Class declarations with object exemplar semantics

object_exemplar_decls.js

//----- Example code:

    // Class declarations:
    // - No data properties allowed
    // - Future: support mixins
    // - Optional: call the initialization method `new` instead of `constructor`

    // Superclass
    class Person {
        constructor(name) {
            this.name = name;
        }
        describe() {
            return "Person called "+this.name;
        }
    }

    // Subclass
    class Employee extends Person {
        constructor(name, title) {
            super.constructor(name);
            this.title = title;
        }
        describe() {
            return super.describe()+" ("+this.title+")";
        }
    });
    
    var jane = new Employee("Jane", "CTO");
    console.log(jane instanceof Person); // true

//----- Desugars to:

    // Advantages:
    // - You can always write Person.* instead of Person.prototype.*
    // - Subtyping becomes very easy
    // More information: http://www.2ality.com/2011/06/prototypes-as-classes.html

    var Person = {
        constructor: function (name) {
            this.name = name;
        },
        describe: function() {
            return "Person called "+this.name;
        }
    };

    var Employee = Object.create(Person);
    Employee.constructor = function (name, title) {
        Employee.super.constructor.call(this, name);
        this.title = title;
    };
    Employee.describe = function () {
        return Employee.super.describe.call(this)+" ("+this.title+")";
    };

    var jane = Object.create(Employee);
    jane.constructor("Jane", "CTO");
    console.log(Person.isPrototypeOf(jane));

//----- Same functionality, via the library Proto.js, https://github.com/rauschma/proto-js

    var Person = Proto.extend({
        constructor: function (name) {
            this.name = name;
        },
        describe: function() {
            return "Person called "+this.name;
        }
    });

    var Employee = Person.extend({
        constructor: function (name, title) {
            Employee.super.constructor.call(this, name);
            this.title = title;
        },
        describe: function () {
            return Employee.super.describe.call(this)+" ("+this.title+")";
        }
    });

    var jane = Employee.new("Jane", "CTO");
    console.log(Person.isPrototypeOf(jane));
    
//----- Compatibility with (legacy) constructor functions

    // - To extend a constructor function Foo, you write: `extends Foo.prototype`
    //   Could be handled automatically, by checking whether the operand of `extends` is a function.
    //   Proto.js comes with support for extending constructor functions.