Determine the average
For each datapoint determine the distance from the average.
Sum all of the distances
Take an average of the sum

Shouldn't step (3) be Take the average of the distances ?!?

assert.equal(mad(arr), expected, 1e-9);

I don't think that does what you think it does.

Did you intend to use assert.approximately?

This just gives a weird error message on failure.