Question

I’m trying to figure out why you might use the following code:

    var myObject = myObject || {};

I’ve seen this used several times, but don’t understand why this would be necessary. Thanks for your responses.

Answer 1

People call JavaScript’s binary or || the defaulting operator

var myObject = myObject || function(){};

is the same as

var myObject = myObject ? myObject : function(){};

The following code

var AppSpace = AppSpace || {};

is used because multiple files are going to set and use the namespace, and you don’t want to overwrite the namespace if it has already been created. That way, it doesn’t matter which file is included first.

Here’s another example of defaulting.

function doSomething (callback)  {
   something();
   code();      
   callback = callback || function() {};
   // Now we can call the callback even if it wasn't passed in.
   callback();         
}

However, be careful of the following problem Why does IE nuke window.ABC variables?

That is if a namespace has been defined using

window.AppSpace = {a: 1};

And another file sets

var AppSpace = AppSpace || {}

In IE, it will overwrite the value of window.AppSpace to the empty object if the two scripts are in different script tags because of variable hoisting and the fact that IE doesn’t realize that window.a and var a at the top level are all pointing at the same variable.