Question

var google = google || {};
google.Test = google.Test || {};
google.Test.Render = google.Test.Render || {};

What is the difference between the above, and the following.

var MyCompany = MyCompany || {
MyApplication: {
Model: {}
} };

And what is the purpose of the || ?

Answer 1

The first snippet tests whether each level of the namespace exists and if not, it creates it (though the first line should probably be var google = google || {};).

The second one only tests whether the top level exists. For example:

var MyCompany = {};

// later in the same scope

var MyCompany = MyCompany || { MyApplication: { Model: {} } };

Since MyCompany is already defined, this expression evaluates to MyCompany = MyCompany, i.e. MyCompany stays an empty element, the nested objects are not created. If the following code relies on MyCompany.MyApplication‘s existence, it will fail.

How || works is described here: In Javascript, what does it mean when there is a logical operator in a variable declaration?