Locally Var.s Affect Global Ones

Say I have script that looks a bit like this:

Array.prototype.getValue = function(ind, dir) {
if (dir !=0) ind[ind.length-1] += dir;
// more script
}

screens = [...// a big array
fnd = [1,2,3];
// 1.
a = findStart(fnd);
// 2.
b = findEnd(fnd);

findStart = function(cs) {
return screens.getValue(cs, 1);
}

findEnd = function(cs) {
return screens.getValue(cs, -1)
}

As I go from 1. to 2. the value of fnd changes. I don't understand how a
locally scoped variable might be affecting a global one.