Javascript Unlocked

Chapter 1. Diving into the JavaScript Core

You may have owned an iPhone for years and regard yourself as an experienced user. At the same time, you keep removing unwanted characters one at a time while typing by pressing delete. However, one day you find out that a quick shake allows you to delete the whole message in one tap. Then you wonder why on earth you didn't know this earlier. The same thing happens with programming. We can be quite satisfied with our coding until, all of sudden, we run into a trick or a lesser-known language feature that makes us reconsider the entire work done over the years. It turns out that we could do this in a cleaner, more readable, more testable, and more maintainable way. So it's presumed that you already have experience with JavaScript; however, this chapter equips you with the best practices to improve your code. We will cover the following topics:

Making your code readable and expressive
Mastering multiline strings in JavaScript
Manipulating arrays in the ES5 way
Traversing an object in an elegant, reliable, safe, and fast way
The most effective way of declaring objects
How to magic methods in JavaScript

Make your code readable and expressive

There are numerous practices and heuristics to make a code more readable, expressive, and clean. We will cover this topic later on, but here we will talk about syntactic sugar. The term means an alternative syntax that makes the code more expressive and readable. In fact, we already had some of this in JavaScript from the very beginning. For instance, the increment/decrement and addition/subtraction assignment operators inherited from C. foo++ is syntactic sugar for foo = foo + 1, and foo += bar is a shorter form for foo = foo + bar. Besides, we have a few tricks that serve the same purpose.

JavaScript applies logical expressions to so-called short-circuit evaluation. This means that an expression is read left to right, but as soon as the condition result is determined at an early stage, the expression tail is not evaluated. If we have true || false || false, the interpreter will know from the first test that the result is true regardless of other tests. So the false || false part is not evaluated, and this opens a way for creativity.

Function argument default value

When we need to specify default values for parameters we can do like that:

function stub( foo ) {
 return foo || "Default value";
}

console.log( stub( "My value" ) ); // My value
console.log( stub() ); // Default value

What is going on here? When foo is true (not undefined, NaN, null, false, 0, or ""), the result of the logical expression is foo otherwise the expression is evaluated until Default value and this is the final result.

Starting with 6th edition of EcmaScript (specification of JavaScript language) we can use nicer syntax:

function stub( foo = "Default value" ) {
 return foo;
}

Conditional invocation

While composing our code we shorten it on conditions:"

var age = 20;
age >= 18 && console.log( "You are allowed to play this game" );
age >= 18 || console.log( "The game is restricted to 18 and over" );

In the preceding example, we used the AND (&&) operator to invoke console.log if the left-hand condition is Truthy. The OR (||) operator does the opposite, it calls console.log if the condition is Falsy.

I think the most common case in practice is the shorthand condition where the function is called only when it is provided:

/**
* @param {Function} [cb] - callback
*/
function fn( cb ) {
 cb && cb();
};

The following is one more example on this:

/**
* @class AbstractFoo
*/
AbstractFoo = function(){
 // call this.init if the subclass has init method
 this.init && this.init();
};

Syntactic sugar was introduced to its full extent to the JavaScript world only with the advance in CoffeeScript, a subset of the language that trans-compiles (compiles source-to-source) into JavaScript. Actually CoffeeScript, inspired by Ruby, Python, and Haskell, has unlocked arrow-functions, spreads, and other syntax to JavaScript developers. In 2011, Brendan Eich (the author of JavaScript) admitted that CoffeeScript influenced him in his work on EcmaScript Harmony, which was finalized this summer in ECMA-262 6th edition specification. From a marketing perspective, the specification writers agreed on using a new name convention that calls the 6th edition as EcmaScript 2015 and the 7th edition as EcmaScript 2016. Yet the community is used to abbreviations such as ES6 and ES7. To avoid confusion further in the book, we will refer to the specifications by these names. Now we can look at how this affects the new JavaScript.

