Simple, decomplected, isomorphic HTML UI library for Clojure and ClojureScript
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Rum is a client/server library for HTML UI. In ClojureScript, it works as React wrapper, in Clojure, it is a static HTML generator.

Table of Contents


Simple semantics: Rum is arguably smaller, simpler and more straightforward than React itself.

Decomplected: Rum is a library, not a framework. Use only the parts you need, throw away or replace what you don’t need, combine different approaches in a single app, or even combine Rum with other frameworks.

No enforced state model: Unlike Om, Reagent or Quiescent, Rum does not dictate where to keep your state. Instead, it works well with any storage: persistent data structures, atoms, DataScript, JavaScript objects, localStorage or any custom solution you can think of.

Extensible: the API is stable and explicitly defined, including the API between Rum internals. It lets you build custom behaviours that change components in significant ways.

Minimal codebase: You can become a Rum expert just by reading its source code (~900 lines).

Comparison to other frameworks


  • does not dictate how to store your state,
  • has server-side rendering,
  • is much smaller.

Who’s using Rum?

Using Rum

Add to project.clj: [rum "0.11.2"]

Defining a component

Use rum.core/defc (short for “define component”) to define a function that returns component markup:

(require [rum.core :as rum])

(rum/defc label [text]
  [:div {:class "label"} text])

Rum uses Hiccup-like syntax for defining markup:

[<tag-n-selector> <attrs>? <children>*]

<tag-n-selector> defines a tag, its id and classes:


By default, if you omit the tag, div is assumed:

  :#id    === :div#id
  :.class === :div.class

<attrs> is an optional map of attributes:

  • Use kebab-case keywords for attributes (e.g. :allow-full-screen for allowFullScreen)
  • You can include :id and :class there as well
  • :class can be a string or a sequence of strings
  • :style, if needed, must be a map with kebab-case keywords
  • event handlers should be arity-one functions
[:input { :type      "text"
          :allow-full-screen true
          :id        "comment"
          :class     ["input_active" "input_error"]
          :style     { :background-color "#EEE"
                       :margin-left      42 }
          :on-change (fn [e]
                       (js/alert (.. e -target -value))) }]

<children> is a zero, one or many elements (strings or nested tags) with the same syntax:

  [:div {} "Text"]         ;; tag, attrs, nested text
  [:div {} [:span]]        ;; tag, attrs, nested tag
  [:div "Text"]            ;; omitted attrs
  [:div "A" [:em "B"] "C"] ;; 3 children, mix of text and tags

Children can include lists or sequences which will be flattened:

  [:div (list [:i "A"] [:b "B"])] === [:div [:i "A"] [:b "B"]]

By default all text nodes are escaped. To embed an unescaped string into a tag, add the :dangerouslySetInnerHTML attribute and omit children:

  [:div { :dangerouslySetInnerHTML {:__html "<span></span>"}}]

Rendering component

Given this code:

(require [rum.core :as rum])

(rum/defc repeat-label [n text]
  [:div (repeat n [:.label text])])

First, we need to create a component instance by calling its function:

(repeat-label 5 "abc")

Then we need to pass that instance to (rum.core/mount comp dom-node):

(rum/mount (repeat-label 5 "abc") js/document.body)

And we will get this result:

      <div class="label">abc</div>
      <div class="label">abc</div>
      <div class="label">abc</div>
      <div class="label">abc</div>
      <div class="label">abc</div>

Usually, mount is used just once in an app lifecycle to mount the top of your component tree to a page. After that, for a dynamic applications, you should either update your components or rely on them to update themselves.

Updating components manually

The simplest way to update your app is to mount it again:

(rum/defc timer []
  [:div (.toISOString (js/Date.))])

(rum/mount (timer) js/document.body)

  #(rum/mount (timer) js/document.body)

Reactive components

Rum offers mixins as a way to hook into a component’s lifecycle and extend its capabilities or change its behaviour.

One very common use-case is for a component to update when some reference changes. Rum has a rum.core/reactive mixin just for that:

(def count (atom 0))

(rum/defc counter < rum/reactive []
  [:div { :on-click (fn [_] (swap! count inc)) }
    "Clicks: " (rum/react count)])

(rum/mount (counter) js/document.body)

Two things are happening here:

  1. We’re adding the rum.core/reactive mixin to the component.
  2. We’re using rum.core/react instead of deref in the component body.

This will set up a watch on the count atom and will automatically call rum.core/request-render on the component each time the atom changes.

Component’s local state

Sometimes you need to keep track of some mutable data just inside a component and nowhere else. Rum provides the rum.core/local mixin. It’s a little trickier to use, so hold on:

  1. Each component in Rum has internal state associated with it, normally used by mixins and Rum internals.
  2. rum.core/local creates a mixin that will put an atom into the component’s state.
  3. rum.core/defcs is used instead of rum.core/defc. It allows you to get hold of the components’s state in the render function (it will be passed as a first argument).
  4. You can then extract that atom from the component’s state and deref/swap!/reset! it as usual.
  5. Any change to the atom will force the component to update.

