Inspired by a tweet from a fellow developer, I decided to take a look at Karax, a nifty framework for developing single page applications in Nim.

After following the basic tutorials and examples, I searched for something more complex and found very sparse documentation, so I’ll write my findings here.

As usual, the complete source code is on my github repo, where you can find also a working live demo.

In this example I wanted to experiment with the component pattern, and create a stateful module that can be reused. So I modeled a nim clock object, here the source:

# source for the Clock component
import karax/[karax, karaxdsl, vdom]
import std/times
import std/dom
import sugar

  KClock* = ref object of VComponent
    currentTime: DateTime
    offset: TimeInterval
    timer: TimeOut
    prefix: string

# return a VNode with the html rendered for the component
proc render(c: VComponent): VNode =
  let self = KClock(c)
    let value = format(self.currentTime+self.offset, "HH:mm:ss")
      text "Local Time " & self.prefix & " => " & value

# update the clock value and re-triggers a timer
proc update(self: KClock) =
  self.currentTime = now()
  self.timer = setTimeout( () => self.update, 100)
  markDirty(self) # need to be re-rendered

# create, initialize and return a new Clock object
proc new*(T: type KClock, tzoffset = 0): KClock =
  let self = newComponent(KClock, render)
  self.currentTime = now()
  self.offset = initTimeInterval(hours = tzoffset)
  self.timer = setTimeout(() => self.update, 100)
  self.prefix = if tzoffset >= 0: "+" else: ""
  self.prefix.add $tzoffset
  return self

this lives in a separate file and can be imported from the main page.

Some attention point goes in the constructor, that requires special newComponent call, and the update() function that contains a Timeout callback in order to re-render itself after some time.

let’s use the component in the main page, and sprinkle some interactivity just for fun:

import karax / [kbase, vdom, kdom, karax, karaxdsl, jstrutils]
import kclock

  clocks: seq[KClock] # keep a list of clocks
  offset: kstring     # value entered in the input box

proc render(): VNode =
      label(`for` = "offset"):
        text "Please enter Timezone offset (-12 .. +12)"
      input(type = "number", id = "offset"):
        proc oninput(ev: Event; n: VNode) =
          offset = n.value
      text "Add a new Clock"
      proc onclick(ev: Event; n: VNode) =
        let tzofs = parseInt(offset)
        if tzofs >= -12 and tzofs <= 12:
      text "Remove last Clock"
      proc onclick(ev: Event; n: VNode) =
        discard clocks.pop()
    for clock in clocks:

setRenderer render

for Nim lovers, Karax looks promising, and programming web pages without writing a single Javascript line sounds very interesting; but currently it needs some polish, more documentation and a larger user base. So try and contribute to the development community!