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# Global variables make the code hard to reason about

This is especially visible when debugging. Say you have a function which errors. The stacktrace tells you where the function got it's arguments, *but not who last modified the global variables it might have read*. They could have been modified literally anywhere in the program, perhaps asynchronously. Reconstructing the chain of events that led to this particular errorous state may be next to impossible.

Use function arguments for passing everything. Keep your functions [refenrentially transparent](https://en.wikipedia.org/wiki/Referential_transparency). I.e. the same arguments should always produce the same output. (I recommend looking into functional programing more broadly too while you're at it.)