Attempt to assign to const or readonly variable

Hello everybody
There is something I don’t understand in my meteor application.

In a package I declared a constant : const myvar =“ACONSTANT”
In another package, which is unrelated to the previous package, I do : myvar =“TOTO”,
And then I got the error : "Attempt to assign to const or readonly variable "

How is it possible between 2 packages and therefore 2 portions of completely independent code??
There is a special statement that I forgot to make.
This necessarily comes from me, such this problem cannot exist in Meteor!

Can someone explain to me?
Thank you
YC