What does "var a = a || {};" mean?

I'm a newer for javascript.
Anyone tell me about this?

Thanks!

There are a couple of principles you need to understand here:

First, the JavaScript logical or (||) operator works by evaluating expressions from left to right and returning the first 'true' result.

Second, a null or undefined variable is considered 'false' in a logical operation, whereas a variable with a value is 'true'.

Therefore, this statement is saying: 
    "If existing variable 'a' (a function argument?) is already defined, reference it with this new variable 'a'. If it is not defined, assign a reference to a new object '{}'."