8 using this pathway should allow scope for parallel works to continue rather than converting the Adapt Framework and its build system in one go. We can now move towards replacing our build system in future versions whilst providing space for the community to modernise and refactor the Adapt Framework using much newer ECMAScript standards.

• You can only use ES6-8 in the /js/ folder of the core and plugins.
• The /required/ folder, the /libraries/ folder and externally required modules do not support ES6-8 as these are not transpiled by our build process and will not work in IE11.
• You should not use the import and export statements as we are still using requirejs as our module bundler.
• If using ES6-8 in a plugin, you must specify "framework": ">=5.3" in the bower.json.

There are a few major concepts worth understanding before using ES6 Classes and Backbone together. Backbone is an ES5 library for providing an easy class abstraction and a few base classes from which to extend and make new, easy to read behaviours. ES6 Classes are the standardised ECMAScript native class abstraction. Both Backbone and ES6 Classes seek to provide class abstractions in similar but subtly different ways.

A class in ECMAScript is simply a function called using the new keyword. This function is called the constructor function and is executed as usual using the () notation. When we call a constructor function using the new keyword, a new object instance is created and passed into the function as its this keyword. We call this process instantiation - creating a new instance of the class.

var Class = function() { // constructor function
  this; // is a new instance of the class
};
var instance = new Class(); // instantiation

The new keyword allows us to specify a set of behaviours on the class which each new object instance will inherit. These behaviours don't live on the instance directly but are instead inherited from the parent Class.

var Class = function Constructor(text) {
  this.savedText = text;
  this.log(text);
};
Class.prototype.log = function(text) { console.log(text); };

var instance = new Class('test1');
instance.log('test2');

var instance2 = new Class('test3');
instance2.log('test4');

If you execute and inspect the above code in a debugger, you will see that savedText lives directly on the new instance, whereas log is inherited via the __proto__ property.

It is possible to create many layers of inheritance such that a class is able to inherit behaviour from another and all behaviour from all classes in the chain of inheritance will be expressed on the instance.

var Class = function ConstructorA() {};
Class.prototype.log = function(text) { console.log(text); };

var Class2 = function ConstructorB() {};
Class2.prototype = Object.create(Class.prototype); // extend/inherit from the Class prototype
Class2.prototype.log2 = function(text) { console.log(`${text}2`); };

var instance = new Class2();
instance.log('test1');
instance.log2('test');

In the above example we can see that the instance inherits log from its grandparent and log2 from is parent. It should be apparent by this point that the syntax is rather messy and unnecessarily complex.

We have the constructor function, the new keyword, the this keyword, the constructor's prototype object, the instance and the instance's prototype inheritance chain.

Backbone and ES6 classes are identical in the above regards but both provide a much nicer syntax for achieving the same ends.

Backbone:

var Class = Backbone.Model.extend({ // returns a constructor function
  // defines the constructor.prototype object
  log: function(text) { console.log(text); }
});

var Class2 = Class.extend({
  log2: function(text) { console.log(`${text}2`); }
});

var instance = new Class2();
instance.log('test1');
instance.log2('test');

ES6:

class Class {  // returns a constructor function
  // defines the constructor.prototype object
  log(text) { console.log(text); }
}

class Class2 extends Class {
  log2(text) { console.log(`${text}2`); }
}

var instance = new Class2();
instance.log('test1');
instance.log2('test');

The named values on the objects passed into the Backbone.Class.extend function are copied property name by property name onto the constructor.prototype and constructor respectively. More on this later...

Aside from being able to define constructor prototype behaviour for each instance, it is possible to assign properties directly to the class constructor, these are called static properties. Class static properties are often helpful for defining behaviour which belongs to a class abstraction but which is not specific to an instance of the class.

var Class = function Constructor() {
  Class.recordInstances(); // record that a new instance was made
};

Class.instances = 0;
Class.recordInstances = function() {
  this.instances++;
};

Contrast String.fromCharCode() with String.prototype.charCodeAt().

Static properties are defined slightly differently in Backbone and ES6.

Backbone:

var Class = new Backbone.Model.extend({
  // defines the constructor.prototype object properties

  log: function() {}

}, {
  // defines class static properties

  recordInstances: function() {}

});

ES6:

class Class {
  
  // define a constructor.prototype object property
  log() {}

  // define a class static property
  static recordInstances() {}

}

Natively Backbone and ES6 differ in the way they treat class static properties. Backbone will copy the parent class static property values and assign them to the child class at the same property name, whereas ES6 will inherit parent class static properties on the child class.

In Adapt Framework we have a polyfill that corrects the Backbone's static property behaviour and brings it inline with ES6 inheritance, such that parent class static properties are now inherited by the child class in Adapt Framework.