Arrow functions

Traditional function expression may look like this:

function( param1, param2 ){ /* function body */ }

When declaring an expression using the arrow function (aka fat arrow function) syntax, we will have this in a less verbose form, as shown in the following:

( param1, param2 ) => { /* function body */ }

In my opinion, we don't gain much with this. But if we need, let's say, an array method callback, the traditional form would be as follows:

function( param1, param2 ){ return expression; }

Now the equivalent arrow function becomes shorter, as shown here:

( param1, param2 ) => expression

We may do filtering in an array this way:

// filter all the array elements greater than 2
var res = [ 1, 2, 3, 4 ].filter(function( v ){
 return v > 2;
})
console.log( res ); // [3,4]

Using an array function, we can do filtering in a cleaner form:

var res  = [ 1, 2, 3, 4 ].filter( v => v > 2 );
console.log( res ); // [3,4]

Besides shorter function declaration syntax, the arrow functions bring the so called lexical this. Instead of creating its own context, it uses the context of the surrounding object as shown here:

"use strict";
/**
* @class View
*/   
let View = function(){
 let button = document.querySelector( "[data-bind=\"btn\"]" );
 /**
  * Handle button clicked event
  * @private 
  */
 this.onClick = function(){
   console.log( "Button clicked" );
 };
 button.addEventListener( "click", () => {
   // we can safely refer surrounding object members
   this.onClick(); 
 }, false );
}

In the preceding example, we subscribed a handler function to a DOM event (click). Within the scope of the handler, we still have access to the view context (this), so we don't need to bind the handler to the outer scope or pass it as a variable through the closure:

var that = this;
button.addEventListener( "click", function(){
  // cross-cutting concerns
  that.onClick(); 
}, false );

Method definitions

As mentioned in the preceding section, arrow functions can be quite handy when declaring small inline callbacks, but always applying it for a shorter syntax is controversial. However, ES6 provides new alternative method definition syntax besides the arrow functions. The old-school method declaration may look as follows:

var foo = {
 bar: function( param1, param2 ) {
 }
}

In ES6 we can get rid of the function keyword and the colon. So the preceding code can be put this way:

let foo = {
 bar ( param1, param2 ) {
 }
}

The rest operator

Another syntax structure that was borrowed from CoffeeScript came to JavaScript as the rest operator (albeit, the approach is called splats in CoffeeScript).

When we had a few mandatory function parameters and an unknown number of rest parameters, we used to do something like this:

"use strict";
var cb = function() {
 // all available parameters into an array
 var args = [].slice.call( arguments ),
     // the first array element to foo and shift
     foo = args.shift(),
     // the new first array element to bar and shift
     bar = args.shift();
 console.log( foo, bar, args );
};
cb( "foo", "bar", 1, 2, 3 ); // foo bar [1, 2, 3]

Now check out how expressive this code becomes in ES6:

let cb = function( foo, bar, ...args ) {
 console.log( foo, bar, args );
}
cb( "foo", "bar", 1, 2, 3 ); // foo bar [1, 2, 3]

Function parameters aren't the only application of the rest operator. For example, we can use it in destructions as well, as follows:

let [ bar, ...others ] = [ "bar", "foo", "baz", "qux" ];
console.log([ bar, others ]); // ["bar",["foo","baz","qux"]]

The spread operator

Similarly, we can spread array elements into arguments:

let args = [ 2015, 6, 17 ],
   relDate = new Date( ...args );
console.log( relDate.toString() );  // Fri Jul 17 2015 00:00:00 GMT+0200 (CEST)

ES6 also provides expressive syntactic sugar for object creation and inheritance, but we will examine this later in The most effective way of declaring objects section.

Mastering multiline strings in JavaScript

Multi-line strings aren't a good part of JavaScript. While they are easy to declare in other languages (for instance, NOWDOC), you cannot just keep single-quoted or double-quoted strings in multiple lines. This will lead to syntax error as every line in JavaScript is considered as a possible command. You can set backslashes to show your intention:

var str = "Lorem ipsum dolor sit amet, \n\
consectetur adipiscing elit. Nunc ornare, \n\
diam ultricies vehicula aliquam, mauris \n\
ipsum dapibus dolor, quis fringilla leo ligula non neque";

This kind of works. However, as soon as you miss a trailing space, you get a syntax error, which is not easy to spot. While most script agents support this syntax, it's, however, not a part of the EcmaScript specification.

In the times of EcmaScript for XML (E4X), we could assign a pure XML to a string, which opened a way for declarations such as these:

var str = <>Lorem ipsum dolor sit amet, 
consectetur adipiscing 
elit. Nunc ornare </>.toString();

Nowadays E4X is deprecated, it's not supported anymore.