In practice, it’s quite convenient to use:

(rum/defcs stateful < (rum/local 0 ::key)
  [state label]
  (let [local-atom (::key state)]
    [:div { :on-click (fn [_] (swap! local-atom inc)) }
      label ": " @local-atom]))

(rum/mount (stateful "Click count") js/document.body)

Optimizing with shouldComponentUpdate

If your component accepts only immutable data structures as arguments, it may be a good idea to add the rum.core/static mixin:

(rum/defc label < rum/static [n text]
  [:.label (repeat n text)])

rum.core/static will check if the arguments of a component’s constructor have changed (using Clojure’s -equiv semantic), and if they are the same, avoid re-rendering.

(rum/mount (label 1 "abc") body)
(rum/mount (label 1 "abc") body) ;; render won’t be called
(rum/mount (label 1 "xyz") body) ;; this will cause a re-render
(rum/mount (label 1 "xyz") body) ;; this won’t

Note that this is not enabled by default because a) comparisons can be expensive, and b) things will go wrong if you pass a mutable reference as an argument.

Writing your own mixin

Many applications have very specific requirements and custom optimization opportunities, so odds are you’ll be writing your own mixins.

Let’s see what a Rum component really is. Each Rum component has:

  • A render function
  • One or more mixins
  • An internal state map
  • A corresponding React component

For example, if we have this component defined:

(rum/defc input [label value]
  [:label label ": "
    [:input { :value value }]])

(input "Your name" "")

It will have the following state:

{ :rum/args ["Your name" ""]
  :rum/react-component <react-component> }

You can read the internal state by using the rum.core/defcs (short for “define component [and pass] state”) macro instead of rum.core/defc. It will pass state to the render function as the first argument:

(rum/defcs label [state label value]
  [:div "My args:" (pr-str (:rum/args state))])

(label "A" 3) ;; => <div>My args: ["A" 3]</div>

The internal state cannot be directly manipulated, except at certain stages of a component’s lifecycle. Mixins are functions that are invoked at these stages to give you and opportunity to modify the state and/or do side effects to the world.

The following mixin will record the component’s mount time:

(rum/defcs time-label
  < { :will-mount (fn [state]
                    (assoc state ::time (js/Date.))) }
  [state label]
  [:div label ": " (str (::time state))])

As you can see, :will-mount is a function from state to state. It gives you a chance to populate, clean or modify state map the moment before the component has been mounted.

Another useful thing you can do in a mixin is to decide when to update a component. If you can get ahold of React component (notice that that’s different from Rum component, unfortunately; sorry), you can call rum.core/request-render to schedule this component’s update at next frame (Rum uses requestAnimationFrame to batch and debounce component update calls). To get React component, just look up :rum/react-component key in a state.

This mixin will update a component each second:

(def periodic-update-mixin
  { :did-mount    (fn [state]
                    (let [comp      (:rum/react-component state)
                          callback #(rum/request-render comp)
                          interval  (js/setInterval callback 1000)]
                       (assoc state ::interval interval)))
    :will-unmount (fn [state]
                    (js/clearInterval (::interval state))
                    (dissoc state ::interval)) })

(rum/defc timer < periodic-update-mixin []
  [:div (.toISOString (js/Date.))])

(rum/mount (timer) js/document.body)

Here’s a full list of callbacks you can define in a mixin:

{ :init                 ;; state, props     ⇒ state
  :will-mount           ;; state            ⇒ state
  :before-render        ;; state            ⇒ state
  :wrap-render          ;; render-fn        ⇒ render-fn
  :render               ;; state            ⇒ [pseudo-dom state]
  :did-catch            ;; state, err, info ⇒ state
  :did-mount            ;; state            ⇒ state
  :after-render         ;; state            ⇒ state
  :did-remount          ;; old-state, state ⇒ state
  :should-update        ;; old-state, state ⇒ boolean
  :will-update          ;; state            ⇒ state
  :did-update           ;; state            ⇒ state
  :will-unmount }       ;; state            ⇒ state

Each component can have any number of mixins:

(rum/defcs component
  < rum/static
    (rum/local 0 ::count)
    (rum/local "" ::text)
  [state label]
  (let [count-atom (::count state)
        text-atom  (::text state)]

One gotcha: don’t forget to return state from the mixin functions. If you’re using them for side-effects only, just return an unmodified state.

Working with atoms

Since Rum relies a lot at components being able to efficiently update themselves in reaction to events, it includes two facilities to build architectures around Atoms and watchers.


If you have a complex state and need a component to interact with only a part of it, create a cursor using (rum.core/cursor-in ref path). Given atom with deep nested value and path inside it, cursor-in will create an atom-like structure that can be used separately from main atom, but will sync changes both ways:

(def state (atom { :color "#cc3333"
                   :user { :name "Ivan" } }))

(def user-name (rum/cursor-in state [:user :name]))

@user-name ;; => "Ivan"

(reset! user-name "Oleg") ;; => "Oleg"

@state ;; => { :color "#cc3333"
       ;;      :user  { :name "Oleg" } }

Cursors implement IAtom and IWatchable and interface-wise are drop-in replacement for regular atoms. They work well with rum/reactive and rum/react too.

