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Karax is a framework for developing single page applications in Nim.


To use Karax you must have nim installed. You can follow the instructions here.

Then you can install karax through nimble: nimble install karax

Try Karax

To try it out, run:

cd ~/projects # Insert your favourite directory for projects

nimble develop karax # This will clone Karax and create a link to it in ~/.nimble

cd karax

cd examples/todoapp

nim js todoapp.nim

open todoapp.html

cd ../..

cd examples/mediaplayer

nim js playerapp.nim

open playerapp.html

It uses a virtual DOM like React, but is much smaller than the existing frameworks plus of course it's written in Nim for Nim. No external dependencies! And thanks to Nim's whole program optimization only what is used ends up in the generated JavaScript code.


  • Leverage Nim's macro system to produce a framework that allows for the development of applications that are boilerplate free.
  • Keep it small, keep it fast, keep it flexible.

Hello World

The simplest Karax program looks like this:

include karax / prelude

proc createDom(): VNode =
  result = buildHtml(tdiv):
    text "Hello World!"

setRenderer createDom

Since div is a keyword in Nim, karax choose to use tdiv instead here. tdiv produces a <div> virtual DOM node.

As you can see, karax comes with its own buildHtml DSL for convenient construction of (virtual) DOM trees (of type VNode). Karax provides a tiny build tool called karun that generates the HTML boilerplate code that embeds and invokes the generated JavaScript code::

nim c karax/tools/karun karax/tools/karun -r helloworld.nim

Via -d:debugKaraxDsl we can have a look at the produced Nim code by buildHtml:

let tmp1 = tree(VNodeKind.tdiv)
add(tmp1, text "Hello World!")

(I shortened the IDs for better readability.)

Ok, so buildHtml introduces temporaries and calls add for the tree construction so that it composes with all of Nim's control flow constructs:

include karax / prelude
import random

proc createDom(): VNode =
  result = buildHtml(tdiv):
    if rand(100) <= 50:
      text "Hello World!"
      text "Hello Universe"

setRenderer createDom


let tmp1 = tree(VNodeKind.tdiv)
if rand(100) <= 50:
  add(tmp1, text "Hello World!")
  add(tmp1, text "Hello Universe")

Event model

Karax does not change the DOM's event model much, here is a program that writes "Hello simulated universe" on a button click:

include karax / prelude
# alternatively: import karax / [kbase, vdom, kdom, vstyles, karax, karaxdsl, jdict, jstrutils, jjson]

var lines: seq[kstring] = @[]

proc createDom(): VNode =
  result = buildHtml(tdiv):
      text "Say hello!"
      proc onclick(ev: Event; n: VNode) =
        lines.add "Hello simulated universe"
    for x in lines:
        text x

setRenderer createDom

kstring is Karax's alias for cstring (which stands for "compatible string"; for the JS target that is an immutable JavaScript string) which is preferred for efficiency on the JS target. However, on the native targets kstring is mapped to string for efficiency. The DSL for HTML construction is also avaible for the native targets (!) and the kstring abstraction helps to deal with these conflicting requirements.

Karax's DSL is quite flexible when it comes to event handlers, so the following syntax is also supported:

include karax / prelude
from sugar import `=>`

var lines: seq[kstring] = @[]

proc createDom(): VNode =
  result = buildHtml(tdiv):
    button(onclick = () => lines.add "Hello simulated universe"):
      text "Say hello!"
    for x in lines:
        text x

setRenderer createDom

The buildHtml macro produces this code for us:

let tmp2 = tree(VNodeKind.tdiv)
let tmp3 = tree(VNodeKind.button)
addEventHandler(tmp3, EventKind.onclick,
                () => lines.add "Hello simulated universe", kxi)
add(tmp3, text "Say hello!")
add(tmp2, tmp3)
for x in lines:
  let tmp4 = tree(VNodeKind.tdiv)
  add(tmp4, text x)
  add(tmp2, tmp4)

As the examples grow larger it becomes more and more visible of what a DSL that composes with the builtin Nim control flow constructs buys us. Once you have tasted this power there is no going back and languages without AST based macro system simply don't cut it anymore.

