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.


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 "0.10.8"]

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

As you can see, :did-mount is a function from state to state. It gives you a chance to populate, clean or modify state map after 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-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")
  3. 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 { ... } }

React context

To define child context, 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/React.PropTypes.string} } }
  [color child]

Server-side rendering

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

  1. Import rum.core as usual.
  2. Define components using rum/defc or other macros as usual.
  3. Instead of mounting, call rum/render-html to render into a string.
  4. Generate the HTML page using that string.
  5. On the client side, mount 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=\"\" data-reactid=\"1\" data-react-checksum=\"-857140882\">hello</div>"

;; on a client
(rum/mount (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
  • scrum, state coordination framework




  • React 15.4.2-0, Sablono 0.7.7
  • Render boolean aria-* values as strings (thx r0man, PR #114)
  • Escape attributes during server-side rendering (thx Alexander Solovyov, PR #115)


  • Fixed server-side rendering discrepancy (issue #99)
  • Sablono 0.7.5, React 15.3.1-0


  • Sablono 0.7.4 fixes the issue with controlling components refusing to change value if non-string value was used
  • React 15.3.0-0
  • Throw error when < is misplaced in defc (thx Martin Klepsch, issue #88, PR #90)


  • Sablono 0.7.3 fixes the issue when IE lost keystrokes in controlled inputs/textarea (#86)
  • React 15.2.1-1
  • Warn when rum.core/react is used without rum.core/reactive (thx Martin Klepsch, issue #82, PR #87)


  • Ability to use :pre and :post checks in rum.core/defc (thx Martin Klepsch, PR #81)


  • Fixed regression of displayName in 0.10.0
  • Bumped React to 15.2.0


  • Fixed a bug when :before-render and :will-update weren’t called on subsequent renders


  • Made rum.core/state public again
  • :before-render should be called on server-side rendering too (thx Alexander Solovyov, PR #79)


A big cleanup/optmization/goodies release with a lot breaking changes. Read carefully!

  • [ BREAKING ] cursor got renamed to cursor-in. New cursor method added that takes single key (as everywhere in Clojure)
  • [ BREAKING ] rum/mount returns nil (because you shouldn’t rely on return value of ReactDOM.render)
  • [ BREAKING ] :transfer-state is gone. All of component’s state is now transferred by default. If you still need to do something fancy on componentWillReceiveProps, new callback is called :did-remount callback
  • [ BREAKING ] removed cursored and cursored-watch mixins. They felt too unnatural to use
  • [ BREAKING ] removed rum/with-props (deprecated since 0.3.0). Use rum/with-key and rum/with-ref instead
  • [ BREAKING ] server-side rendering no longer calls :did-mount (obviously, that was a mistake)
  • [ BREAKING ] :rum/id is gone. If you need an unique id per component, allocate one in :init as store it in state under namespaced key

When upgrading to 0.10.0, check this migration checklist:

  • Change all rum/cursor calls to rum/cursor-in
  • Find all :transfer-state mixins.
    • If the only thing they were doing is something like (fn [old new] (assoc new ::key (::key old))), just delete them.
    • If not, rename to :did-remount
  • Check if you were using rum/mount return value. If yes, find another way to obtain component (e.g. via ref, defcc etc)
  • Replace rum/with-props with rum/with-key, rum/with-ref or :key-fn
  • Check that you weren’t relying on :did-mount in server-side rendering

Now for the good stuff:

  • Cursors now support metadata, alter-meta! etc
  • Cursors can be used from Clojure
  • Added :key-fn to mixins. That function will be called before element creation, with same arguments as render fn, and its return value will be used as a key on that element
  • Mixins can specify :before-render (triggered at componentWillMount and componentWillUpdate) and :after-render (componentDidMount and componentDidUpdate) callback
  • Added rum/ref and rum/ref-node helpers, returning backing component and DOM node
  • Some client-side API functions added to server version (dom-node, unmount, request-render etc). Their implementation just throws an exception. This is to help you write less conditional directives in e.g. :did-mount or :will-unmount mixins. They will never be called, but won’t stop code from compiling either.

