In the second part of the "Solving Problems the Clojure Way"[0] talk, the speaker shows step-by-step transformation of the heavy imperative (book-keepish) JavaScript code to a more functional approach.

As a result of transformation, ~95% of the code is easily unit-testable and the only impure code is the call to main which basically starts all the domino falling.

[0] https://www.youtube.com/watch?v=vK1DazRK_a0