js inheritance

subclass.js

// Person super class
function Person(name) {
    this.name = name;
}
Person.prototype.describe = function () {
    return 'Person called '+this.name;
};

// Employee sub class
function Employee(name, title) {
    Employee._super.constructor.call(this, name);
    this.title = title;
}
Employee.prototype.describe = function () {
    return Employee._super.describe.call(this)+' ('+this.title+')';
};

// inheritance
subclasses(Employee, Person);

// usage
var jane = new Employee('Jane', 'CTO');
jane.describe();


/**
 * 
 */
function subclasses(SubC, SuperC) {
    var subProto = Object.create(SuperC.prototype);
    // Save `constructor` and, possibly, other methods
    copyOwnPropertiesFrom(subProto, SubC.prototype);
    SubC.prototype = subProto;
    SubC._super = SuperC.prototype;
}


/**
 * (1) Get an array with the keys of all own properties of source.
 * (2) Iterate over those keys.
 * (3) Retrieve a property descriptor.
 * (4) Use that property descriptor to create an own property in target.
 * 
 * Note that this function is very similar to the function _.extend() in the Underscore.js library.
 */
function copyOwnPropertiesFrom(target, source) {
    Object.getOwnPropertyNames(source)  // (1)
    .forEach(function(propKey) {  // (2)
        var desc = Object.getOwnPropertyDescriptor(source, propKey); // (3)
        Object.defineProperty(target, propKey, desc);  // (4)
    });
    return target;
}