Concatenation versus array join

We can also use string concatenation. It may feel clumsy, but it's safe:

var str = "Lorem ipsum dolor sit amet, \n" +
 "consectetur adipiscing elit. Nunc ornare,\n" +
 "diam ultricies vehicula aliquam, mauris \n" +
 "ipsum dapibus dolor, quis fringilla leo ligula non neque";

You may be surprised, but concatenation is slower than array joining. So the following technique will work faster:

var str = [ "Lorem ipsum dolor sit amet, \n",
 "consectetur adipiscing elit. Nunc ornare,\n",
 "diam ultricies vehicula aliquam, mauris \n",
 "ipsum dapibus dolor, quis fringilla leo ligula non neque"].join( "" );

Template literal

What about ES6? The latest EcmaScript specification introduces a new sort of string literal, template literal:

var str = `Lorem ipsum dolor sit amet, \n
consectetur adipiscing elit. Nunc ornare, \n
diam ultricies vehicula aliquam, mauris \n
ipsum dapibus dolor, quis fringilla leo ligula non neque`;

Now the syntax looks elegant. But there is more. Template literals really remind us of NOWDOC. You can refer any variable declared in the scope within the string:

"use strict";
var title = "Some title",
   text = "Some text",
   str = `<div class="message">
<h2>${title}</h2>
<article>${text}</article>
</div>`;
console.log( str );

The output is as follows:

<div class="message">
<h2>Some title</h2>
<article>Some text</article>
</div>

If you wonder when can you safely use this syntax, I have a good news for you—this feature is already supported by (almost) all the major script agents (http://kangax.github.io/compat-table/es6/).

Multi-line strings via transpilers

With the advance of ReactJS, Facebook's EcmaScript language extension named JSX (https://facebook.github.io/jsx/) is now really gaining momentum. Apparently influenced by previously mentioned E4X, they proposed a kind of string literal for XML-like content without any screening at all. This type supports template interpolation similar to ES6 templates:

"use strict";
var Hello = React.createClass({
 render: function() {
   return <div class="message">
<h2>{this.props.title}</h2>
<article>{this.props.text}</article>
</div>;
 }
});

React.render(<Hello title="Some title" text="Some text" />, node);

Another way to declare multiline strings is by using CommonJS Compiler (http://dsheiko.github.io/cjsc/). While resolving the 'require' dependencies, the compiler transforms any content that is not .js/.json content into a single-line string:

foo.txt

Lorem ipsum dolor sit amet,
consectetur adipiscing elit. Nunc ornare,
diam ultricies vehicula aliquam, mauris
ipsum dapibus dolor, quis fringilla leo ligula non neque

consumer.js

var str = require( "./foo.txt" );
console.log( str );

You can find an example of JSX use in Chapter 6, A Large-Scale JavaScript Application Architecture.

Manipulating arrays in the ES5 way

Some years ago when the support of ES5 features was poor (EcmaScript 5th edition was finalized in 2009), libraries such as Underscore and Lo-Dash got highly popular as they provided a comprehensive set of utilities to deal with arrays/collections. Today, many developers still use third-party libraries (including jQuery/Zepro) for methods such as map, filter, every, some, reduce, and indexOf, while these are available in the native form of JavaScript. It still depends on how you use such libraries, but it may likely happen that you don't need them anymore. Let's see what we have now in JavaScript.

Array methods in ES5

Array.prototype.forEach is probably the most used method of the arrays. That is, it is the native implementation of _.each, or for example, of the $.each utilities. As parameters, forEach expects an iteratee callback function and optionally a context in which you want to execute the callback. It passes to the callback function an element value, an index, and the entire array. The same parameter syntax is used for most array manipulation methods. Note that jQuery's $.each has the inverted callback parameters order:

"use strict";
var data = [ "bar", "foo", "baz", "qux" ];
data.forEach(function( val, inx ){
  console.log( val, inx ); 
});

Array.prototype.map produces a new array by transforming the elements of a given array:

"use strict";
var data = { bar: "bar bar", foo: "foo foo" },
   // convert key-value array into url-encoded string
   urlEncStr = Object.keys( data ).map(function( key ){
     return key + "=" + window.encodeURIComponent( data[ key ] );
   }).join( "&" );

console.log( urlEncStr ); // bar=bar%20bar&foo=foo%20foo

Array.prototype.filter returns an array, which consists of given array values that meet the callback's condition:

"use strict";
var data = [ "bar", "foo", "", 0 ],
   // remove all falsy elements
   filtered = data.filter(function( item ){
     return !!item;
   });

console.log( filtered ); // ["bar", "foo"]

Array.prototype.reduce/Array.prototype.reduceRight retrieves the product of values in an array. The method expects a callback function and optionally the initial value as arguments. The callback function receive four parameters: the accumulative value, current one, index and original array. So we can, for an instance, increment the accumulative value by the current one (return acc += cur;) and, thus, we will get the sum of array values.

