In JavaScript, the API could be fairly complex. Thing is, JavaScript doesn't have arrays of numbers. So, what should the function do if the array passed in contains a null, an undefined, a string such as "7", "012", "008", or "2+3", a function, another array, a hash, etc?

I can easily see this grow into a function accepting options indicating such things as "null values are zeroes", "ignore null values", "evaluate string arguments", etc, or maybe into a factory that produces customized averaging functions.

For example, average([null,1,2,3],[4,5]) could either be 3 (the average of all numbers), 2.5 (sum of numbers divided by number of 'root' items), undefined (input format error), 3.25 (average of average of [1,2,3] and average of [4,5]), etc.

And what if it gets passed arrays of equal size? "average([1,2,3],[7,4,5])" could easily be interpreted as a vector average, and produce [4,3,4]

Silly? Yes, but if you want to write a robust library in JavaScript, you have to think about what to do with all these kinds of input.

And of course, there are the simple variants of harmonic mean, arithmetic mean, and geometric mean, and several others (https://en.m.wikipedia.org/wiki/Average)