Attaching data to an event handler

Since the type of an event handler is (ev: Event; n: VNode) or () any additional data that should be passed to the event handler needs to be done via Nim's closures. In general this means a pattern like this:

proc menuAction(menuEntry: kstring): proc() =
  result = proc() =
    echo "clicked ", menuEntry

proc buildMenu(menu: seq[kstring]): VNode =
  result = buildHtml(tdiv):
    for m in menu:
      nav(class="navbar is-primary"):
          a(class="navbar-item", onclick = menuAction(m)):

DOM diffing

Ok, so now we have seen DOM creation and event handlers. But how does Karax actually keep the DOM up to date? The trick is that every event handler is wrapped in a helper proc that triggers a redraw operation that calls the renderer that you initially passed to setRenderer. So a new virtual DOM is created and compared against the previous virtual DOM. This comparison produces a patch set that is then applied to the real DOM the browser uses internally. This process is called "virtual DOM diffing" and other frameworks, most notably Facebook's React, do quite similar things. The virtual DOM is faster to create and manipulate than the real DOM so this approach is quite efficient.

Form validation

Most applications these days have some "login" mechanism consisting of username and password and a login button. The login button should only be clickable if username and password are not empty. An error message should be shown as long as one input field is empty.

To create new UI elements we write a loginField proc that returns a VNode:

proc loginField(desc, field, class: kstring;
                validator: proc (field: kstring): proc ()): VNode =
  result = buildHtml(tdiv):
    label(`for` = field):
      text desc
    input(class = class, id = field, onchange = validator(field))

We use the karax / errors module to help with this error logic. The errors module is mostly a mapping from strings to strings but it turned out that the logic is tricky enough to warrant a library solution. validateNotEmpty returns a closure that captures the field parameter:

proc validateNotEmpty(field: kstring): proc () =
  result = proc () =
    let x = getVNodeById(field).getInputText
    if x.isNil or x == "":
      errors.setError(field, field & " must not be empty")
      errors.setError(field, "")

This indirection is required because event handlers in Karax need to have the type proc () or proc (ev: Event; n: VNode). The errors module also gives us a handy disableOnError helper. It returns "disabled" if there are errors. Now we have all the pieces together to write our login dialog:

# some consts in order to prevent typos:
  username = kstring"username"
  password = kstring"password"

var loggedIn: bool

proc loginDialog(): VNode =
  result = buildHtml(tdiv):
    if not loggedIn:
      loginField("Name :", username, "input", validateNotEmpty)
      loginField("Password: ", password, "password", validateNotEmpty)
      button(onclick = () => (loggedIn = true), disabled = errors.disableOnError()):
        text "Login"
        text errors.getError(username)
        text errors.getError(password)
        text "You are now logged in."

setRenderer loginDialog

(Full example here.)

This code still has a bug though, when you run it, the login button is not disabled until some input fields are validated! This is easily fixed, at initialization we have to do:

setError username, username & " must not be empty"
setError password, password & " must not be empty"

There are likely more elegant solutions to this problem.


For routing setRenderer can be called with a callback that takes a parameter of type RouterData. Here is the relevant excerpt from the famous "Todo App" example:

proc createDom(data: RouterData): VNode =
  if data.hashPart == "#/": filter = all
  elif data.hashPart == "#/completed": filter = completed
  elif data.hashPart == "#/active": filter = active
  result = buildHtml(tdiv(class="todomvc-wrapper")):
    section(class = "todoapp"):

setRenderer createDom

(Full example here.)

Server Side HTML Rendering

Karax can also be used to render HTML on the server. Only a subset of modules can be used since there is no JS interpreter.

import karax / [karaxdsl, vdom]

const places = @["boston", "cleveland", "los angeles", "new orleans"]

proc render*(): string =
  let vnode = buildHtml(tdiv(class = "mt-3")):
    h1: text "My Web Page"
    p: text "Hello world"
      for place in places:
        li: text place
      dt: text "Can I use Karax for client side single page apps?"
      dd: text "Yes"

      dt: text "Can I use Karax for server side HTML rendering?"
      dd: text "Yes"
  result = $vnode

echo render()

You can embed raw html using the verbatim proc:

let vg = """
<svg height="100" width="100">
<circle cx="50" cy="50" r="40" stroke="black" stroke-width="3" fill="red" />
Sorry, your browser does not support inline SVG.
let wrap = buildHtml(tdiv(class="wrapper")):

echo wrap

Generate HTML with event handlers

If you are writing a static site generator or do server-side HTML rendering via nim c, you may want to override addEventHandler when using event handlers to avoid compiler complaints.

Here's an example of auto submit a dropdown when a value is selected:

template kxi(): int = 0
template addEventHandler(n: VNode; k: EventKind; action: string; kxi: int) =
  n.setAttr($k, action)

  names = @["nim", "c", "python"]
  selected_name = request.params.getOrDefault("name")
  hello = buildHtml(html):
    form(`method` = "get"):
      select(name="name", onchange="this.form.submit()"):
        for name in names:
          if name == selected_name:
            option(selected = ""): text name
            option: text name


MIT License. See here.