Besides calculating with these methods, we can concatenate string values or arrays:

"use strict";
var data = [[ 0, 1 ], [ 2, 3 ], [ 4, 5 ]],
   arr = data.reduce(function( prev, cur ) {
     return prev.concat( cur );
   }),
   arrReverse = data.reduceRight(function( prev, cur ) {
     return prev.concat( cur );
   });

console.log( arr ); //  [0, 1, 2, 3, 4, 5]
console.log( arrReverse ); // [4, 5, 2, 3, 0, 1]

Array.prototype.some tests whether any (or some) values of a given array meet the callback condition:

"use strict";
var bar = [ "bar", "baz", "qux" ],
   foo = [ "foo", "baz", "qux" ],
   /**
    * Check if a given context (this) contains the value
    * @param {*} val
    * @return {Boolean}
    */
   compare = function( val ){
     return this.indexOf( val ) !== -1; 
   };

console.log( bar.some( compare, foo ) ); // true

In this example, we checked whether any of the bar array values are available in the foo array. For testability, we need to pass a reference of the foo array into the callback. Here we inject it as context. If we need to pass more references, we would push them in a key-value object.

As you probably noticed, we used in this example Array.prototype.indexOf. The method works the same as String.prototype.indexOf. This returns an index of the match found or -1.

Array.prototype.every tests whether every value of a given array meets the callback condition:

"use strict";
var bar = [ "bar", "baz" ],
   foo = [ "bar", "baz", "qux" ],
   /**
    * Check if a given context (this) contains the value
    * @param {*} val
    * @return {Boolean}
    */
   compare = function( val ){
     return this.indexOf( val ) !== -1; 
   };

console.log( bar.every( compare, foo ) ); // true

If you are still concerned about support for these methods in a legacy browser as old as IE6-7, you can simply shim them with https://github.com/es-shims/es5-shim.

Array methods in ES6

In ES6, we get just a few new methods that look rather like shortcuts over the existing functionality.

Array.prototype.fill populates an array with a given value, as follows:

"use strict";
var data = Array( 5 );
console.log( data.fill( "bar" ) ); // ["bar", "bar", "bar", "bar", "bar"]

Array.prototype.includes explicitly checks whether a given value exists in the array. Well, it is the same as arr.indexOf( val ) !== -1, as shown here:

"use strict";
var data = [ "bar", "foo", "baz", "qux" ];
console.log( data.includes( "foo" ) );

Array.prototype.find filters out a single value matching the callback condition. Again, it's what we can get with Array.prototype.filter. The only difference is that the filter method returns either an array or a null value. In this case, this returns a single element array, as follows:

"use strict";
var data = [ "bar", "fo", "baz", "qux" ],
   match = function( val ){
     return val.length < 3;
   };
console.log( data.find( match ) ); // fo

Traversing an object in an elegant, reliable, safe, and fast way

It is a common case when we have a key-value object (let's say options) and need to iterate it. There is an academic way to do this, as shown in the following code:

"use strict";
var options = {
    bar: "bar",
    foo: "foo"
   },
   key;
for( key in options ) {
 console.log( key, options[ key] );
}

The preceding code outputs the following:

bar bar
foo foo

Now let's imagine that any of the third-party libraries that you load in the document augments the built-in Object:

Object.prototype.baz = "baz";

Now when we run our example code, we will get an extra undesired entry:

bar bar
foo foo
baz baz

The solution to this problem is well known, we have to test the keys with the Object.prototype.hasOwnProperty method:

//…
for( key in options ) {
 if ( options.hasOwnProperty( key ) ) {
   console.log( key, options[ key] );
 }
}

Iterating the key-value object safely and fast

Let's face the truth—the structure is clumsy and requires optimization (we have to perform the hasOwnProperty test on every given key). Luckily, JavaScript has the Object.keys method that retrieves all string-valued keys of all enumerable own (non-inherited) properties. This gives us the desired keys as an array that we can iterate, for instance, with Array.prototype.forEach:

"use strict";
var options = {
    bar: "bar",
    foo: "foo"
   };
Object.keys( options ).forEach(function( key ){
 console.log( key, options[ key] );
});

Besides the elegance, we get a better performance this way. In order to see how much we gain, you can run this online test in distinct browsers such as: http://codepen.io/dsheiko/pen/JdrqXa.

