Create an empty object in JavaScript with {} or new Object()?

Objects

There is no benefit to using new Object(); - whereas {}; can make your code more compact, and more readable.

For defining empty objects they're technically the same. The {} syntax is shorter, neater (less Java-ish), and allows you to instantly populate the object inline - like so:

var myObject = {
        title:  'Frog',
        url:    '/img/picture.jpg',
        width:  300,
        height: 200
      };

Arrays

For arrays, there's similarly almost no benefit to ever using new Array(); over []; - with one minor exception:

var emptyArray = new Array(100);

creates a 100 item long array with all slots containing undefined - which may be nice/useful in certain situations (such as (new Array(9)).join('Na-Na ') + 'Batman!').

My recommendation

1. Never use new Object(); - it's clunkier than {}; and looks silly.
2. Always use []; - except when you need to quickly create an "empty" array with a predefined length.

These have the same end result, but I would simply add that using the literal syntax can help one become accustomed to the syntax of JSON (a string-ified subset of JavaScript literal object syntax), so it might be a good practice to get into.

One other thing: you might have subtle errors if you forget to use the new operator. So, using literals will help you avoid that problem.

Ultimately, it will depend on the situation as well as preference.

var objectA = {}

is a lot quicker and, in my experience, more commonly used, so it's probably best to adopt the 'standard' and save some typing.

The object and array literal syntax {}/[] was introduced in JavaScript 1.2, so is not available (and will produce a syntax error) in versions of Netscape Navigator prior to 4.0.

My fingers still default to saying new Array(), but I am a very old man. Thankfully Netscape 3 is not a browser many people ever have to consider today...

I believe {} was recommended in one of the Javascript vids on here as a good coding convention. new is necessary for pseudoclassical inheritance. the var obj = {}; way helps to remind you that this is not a classical object oriented language but a prototypal one. Thus the only time you would really need new is when you are using constructors functions. For example:

var Mammal = function (name) {
  this.name = name;
};

Mammal.prototype.get_name = function () {
  return this.name;
}

Mammal.prototype.says = function() {
  return this.saying || '';
}

Then it is used like so:

var aMammal = new Mammal('Me warm-blooded');
var name = aMammal.get_name();

Another advantage to using {} as oppose to new Object is you can use it to do JSON-style object literals.

This is essentially the same thing. Use whatever you find more convenient.

Var Obj = {};

This is the most used and simplest method.

Using

var Obj = new Obj() 

is not preferred.

There is no need to use new Object().

For simplicity, readability and execution speed, use the first one (the object literal method).