Object vs Class vs Function

As you must already be aware by now there are no classes in JavaScript. Instead functions in JavaScript may be made to behave like constructors by preceding a function call with the new keyword. This is known as the constructor pattern.

In JavaScript everything is an object except for the primitive data types (boolean, number and string), and undefined. On the other hand null is actually an object reference even though you may at first believe otherwise. This is the reason typeof null returns "object".

Functions in JavaScript are similar to functables in Lua (i.e. they are callable objects). Hence a function can be used in place of an object. Similarly arrays are also objects in JavaScript. On the other hand objects can be thought of as associative arrays.

The most important point however is that there are no classes in JavaScript because JavaScript is a prototypal object oriented language. This means that objects in JavaScript directly inherit from other objects. Hence we don't need classes. All we need is a way to create and extend objects.

Read the following thread to learn more about prototypal inheritance in JavaScript: Benefits of prototypal inheritance over classical?

Update 2015

There are classes in JavaScript they just aren't used on older browsers:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes

It has constructors, extensions, and the like.

class Cat { 
  constructor(name) {
    this.name = name;
  }
  
  speak() {
    console.log(this.name + ' makes a noise.');
  }
}

class Lion extends Cat {
  speak() {
    super.speak();
    console.log(this.name + ' roars.');
  }
}

A Class in JS:

function Animal(){  

    // Private property
    var alive=true;

    // Private method
    function fight(){ //... }   

    // Public method which can access private variables
    this.isAlive = function() { return alive; } 

    // Public property
    this.name = "Joe";
}

// Public method
Animal.prototype.play = function() { alert("Bow wow!"); }

// .. and so on

Now when you create it's object

var obj = new Animal();

You can expect anything of this object as you would from objects in other language. Just the efforts to achieve it, was a bit different. You should also be looking at inheritance in JS.

Getting back too your question, I'll reword it as:

Class  : A representation of a set with common properties.
object : One from the set with the same properties.


var Class = function() {alert('bar');}; // A set of function which alert 'bar'
var object = new Class();               // One of the functions who alert's 'bar'.

JavaScript does not have classes, and functions are actually objects in JavaScript (first-class citizens). The only difference that function objects have is that they are callable.

function func() { alert('foo'); } // a function - Correct

func(); // call the function - alerts 'foo' - Correct

var func2 = function () { alert('foo'); } // same as 'func' surely? - Nope, func2 is a different object, that apparently does the same thing when called.

var Class = function() { alert('bar'); }; - It's a function with no name stored in variable Class.

var c = new Class(); - Calls function stored in Class supplying new empty object as this and returning that object. Functions called as new functionA() are expected to work as constructors and prepare a newly created object (this). In your case - constructor does nothing with the object and just alerts bar.

You also get classes in ES6 that look like this:

//class
class Cat {
    //constructor
    constructor() {
        this.name = 'Snowball';
    }

    //method
    meow() {
        console.log('Hello, nyah! My name is ' + this.name + ' nyah!~');
    }
};