What is ["null"] in javascript?

I am migrating an asp code to .net. I came across the following code but not sure what is it doing.

var collection = {};if (typeof(collection["null"]) != "undefined"&&     typeof(collection["null"][id]) != "undefined")

Can any well tell that what does collection["null"] does? If possible, how to define it in .Net

Just to give an idea, I made closedMealArea as a List in .Net

Thanks in advance

Cheers,wildanjel