package ocaml-vdom

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DOM and VDOM for OCaml


Dune Dependency






This package contains:

  • OCaml bindings to DOM and other client-side Javascript APIs (using gen_js_api).
  • An implementation of the Elm architecture, where the UI is specified as a functional "view" on the current state.

Published: 29 May 2020


ocaml-vdom: Elm architecture and (V)DOM for OCaml


This package contains:

  • OCaml bindings to DOM and other client-side Javascript APIs (using gen_js_api).

  • An implementation of the Elm architecture, where the UI is specified as a functional "view" on the current state.


  • OCaml

  • js_of_ocaml

  • gen_js_api

Installation (with OPAM)

opam install ocaml-vdom

Manual installation

git clone
cd ocaml-vdom
make all
make doc
make examples   # Optional (browse index.html files in _build/default/examples to try out)
make install

DOM bindings

Js_browser exposes (partial) OCaml bindings of the browser's DOM and other common client-side Javascript APIs.

It is implemented with gen_js_api, making it realistic to have it working with Bucklescript in the future. This would open the door to writing client-side web applications in OCaml that could be compiled to Javascript either with js_of_ocaml or Bucklescript.


The Elm architecture is a functional way to describe UI applications. In this architecture, the current state of the UI is represented with a single data type and a "view" function projects this state to a concrete rendering. In our case, this rendering is done to a tree-like abstraction of the browser DOM, called a VDOM (Virtual DOM). This VDOM can itself be rendered to a concrete DOM. Whenever the state changes, the view function produces a new VDOM tree, which is then diffed with the previous one to update the concrete DOM accordingly. The VDOM also specifies how DOM events are wrapped into "messages" that are processed by an "update" function to modify the current state. This function can also spawn "commands" (such as AJAX calls) whose outcome is also notified by messages.

The implementation of this architecture relies on two modules:

  • Vdom : definition of the VDOM tree and of "virtual applications". This is a "pure" module, which does not depend on any Javascript bindings (it could be executed on the server-side, e.g. for automated testing).

  • Vdom_blit : rendering of virtual applications into the actual DOM. This modules implements the initial "blit" operation (rendering a VDOM tree to the DOM) and the "diff/synchronization" algorithm. It also manages the state of a running application. Vdom_blit is implemented on top of Js_browser.

This implementation of VDOM has some specificities:

  • Each node in the VDOM tree has a "key" string field. By default, the key corresponds to the tag name for elements but it can be overriden. The key is used by the synchronization algorithm as follows: when synchronizing the old and new children of an element, the children are first grouped by key. Two children with different keys are never synchronized, and the sequence of old and new children with a given key are synchronized in a pairwise way (first old child with key K against first new child with key K; etc...), adding or removing extra/missing children if needed. Children are also reordered in the DOM, if needed, to match the new ordering.

  • Event handlers are not attached on DOM nodes created when a VDOM tree is rendered. Instead, we attach fixed event handlers on the root container, and rely on event delegation. The handler corresponding to a given element and responsible for a given kind of event is searched directly in the VDOM. The rationale for this design choice is that comparing functional values is not well-defined in OCaml, so it would not be clear, when the "old" and "new" VDOMs are diffed, if the event handler on the DOM node should be refreshed.

  • A "bridge" structure in created in Vdom_blit to represent the correspondence between VDOM and DOM nodes. This structure mimics the shape of both trees and avoids having to query the concrete DOM to navigate in the tree.

  • No data structure is created to represent the "diff" between old and new VDOMs. Instead, the synchronization algorithm detects VDOM changes and apply them on the fly to the corresponding DOM node.

  • There is some special support for the "value" property. When this property is explicitly bound in the VDOM (typically on an input field), the value is forced on the element: whenever the DOM value changes, the event is potentially dispatched to an event handler, and the new VDOM property is forced on the DOM element. In particular, if the internal state is not updated by the event handler, the field becomes in practice read-only.

  • Some special VDOM node attributes are provided to present "superficial state changes" that are not reflected in the proper functional state (currently: giving focus to an element, or ensuring an element is visible by y-scrolling its parent). These attributes produce the corresponding DOM action when they are first put on an element (which is not completely well-defined, since this depends on the synchronization algorithm).

  • The "view" function is not applied synchronously when the state ("model") changes. Instead, a rendering (applying the "view" function and updating the actual DOM accordingly) is scheduled. This means that multiple changes can be grouped without triggering a redraw. The current strategy is to delay redrawing with window.requestAnimationFrame, which is supposed to be available (natively, or through a polyfill).


A simple one-module application would look like:

open Vdom

(* Definition of the vdom application *)

type model = .... (* the state of the application *)
let view model =  ...  (* the state->vdom rendering function *)
let init = return ... (* the initial state *)
let update model = function .... (* the state-updating function *)
let my_app = app ~init ~update ~view ()

(* Driver *)

open Js_browser

let run () = my_app   (* run the application *)
  |> Vdom_blit.dom    (* get its root DOM container *)
  |> Element.append_child (Document.body document)   (* insert the DOM in the document *)

let () = Window.set_onload window run

Compiling this to Javascript:

ocamlfind ocamlc -package ocaml-vdom -no-check-prims -linkpkg -o myprog.exe
js_of_ocaml +gen_js_api/ojs_runtime.js -o myprog.js myprog.exe

The Javascript code can then be used from a simple HTML file such as:

    <script src="myprog.js"></script>

Examples: Demo, Counters

Third-party examples: TodoMVC (source, demo), With Eliom service


This project has been created by LexiFi initially for its internal use. It is already used in production but it is still relatively new and no commitment is made on the stability of its interface. So please let us know if you consider using it!

This ocaml-vdom package is licensed by LexiFi under the terms of the MIT license.


Dependencies (4)

  1. gen_js_api >= "1.0.6"
  2. js_of_ocaml-compiler
  3. dune >= "2.0"
  4. ocaml >= "4.08.0"

Dev Dependencies


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