Question

Looking at Google’s code for their bookmark bubble library, I came across this:

var google = google || {};
google.bookmarkbubble = google.bookmarkbubble || {};

Could someone explain what it is they’re doing here and why they are doing it? Since JS is an interpreted language why would you ever need to assign the same google object into the google variable? Wouldn’t the script this is contained in only get executed once each time a page is loaded?

Answer 1

They are setting up a namespace-like object. In case there is already a google object or google.bookmarkbubble object, they are making sure they don’t replace it. For example, if you had another Google library added, it may have already set up a google object that looks like this:

{
    somelibrary: {
        ...
    }
}

So if they just had:

var google = { bookmarkbubble: { ... } }

That would break any code that referenced google.somelibrary. Likewise, if they didn’t have var google = google || {}; and you didn’t have google already defined, then google.bookmarkbubble would throw an error.

In short, this code ensures that a google.bookmarkbubble object exists without overwriting any previously-defined google or google.bookmarkbubble objects.