When the extend function is called from a Backbone class, the extend function copies the values of the enumerable properties by name from both the prototype and static objects. As the extend function copies values only it disregards property descriptions. This means that a getter defined for Backbone extend to copy will only copy the getter's value and not the getter definition.

var constructorPrototype = {};
Object.defineProperty(constructorPrototype, 'test', {
  get: function() {
    // perform tasks
    return 1;
  }
});

var Class = new Backbone.Model.extend(constructorPrototype);

The above definition would only copy the value of the test property to the constructor prototype rather than copying the property definition.

It will produce:

Class.prototype === {
  test: 1
};

var instance = new Class();
// the defined getter function isn't called
instance.test === 1;

And not:

Class.prototype === {
  get test: function() {
    // perform tasks
    return 1;
  }
};

The way to define a getter or setter on a Backbone class is to perform the Object.defineProperty on the constructor.prototype after the class creation.

var Class = new Backbone.Model.extend({});
Object.defineProperty(Class.prototype, 'test', {
  get: function() {
    // perform tasks
    return 1;
  }
});

With Backbone classes it is possible to assign any value or reference to the constructor's prototype object or to the class statically and for it to be inherited on the instance. It is however more complicated to add a property getter/setter.

var Class = Backbone.Model.extend({
  a: null,
  b: 1,
  c: "string",
  d: function() {},
  e: {},
  f: []
}, {
  A: null,
  B: 1,
  C: "string",
  D: function() {},
  E: {},
  F: []
});

// prototype getter/setter definition
Object.defineProperty(Class.prototype.g, {
  get: function() {},
  set: function(value) {}
});

// static getter/setter definition
Object.defineProperty(Class.G, {
  get: function() {},
  set: function(value) {}
});

var instance = new Class();
// the value of a is inherited from the class prototype
instance.a === null;

In ES6 it is only possible to define a getter, setter or function on both the constructor's prototype object and on the class statically. It is more difficult to assign inherited default values on ES6 classes but much easier to define getters and setters.

class Class {

  d() {}
  get g() {}
  set g(value) {}
 
  static D() {}
  static get G() {}
  static set G(value) {}

}

// assign inheritable defaults
Class.prototype.a = null;
Class.prototype.b = 1;
Class.prototype.c = "string";
Class.prototype.e = {};
Class.prototype.f = [];

// assign static defaults
Class.A = null;
Class.B = 1;
Class.C = "string";
Class.E = {};
Class.F = [];

var instance = new Class();
// the value of a is inherited from the class prototype
instance.a === null;

The best way to set custom default values on ES6 classes using Backbone, is to do that in the constructor, preinitialize or initialize functions. These values will not inherited from the constructor prototype but will instead exist on the instance. See more on these functions next.

class Class {
  preinitialize() {
    this.customDefault = 1;
  }
}

Backbone classes come with predefined constructors which provide default class instantiation behaviours. The Backbone.Model class has a constructor which initializes model defaults, Backbone.View has behaviour which constructs the parent element attached to the view instance at this.$el. It is possible to override the constructor function in both ES6 and Backbone classes as follows.

Backbone:

var Class = Backbone.View.extend({
  constructor: function() {}
});

ES6:

class Class {
  constructor() {}
}

It is very unlikely however that anyone would want to override the default constructor behaviour of Backbone classes and so the Backbone constructor functions call a series of functions from which the default constructor can be extended rather than being overridden.

var Class = Backbone.View.extend({
  preinitialize: function() {
    // executed before default constructor behaviour
  },
  initialize: function() {
    // executed after default constructor behaviour
  }
});

The same pattern continues to apply when using ES6 syntax.

class Class extends Backbone.View {
  preinitialize() {
   // executed before default constructor behaviour
  }
  initialize() {
   // executed after default constructor behaviour
  }
}

As Backbone.extend doesn't copy property definitions and as ES6 classes cannot have easily defined default values, it is easiest to convert all Backbone initializing properties to their alternative syntax, as function definitions.

var Class = Backbone.Model.extend({
  defaults: {
    // defaults definition
  }
});

var Class = Backbone.View.extend({
  attributes: {
    // defaults definition
  }
});

The above code would be translated into the following in ES6.

class Class extends Backbone.Model {
   defaults() {
     return {
       // defaults definition
     };
   }
}

class Class extends Backbone.View {
   attributes() {
     return {
       // defaults definition
     };
   }
}

Previously if we needed to call a parent class function which had been overridden on the child class, we would need a reference to the parent class.

var Class = Backbone.Model.extend({

  test: function() {
    console.log('parent');
  }

});

var Class1 = Class.extend({

  initialize: function() {
    this.test();
  },

  test: function() {
    // Call the test function on the parent class
    Class.prototype.test.apply(this, arguments);
    console.log('child');
  }

});

In ES6 it becomes easier to do this with the super keyword.

class Class extends Backbone.Model {

  test() {
    console.log('parent');
  }

}

class Class1 extends Class {

  initialize() {
    this.test();
  }

  test(...args) {
    // Call the test function on the parent class
    super.test(...args);
    console.log('child');
  }

}

Happy coding!