Derived atoms

Use derived atoms to create “chains” and acyclic graphs of dependent atoms. derived-atom will:

  • Take N “source” refs
  • Set up a watch on each of them
  • Create “sink” atom
  • When any of source refs changes:
    • re-run function f, passing N dereferenced values of source refs
    • reset! result of f to the sink atom
  • return sink atom
  (def *a (atom 0))
  (def *b (atom 1))
  (def *x (derived-atom [*a *b] ::key
            (fn [a b]
              (str a \":\" b))))
  (type *x) ;; => clojure.lang.Atom
  @*x     ;; => 0:1
  (swap! *a inc)
  @*x     ;; => 1:1
  (reset! *b 7)
  @*x     ;; => 1:7

Derived atoms are like cursors, but can “depend on” multiple references and won’t sync changes back to the source if you try to update derived atom (don’t).

Interop with React

Native React component

You can access the raw React component by reading the state’s :rum/react-component attribute:

{ :did-mount (fn [state]
               (let [comp     (:rum/react-component state)
                     dom-node (js/ReactDOM.findDOMNode comp)]
                 (set! (.-width (.-style dom-node)) "100px"))
               state) }

React keys and refs

There’re three ways to specify React keys:

  1. If you need a key on Sablono tag, put it into attributes: [:div { :key "x" }]
  2. If you need a key on Rum component, use with-key:
(rum/defc my-component [str]

(rum/with-key (my-component "args") "x")
  1. or, you can specify :key-fn in a mixin to calculate key based on args at component creation time:
(rum/defc my-component
  < { :key-fn (fn [x y z]
                (str x "-" y "-" z)) }
  [x y z]

(my-component 1 2 3) ;; => key == "1-2-3"

:key-fn must accept same arguments your render function does.

Refs work the same way as options 1 and 2 for keys work:

  1. [:div { :ref "x" }]
  2. (rum/with-ref (my-component) "x")

Accessing DOM

There’re couple of helpers that will, given state map, find stuff in it for you:

(rum/dom-node state)     ;; => top-level DOM node
(rum/ref      state "x") ;; => ref-ed React component
(rum/ref-node state "x") ;; => top-level DOM node of ref-ed React component

Custom class properties

To define arbitrary properties and methods on a component class, specify a :class-properties map in a mixin:

(rum/defc comp
  < { :class-properties { ... } }

To define static properties on a component class, specify a :static-properties map in a mixin:

(rum/defc comp
  < { :static-properties { ... } }

React context

To define child context

  1. Add dependency [cljsjs/prop-types "15.5.10-1"]
  2. (require [cljsjs.prop-types])
  3. Specify a :child-context function taking state and returning context map in a mixin:
(rum/defc theme
  < { :child-context
      (fn [state]
        (let [[color] (:rum/args state)]
          { :color color }))
      { :childContextTypes {:color js/PropTypes.string} } }
  [color child]

Server-side rendering

If used from clj/cljc, Rum works as a traditional template engine à la Hiccup:

  1. Rum’s project.clj dependency becomes [rum "0.11.2" :exclusions [cljsjs/react cljsjs/react-dom sablono]
  2. Import rum.core as usual.
  3. Define components using rum/defc or other macros as usual.
  4. Instead of mounting, call rum/render-html to render into a string.
  5. Generate the HTML page using that string.
  6. On the client side, mount (but using rum/hydrate) the same component over the node where you rendered your server-side component.
(require '[rum.core :as rum])

(rum/defc my-comp [s]
  [:div s])

;; on a server
(rum/render-html (my-comp "hello"))
;; => "<div data-reactroot=\"\">hello</div>"

;; on a client
(rum/hydrate (my-comp "hello") js/document.body)

Use rum/render-static-markup if you’re not planning to connect your page with React later:

(rum/render-static-markup (my-comp "hello")) ;; => <div>hello</div>

Rum server-side rendering does not use React or Sablono, it runs completely in JVM, without involving JavaScript at any stage.

As of [rum "0.8.3"] and [hiccup "1.0.5"], Rum is ~3× times faster than Hiccup.

Server-side components do not have full lifecycle support, but :init and :will-mount from mixins would be called at the component’s construction time.



App templates


  • Reforms, Bootstrap 3 forms
  • rum-mdl, Material design lite components
  • derivatives, creates chains of derived values from an atom
  • citrus, state coordination framework (previously known as scrum)
  • Antizer Ant Design component library



Rum was build on inspiration from Quiescent, Om and Reagent.

All heavy lifting done by React, Ŝablono and ClojureScript.


Copyright © 2014 Nikita Prokopov

Licensed under Eclipse Public License (see LICENSE).