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    <p>In the first one you can only create the object once, while with the second one you can create as many objects as you like. I.E. the first one is effectively a singleton.</p> <p>Note that closures are not ok for the second one. Every time you instantiate it you are creating the functions all over again and waste a ton of memory. The prototype object is intended to counter this, where you can create the functions once outside a function scope and no accidental closures are created.</p> <pre><code>function foo2(){ this._bar = 0; } foo2.prototype = { constructor: foo2, getBar: function(){ return this._bar; }, addOne: function(){ this._bar++; }, addRandom:function(rand){ this._bar += rand; } }; </code></pre> <p>Then:</p> <pre><code>var a = new foo2, b = new foo2, c = new foo2; </code></pre> <p>Creates three instances which have their own <code>_bar</code> but share the same functionality.</p> <p><a href="http://jsperf.com/object-literal-vs-new-operate/4" rel="nofollow noreferrer">jsperf</a></p> <p>You can "compare" all of this to PHP, some of the code won't even run but it's "equivalent" in principle:</p> <hr> <pre><code>var foo = (function(){ var bar = 0; return { getBar: function(){ return bar; }, addOne: function(){ bar++; }, addRandom: function(rand){ bar += rand; } } })(); </code></pre> <p>is roughly "equivalent" to this in PHP:</p> <pre><code>$foo = new stdClass; $foo-&gt;bar = 0; $foo-&gt;getBar = function(){ return $this-&gt;bar; }; $foo-&gt;addOne = function(){ $this-&gt;bar++; } $foo-&gt;addRandom = function($rand){ $this-&gt;bar += $rand; } </code></pre> <hr> <pre><code>var foo2 = function(){ var bar = 0; this.getBar = function(){ return bar; }; this.addOne = function(){ bar++; }; this.addRandom = function(rand){ bar += rand; } }; </code></pre> <p>Is roughly "equivalent" to this in PHP:</p> <pre><code>Class foo2 { public function __construct(){ $bar = 0; $this-&gt;getBar = function(){ return $bar; }; $this-&gt;addOne = function(){ $bar++; }; $this-&gt;addRandom = function($rand){ $bar += rand; }; } } </code></pre> <hr> <pre><code>function foo2(){ this._bar = 0; } foo2.prototype = { constructor: foo2, getBar: function(){ return this._bar; }, addOne: function(){ this._bar++; }, addRandom:function(rand){ this._bar += rand; } }; </code></pre> <p>Is roughly "equivalent" to this in PHP:</p> <pre><code>Class foo2 { public $_bar; public function __construct(){ $this-&gt;_bar = 0; } public function getBar(){ return $this-&gt;_bar; } public function addOne(){ $this-&gt;_bar++ } public function addRandom($rand){ $this-&gt;_bar += $rand; } } </code></pre> <p>...and is the only one that is close to OOP in the three above examples</p> <hr>
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    1. COThere is no singleton. In OP's post, `foo` is a function that returns a dynamic object with three functions (`getBar`, `addOne`, `addRandom`). You can make as many instances of that object as you want, it's not a singleton.
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    2. CO@FritsvanCampen, take a closer look in the OP, the function is executed immediately so `foo` is just an object created once, not a function that creates "dynamic" objects. You can even run it in your console as is...
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    3. CO+1 for mentioning the shared nature of the private vars, which makes bar in example one a static variable, whereas in the second example it's a private var unique to the object instance
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