I've joined several projects in their maintenance phase which had no tests, and introduced some as I went about debugging, adding features, etc. Since the code was often untestable, I'd make a few refactorings over and over again to allow testing, and these just-so-happen to tease apart the pure computation from the effects; in essence making the code more functional.
An obvious example is to turn implicit state into explicit parameters, eg.
// BEFORE
function foo(x, y) {
b = x + y + a;
}
// AFTER
function foo(x, y) {
b = foo_pure(x, y, a);
}
function foo_pure(x, y, z) {
return x + y + z;
}
This doesn't make `foo` more functional or easier to test; but testing `foo_pure` is trivial. In particular this makes debugging logic errors much easier; since there's no need for elaborate/brittle test setup (eg. setting all the right globals, creating and cleaning up files, etc.).
