Calling .cs function from aspx, with arguments

I have a function inside a class file (.cs), and it works when I call it from my aspx.cs file. 
However I wish to call it from my aspx file, as I want the data to be posted inside a javascript variable.
var land = <% =am.getLandFields(mid) %>;
mid is an ID that the aspx.cs has but the aspx does not:  "The name 'mid' does not exist in the current context" is my error. 
I know that the aspx knows where to call the function from, as if I remove the "mid" I get an error stating that no method with that name accepts 0 arguments. 
So basically, I need mid to be passed from the aspx.cs to the cs.
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