Enumerating an array-like object

Objects such as arguments and nodeList (node.querySelectorAll, document.forms) look like arrays, in fact they are not. Similar to arrays, they have the length property and can be iterated in the for loop. In the form of objects, they can be traversed in the same way that we previously examined. But they do not have any of the array manipulation methods (forEach, map, filter, some and so on). The thing is we can easily convert them into arrays as shown here:

"use strict";
var nodes = document.querySelectorAll( "div" ),
   arr = Array.prototype.slice.call( nodes );

arr.forEach(function(i){
 console.log(i);
});

The preceding code can be even shorter:

arr = [].slice.call( nodes )

It's a pretty convenient solution, but looks like a trick. In ES6, we can do the same conversion with a dedicated method:

arr = Array.from( nodes );

The collections of ES6

ES6 introduces a new type of objects—iterable objects. These are the objects whose elements can be retrieved one at a time. They are quite the same as iterators in other languages. Beside arrays, JavaScript received two new iterable data structures, Set and Map. Set which are a collection of unique values:

"use strict";
let foo = new Set();
foo.add( 1 );
foo.add( 1 );
foo.add( 2 );
console.log( Array.from( foo ) ); // [ 1, 2 ]

let foo = new Set(), 
   bar = function(){ return "bar"; };
foo.add( bar );
console.log( foo.has( bar ) ); // true

The map is similar to a key-value object, but may have arbitrary values for the keys. And this makes a difference. Imagine that we need to write an element wrapper that provides jQuery-like events API. By using the on method, we can pass not only a handler callback function but also a context (this). We bind the given callback to the cb.bind( context ) context. This means addEventListener receives a function reference different from the callback. How do we unsubscribe the handler then? We can store the new reference in Map by a key composed from an event name and a callback function reference:

"use strict";
/**
* @class
* @param {Node} el
*/
let El = function( el ){
 this.el = el;
 this.map = new Map();
};
/**
* Subscribe a handler on event
* @param {String} event
* @param {Function} cb
* @param {Object} context
*/
El.prototype.on = function( event, cb, context ){
 let handler = cb.bind( context || this );
 this.map.set( [ event, cb ], handler );
 this.el.addEventListener( event, handler, false );
};
/**
* Unsubscribe a handler on event
* @param {String} event
* @param {Function} cb
*/

El.prototype.off = function( event, cb ){
 let handler = cb.bind( context ),
     key = [ event, handler ];
 if ( this.map.has( key ) ) {
   this.el.removeEventListener( event, this.map.get( key ) );
   this.map.delete( key );
 }
};

Any iterable object has methods, keys, values, and entries, where the keys work the same as Object.keys and the others return array values and an array of key-value pairs respectively. Now let's see how we can traverse the iterable objects:

"use strict";
let map = new Map()
 .set( "bar", "bar" )
 .set( "foo", "foo" ),
   pair;
for ( pair of map ) {
 console.log( pair );
}

// OR 
let map = new Map([
   [ "bar", "bar" ],
   [ "foo", "foo" ],
]);
map.forEach(function( value, key ){
 console.log( key, value );
});

Iterable objects have manipulation methods such as arrays. So we can use forEach. Besides, they can be iterated by for...in and for...of loops. The first one retrieves indexes and the second, the values.

The most effective way of declaring objects

How do we declare an object in JavaScript? If we need a namespace, we can simply use an object literal. But when we need an object type, we need to think twice about what approach to take, as it affects the maintainability of our object-oriented code.

Classical approach

We can create a constructor function and chain the members to its context:

"use strict"; 
/**
 * @class
 */
var Constructor = function(){
   /**
   * @type {String}
   * @public
   */
   this.bar = "bar";
   /**
   * @public
   * @returns {String}
   */
   this.foo = function() {
    return this.bar;
   };
 },
 /** @type Constructor */
 instance = new Constructor();

console.log( instance.foo() ); // bar
console.log( instance instanceof Constructor ); // true

We can also assign the members to the constructor prototype. The result will be the same as follows:

"use strict";
/**
* @class
*/
var Constructor = function(){},
   instance;
/**
* @type {String}
* @public
*/
Constructor.prototype.bar = "bar";
/**
* @public
* @returns {String}
*/
Constructor.prototype.foo = function() {
 return this.bar;
};
/** @type Constructor */
instance = new Constructor();

console.log( instance.foo() ); // bar
console.log( instance instanceof Constructor ); // true

In the first case, we have the object structure within the constructor function body mixed with the construction logic. In the second case by repeating Constructor.prototype, we violate the Do Not Repeat Yourself (DRY) principle.

