Countdown Clock

• Quickstart
• Learnings
  • as->
  • Declare

QuickStart

clojure -m figwheel.main -b dev -r

Learnings

as->

Use this when we want to have finer control over where our result goes. For example, if we look at how set interval need to work:

; original
(js/Math (.round (/ (- then (-> js/Date (.now))) 1000)))

;; next iteration
(->> (-> js/Date (.now))
     (- then)
     (/ 1000) ;; oops, we need the value from the above to come before :(
     (-> js/Math .round))

;; another iteration
(-> (-> js/Date (.now))
    (/ (- then ??) 1000) ;; okay, we need it to come after next :(
    (-> js/Math .round))

;; good stuff
(as-> (-> js/Date (.now)) time
      (- then time)
      (/ time 1000)
      (-> js/Math (.round time)))

At the end, we have something a lot cleaner and easier to read

Declare

One thing done in the video is wes declared display-time-left below the function that uses it. This is no bueno in clojure. But how could we do this if we wanted to?

1. Move display-time-left above timer
2. Declare display-time-left

This is all because a function has to be defined before we use it. This is often why you start reading Clojure from the bottom up.

To perform the second you need to declare the function before it is used. This is often reserved for specific scenarios, but I have done it here just as an example:

(declare display-time-left)

(defn timer [seconds]
  ;; ...
 (display-time-left seconds))

(defn display-time-left [seconds]
  ;; do stuff)