And couple of optimizations:

  • Rum now makes use of staless components (nothing for you to do, if your component is defined via defc with no mixins, it’ll be automatically compiled to stateless component)
  • Rum will use React’s batched updates to perform rendering on requestAnimationFrame in a single chunk
  • Streamlined internals of component construction, removed render->mixin, args->state, element and ctor->class


  • Added rum.core/derived-atom, a function that let you build reactive chains and directed acyclic graphs of dependent atoms. E.g. you want *c to always contain a value of *a plus a value of *b and update whenever any of them changes
  • Added rum.core/dom-node helper that takes state and finds corresponding top DOM node of a component. Can be called in mixins after initial render only
  • Fixed compatibility of with-key on nil-returning component in server rendering (thx Alexander Solovyov, PR #73)


  • Better support for server-side rendering of SVG
  • [ BREAKING ] Rum used to support multiple ways to specify attributes. You would expect that both :allow-full-screen, :allowFullScreen and "allowFullScreen" would be normalized to allowfullscreen. As a result, you have to face three problems:
    • how do I decide which variant to use?
    • how do I ensure consistency accross my team and our codebase?
    • find & replace become harder

Starting with 0.9.0, Rum will adopt “There’s Only One Way To Do It” policy. All attributes MUST be specified as kebab-cased keywords:

Attribute What to use What not to use
class :class :class-name :className
for :for :html-for :htmlFor
unescaped innerHTML :dangerouslySetInnerHTML { :__html { "..." }}
uncontrolled value :default-value :defaultValue
uncontrolled checked :default-checked :defaultChecked
itemid, classid :item-id, :class-id :itemID :itemId :itemid
xml:lang etc :xml-lang :xml/lang :xmlLang "xml:lang"
xlink:href etc :xlink-href :xlink/href :xlinkHref "xlink:href"
xmlns not supported

To migrate to 0.9.0 from earlier versions, just do search-and-replace for non-standard variants and replace them with recommended ones.


  • Improved server-side rendering for inputs (issue #67 & beyond)
  • Compatible server-side rendering of components that return nil (issue #64)
  • Upgraded React to 15.1.0


  • rum/render-static-markup call for pure HTML templating. Use it if you’re not planning to connect your page with React later
  • rum/def* macros now correctly retain metadata that already exists on a symbol (thx aJchemist, PR #62)


  • Add rum.core/unmount function (thx emnh, issue #61)


  • Retain :arglists metadata on vars defined by rum/def* macros (thx aJchemist, PR #60)


  • Migrated to React 15.0.1
  • Optimized server-side rendering (~4× faster than Rum 0.7.0, ~2-3× faster than Hiccup 1.0.5)




  • Added :class-properties to define arbitrary properties on a React class (thx Karanbir Toor, PR #44)
  • [ BREAKING ] Removed support for :child-context-types and :context-types. Use { :class-properties { :childContextTypes ..., :contextTypes ... } } instead.


  • Check for setTimeout in global scope instead of in window (thx Alexander Solovyov, PR #43)


  • Fixed bug with rum macros emitting wrong namespace. You can now require rum.core under any alias you want (thx Stuart Hinson, PR #42)


  • [ BREAKING ] Core namespace was renamed from rum to rum.core to supress CLJS warnings


  • Upgraded to React 0.13.3, Sablono 0.3.6, ClojueScript 1.7.48
  • New API to access context: child-context, child-context-types, context-types (thx Karanbir Toor, PR #37)
  • New defcc macro for when you only need React component, not the whole Rum state
  • [ BREAKING ] Component inner state (:rum/state) was moved from props to state. It doesn’t change a thing if you were using Rum API only, but might break something if you were relaying on internal details
  • Deprecated rum/with-props macro, use rum/with-key or rum/with-ref fns instead


  • Allow components to refer to themselves (thx Kevin Lynagh, pull request #30)
  • Support for multi-arity render fns (issue #23)


  • Added local mixin


  • Fixed argument destructuring in defc macro (issue #22)


  • will-update and did-update lifecycle methods added (thx Andrey Vasenin, pull request #18)


  • Components defined via defc/defcs will have displayName defined (thx Ivan Dubrov, pull request #16)
  • Not referencing requestAnimationFrame when used in headless environment (thx @whodidthis, pull request #14)


  • Compatibility with clojurescript 0.0-2758, macros included automatically when (:require rum)


  • Updated deps to clojurescript 0.0-2727, react 0.12.2-5 and sablono 0.3.1


  • [ BREAKING ] New syntax for mixins: (defc name < mixin1 mixin2 [args] body...)
  • New defcs macro that adds additional first argument to render function: state
  • Ability to specify key and ref to rum components via with-props


  • Fixed a bug when render-loop tried to .forceUpdate unmounted elements
  • Fixed a cursor leak bug in reactive mixin
  • Removed :should-update from reactive, it now will be re-rendered if re-created by top-level element
  • Combine reactive with static to avoid re-rendering if component is being recreated with the same args


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

All heavy lifting done by React, Ŝablono and ClojureScript.


Copyright © 2014–2016 Nikita Prokopov

Licensed under Eclipse Public License (see LICENSE).