Approach with the private state

So how can we do it otherwise? We can return an object literal by the constructor function:

"use strict";
/**
 * @class
 */
var Constructor = function(){
     /**
     * @type {String}
     * @private
     */
     var baz = "baz";
     return {
       /**
       * @type {String}
       * @public
       */
       bar: "bar",
       /**
       * @public
       * @returns {String}
       */
       foo: function() {
        return this.bar + " " + baz;
       }
     };
   },
   /** @type Constructor */
   instance = new Constructor();

console.log( instance.foo() ); // bar baz
console.log( instance.hasOwnProperty( "baz") ); // false
console.log( Constructor.prototype.hasOwnProperty( "baz") ); // false
console.log( instance instanceof Constructor ); // false

The advantage of this approach is that any variables declared in the scope of the constructor are in the same closure as the returned object, and therefore, available through the object. We can consider such variables as private members. The bad news is that we will lose the constructor prototype. When a constructor returns an object during instantiation, this object becomes the result of a whole new expression.

Inheritance with the prototype chain

What about inheritance? The classical approach would be to make the subtype prototype an instance of supertype:

"use strict";
 /**
 * @class
 */
var SuperType = function(){
       /**
       * @type {String}
       * @public
       */
       this.foo = "foo";
     },
     /**
      * @class
      */
     Constructor = function(){
       /**
       * @type {String}
       * @public
       */
       this.bar = "bar";
     },
     /** @type Constructor */
     instance;

 Constructor.prototype = new SuperType();
 Constructor.prototype.constructor = Constructor;

 instance = new Constructor();
 console.log( instance.bar ); // bar
 console.log( instance.foo ); // foo
 console.log( instance instanceof Constructor ); // true
 console.log( instance instanceof SuperType ); // true

You may run into some code, where for instantiation Object.create is used instead of the new operator. Here you have to know the difference between the two. Object.create takes an object as an argument and creates a new one with the passed object as a prototype. In some ways, this reminds us of cloning. Examine this, you declare an object literal (proto) and create a new object (instance) with Object.create based on the first one. Whatever changes you do now on the newly created object, they won't be reflected on the original (proto). But if you change a property of the original, you will find the property changed in its derivative (instance):

"use strict";
var proto = {
 bar: "bar",
 foo: "foo"
}, 
instance = Object.create( proto );
proto.bar = "qux",
instance.foo = "baz";
console.log( instance ); // { foo="baz",  bar="qux"}
console.log( proto ); // { bar="qux",  foo="foo"}

Inheriting from prototype with Object.create

In contrast to the new operator, Object.create does not invoke the constructor. So when we use it to populate a subtype prototype, we are losing all the logic located in a supertype constructor. This way, the supertype constructor is never called:

// ...
SuperType.prototype.baz = "baz";
Constructor.prototype = Object.create( SuperType.prototype );
Constructor.prototype.constructor = Constructor;

instance = new Constructor();

console.log( instance.bar ); // bar
console.log( instance.baz ); // baz
console.log( instance.hasOwnProperty( "foo" ) ); // false
console.log( instance instanceof Constructor ); // true
console.log( instance instanceof SuperType ); // true

Inheriting from prototype with Object.assign

When looking for an optimal structure, I would like to declare members via an object literal, but still have the link to the prototype. Many third-party projects leverage a custom function (extend) that merge the structure object literal into the constructor prototype. Actually, ES6 provides an Object.assign native method. We can use it as follows:

"use strict";
   /**
    * @class
    */
var SuperType = function(){
     /**
     * @type {String}
     * @public
     */
     this.foo = "foo";
   },
   /**
    * @class
    */
   Constructor = function(){
     /**
     * @type {String}
     * @public
     */
     this.bar = "bar";
   },
   /** @type Constructor */
   instance;

Object.assign( Constructor.prototype = new SuperType(), {
 baz: "baz"
});
instance = new Constructor();
console.log( instance.bar ); // bar
console.log( instance.foo ); // foo
console.log( instance.baz ); // baz
console.log( instance instanceof Constructor ); // true
console.log( instance instanceof SuperType ); // true

This looks almost as required, except there is one inconvenience. Object.assign simply assigns the values of source objects to the target ones regardless of their type. So if you have a source property with an object (for instance, an Object or Array instance), the target object receives a reference instead of a value. So you have to reset manually any object properties during initialization.

