Fails when a default is on last parameter of function

function (a,b,c=1) {
.. code here ...
}

returns a,b, ...first line of code...

This is because the following regexp doesn't end on a bracket....
const DEFAULT_PARAMS = /=[^,]+/mg;

Changing it to: /=[^,)]+/ fixes this.
(Note, this won't capture funky defaults which contain commas, or brackets)

Also maybe consider changing:
var code = fn.toString()
to
const code = fn.prototype.constructor.toString()
This will prevent the whole function being converted to string.

Unless this is for parsing functions that are already strings?