JavaScript Syntax empty {}

Question

I'm trying to figure out what does the empty {} mean.

var $sb = $sb || {};

Does this mean that varaible $sb's value is either copied to itself or it is a function literal?

full context:

var $sb = $sb || {};
$sb.xxx = function() {
    // code
}

Answer 1

It's shortcut for

new Object()

So this line

var $sb = $sb || {};

Will check if variable $sb exists and if not create new object and assign it to $sb variable.

So in other way you can write this like:

if( !$sb ) {
    var $sb = new Object();
}

Answer 2

var a = {} is called the object literal notation. It's a faster than var a = new Object() because it needs no scope resolution (ie you could have defined a constructor with the same name and therefor the JavaScript engine must do such a lookup).

The pattern var a = a || {}; is used to avoid replacing a in case you have already defined a. In this pattern, the or-operator: || functions as a coalescing operator. If a is null or undefined it will execute the expression at the right-hand of the statement: {}

Using this pattern ensures you that a will always be defined as an object and, in case it already exist, will not be overwritten.