Expression: I am stuck

Hi, could someone please tell me whats going on here:

var i=1;

var Banana;

if (comp("Comp 1").layer("Calender_Pre").effect("STAY!")("Checkbox") == 0)

//i = 0

Banana = 20

else if (i == 0)

//i++

Banana = 25

else if (i >= 0)

Banana = 13;

(Banana);

This code seems to work fine until i uncomment either "i=0" on line 4, or "i++" on line 8. If i uncomment "i=0" i get "Error at line 7...illegal use of reserved keyword 'else'", when i uncomment "i++" i get the same Error at line 11.

Why am i allowed to assign a value to "Banana" inside the if statements, but not to "i"?

Thanks and kind regards,