Approach with ExtendClass

ExtendClass, proposed by Simon Boudrias, is a solution that seems flawless (https://github.com/SBoudrias/class-extend). His little library exposes the Base constructor with the extend static method. We use this method to extend this pseudo-class and any of its derivatives:

"use strict";
   /**
    * @class
    */
var SuperType = Base.extend({
     /**
      * @pulic
      * @returns {String}
      */
     foo: function(){ return "foo public"; },
     /**
      * @constructs SuperType
      */
     constructor: function () {}
   }),
   /**
    * @class
    */
   Constructor = SuperType.extend({
     /**
      * @pulic
      * @returns {String}
      */      
     bar: function(){ return "bar public"; }
   }, {
     /**
      * @static
      * @returns {String}
      */      
     bar: function(){ return "bar static"; }
   }),
   /** @type Constructor */
   instance = new Constructor();
   
console.log( instance.foo() ); // foo public
console.log( instance.bar() ); // bar public
console.log( Constructor.bar() ); // bar static
console.log( instance instanceof Constructor ); // true
console.log( instance instanceof SuperType ); // true

Classes in ES6

TC39 (the EcmaScript working group) is pretty aware of the problem, so the new language specification provides extra syntax to structure object types:

"use strict";
class AbstractClass {
 constructor() {
   this.foo = "foo";
 }
}
class ConcreteClass extends AbstractClass {
 constructor() {
   super();
   this.bar = "bar";
 }
 baz() {
   return "baz";
 }
}

let instance = new ConcreteClass();
console.log( instance.bar ); // bar
console.log( instance.foo ); // foo
console.log( instance.baz() ); // baz
console.log( instance instanceof ConcreteClass ); // true
console.log( instance instanceof AbstractClass ); // true

The syntax looks class-based, but in fact this a syntactic sugar over existing prototypes. You can check with the type of ConcreteClass, and it will give you function because ConcreteClass is a canonical constructor. So we don't need any trick to extend supertypes, no trick to refer the supertype constructor from subtype, and we have a clean readable structure. However, we cannot assign properties the same C-like way as we do now with methods. This is still in discussion for ES7 (https://esdiscuss.org/topic/es7-property-initializers). Besides this, we can declare a class's static methods straight in its body:

class Bar {
 static foo() {
   return "static method";
 }
 baz() {
   return "prototype method";
 }
}
let instance = new Bar();
console.log( instance.baz() ); // prototype method
console.log( Bar.foo()) ); // static method

Actually, there are many in the JavaScript community who consider the new syntax as a deviation from the prototypical OOP approach. On the other hand, the ES6 classes are backwards compatible with most of the existing code. Subclasses are now supported by the language and no extra libraries are required for inheritance. And what I personally like the most is that this syntax allows us to make the code cleaner and more maintainable.

How to – magic methods in JavaScript

In the PHP world, there are things such as overloading methods, which are also known as magic methods (http://www.php.net/manual/en/language.oop5.overloading.php). These methods allow us to set a logic that triggers when a nonexisting property of a method is being accessed or modified. In JavaScript, we control access to properties (value members). Imagine we have a custom collection object. In order to be consistent in the API, we want to have the length property that contains the size of the collection. So we declare a getter (get length), which does the required computation whenever the property is accessed. On attempting to modify the property value, the setter will throw an exception:

"use strict";
var bar = {
 /** @type {[Number]} */
 arr: [ 1, 2 ],
 /**
  * Getter
  * @returns {Number}
  */
 get length () {
   return this.arr.length;
 },
 /**
  * Setter
  * @param {*} val
  */
 set length ( val ) {
   throw new SyntaxError( "Cannot assign to read only property 'length'" );
 }
};
console.log ( bar.length ); // 2
bar.arr.push( 3 );
console.log ( bar.length ); // 3
bar.length = 10; // SyntaxError: Cannot assign to read only property 'length'

If we want to declare getters/setters on an existing object, we can use the following:

Object.defineProperty:
"use strict";
var bar = {
 /** @type {[Number]} */
 arr: [ 1, 2 ]
};

Object.defineProperty( bar, "length", {
 /**
  * Getter
  * @returns {Number}
  */
 get: function() {
   return this.arr.length;
 },
 /**
  * Setter
  */
 set: function() {
   throw new SyntaxError( "Cannot assign to read only property 'length'" );
 }
});

console.log ( bar.length ); // 2
bar.arr.push( 3 );
console.log ( bar.length ); // 3
bar.length = 10; // SyntaxError: Cannot assign to read only property 'length'

Object.defineProperty as well as the second parameter of Object.create specifies a property configuration (whether it is enumerable, configurable, immutable, and how it can be accessed or modified). So, we can achieve a similar effect by configuring the property as read-only:

