Firstly, I have defined a variable `count=10` and made it readonly. Now, how can I unset it?

Secondly, on unsetting a variable it sets it to `NULL` string. I have set `dir=/home/kundan`
Now on unsetting dir, if I do `echo $dir`, output is null, but if I give `cd $dir` it goes to `/home/kundan`.

Why is this happening?

I expected that after unsetting `dir` it should not go to `/home/kundan`