Unused function parameters make code harder to maintain and use. Clients that are unaware that the parameter is unused may perform nontrivial computations to compute a value that is ultimately unused. Even if the parameter is known to be unused, passing in default values such as null or undefined makes the code hard to read.

Remove the unused parameter and adjust calls accordingly.

In this code, the function f does not use its second argument y, instead computing it from the first argument using function expensiveComputation. The code invoking f is not aware of this and performs the same computation again, which is wasteful and introduces unnecessary dependencies between otherwise unrelated pieces of code. Instead, the second argument should simply be dropped.

Coding Horror: Code Smells.