"use strict";
var bar = {};

Object.defineProperty( bar, "length", {
 /**
  * Data descriptor
  * @type {*}
  */
 value: 0,
 /**
  * Data descriptor
  * @type {Boolean}
  */
 writable: false
});

bar.length = 10; // TypeError: "length" is read-only

By the way, if you want to get rid of the property accessor in the object, you can simply remove the property:

delete bar.length;

Accessors in ES6 classes

Another way by which we can declare accessors is using the ES6 classes:

"use strict";
/** @class */
class Bar {
 /** @constructs Bar */
 constructor() {
   /** @type {[Number]} */
   this.arr = [ 1, 2 ];
 }
 /**
  * Getter
  * @returns {Number}
  */
 get length() {
   return this.arr.length;
 }
 /**
  * Setter
  * @param {Number} val
  */
 set length( val ) {
    throw new SyntaxError( "Cannot assign to read only property 'length'" );
 }
}

let bar = new Bar();
console.log ( bar.length ); // 2
bar.arr.push( 3 );
console.log ( bar.length ); // 3
bar.length = 10; // SyntaxError: Cannot assign to read only property 'length'

Besides public properties, we can control access to static ones as well:

"use strict";

class Bar {
   /**
    * @static
    * @returns {String}
    */
   static get baz() {
       return "baz";
   }
}

console.log( Bar.baz ); // baz

Controlling access to arbitrary properties

All these examples show access control to known properties. However, there might be a case when I want a custom storage with a variadic interface similar to localStorage. This must be a storage that has the getItem method to retrieve stored values and the setItem method to set them. Besides, this must work the same way as when you directly access or set a pseudo-property (val = storage.aKey and storage.aKey = "value"). These can be achieved by using the ES6 Proxy:

"use strict";
/**
* Custom storage
*/
var myStorage = {
     /** @type {Object} key-value object */
     data: {},
     /**
      * Getter
      * @param {String} key
      * @returns {*}
      */
     getItem: function( key ){
       return this.data[ key ];
     },
     /**
      * Setter
      * @param {String} key
      * @param {*} val
      */
     setItem: function( key, val ){
       this.data[ key ] = val;
     }
   },
   /**
    * Storage proxy
    * @type {Proxy}
    */
   storage = new Proxy( myStorage, {
     /**
      * Proxy getter
      * @param {myStorage} storage
      * @param {String} key
      * @returns {*}
      */
     get: function ( storage, key ) {
       return storage.getItem( key );
     },
     /**
      * Proxy setter
      * @param {myStorage} storage
      * @param {String} key
      * @param {*} val
      * @returns {void}
      */
     set: function ( storage, key, val ) {
       return storage.setItem( key, val );
   }});

storage.bar = "bar";
console.log( myStorage.getItem( "bar" ) ); // bar
myStorage.setItem( "bar", "baz" );
console.log( storage.bar ); // baz

Amazon Customer Feb 05, 2016

[Disclosure: I am the technical reviewer for this book]Let me start by saying, if you want to maximize the benefit from reading "JavaScript Unlocked" you better have understanding of JavaScript 101. That said, take it from someone who has done a ton of reading on the subject matter that the author covers in this book. It's simply an exceptional resource for modern JS programmers.It gets you started with JavaScript core and then covers one specific but practically vital topic for modern Web App development. My personal favorite chapters are "Modular Programming with JavaScript", "Asynchronous JavaScript" and "A Large Scale JavaScript Application Architecture". Examples are pretty realistic to which you can relate to and that helps you understand the concept completely so that it can be used in your day to day coding.Overall, its a great packaging of cherry picked topics which JS programmers require today to build scalable and realistic Web App.

Amazon Verified review

Michael Apr 25, 2016

This is my kind of technical book! I love the no-fluff, just-the-facts approach. The author makes a statement, then follows it up with a concise example that makes the concept crystal clear. I'm able to put the concepts immediately to use in my own applications. I'm not forced to wade through pages of tutorials that make a single point, like other books. Many thanks to the author for making efficient use of my time. I wish more books were like this. The topic coverage it fantastic, and its packed with tips and tricks I've never seen before.Best of all, it illustrates techniques and best practices for both ES5 and ES2015, which was a pleasant surprise for me. The book will remain relevant for some time to come. There's nothing here that I will have to unlearn in a few months. Love this book. 5 stars!

Javascript Unlocked: Improve your code maintainability, performance, and security through practical expert insights and unlock the full potential of JavaScript

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