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This repository is meant to document and explore the implementation of what is known as "the Elm architecture". A simple functional architecture for building frontend applications.

High level overview

The entire state is contained in a single data structure. Things can happen and the state should change accordingly. The number of things that can happen is described as a set of actions. Actions flow unidirectionally down the application. Actions are handled by pure update functions. Such a function takes an action and a state and returns a new state. The state is handed to a view function that returns a virtual DOM representation. A module is an encapsulated set of actions, an update function and a view function. Modules can be nested inside other modules and modules can contain other modules. This makes the architecture nestable and modular.


  • As few mutations in application code as possible. The vast majority of your application can be completely pure.
  • Time changing values and interactions with the world is introduced in a controlled manner through FRP.
  • Testing should be easy! And nothing is easier to test than pure side-effect free functions.
  • State should be easily inspectable for debugging and serialization. Also, time travel.
  • Minimalism and simplicity are center pieces in every used library.
  • Actions should be expressed as union types.
  • Everything should be modular and nestable.
  • Simple live code editing thanks to hot module replacement.



  • Counters example without FRP – This is several versions of a counters application. It starts out very simple and then gradualy increases in complexity. This is implemented with Snabbdom, union-type and Ramda.
  • Counters example with FRP – This is similair to the above example but the architecture is bootstraped with Flyd as and FRP library.
  • Who to follow – A small who to follow box using GitHubs API. It can be compared with a plain FRP version using Flyd and one using Rx.
  • Zip codes – This is a translation of Elm's zip-codes example, designed to show how to use asyncronous tasks, specifically http requests. ES6 Promises are used here as stand-ins for Elm's Tasks.
  • TodoMVC – A TodoMVC implementation built with Snabbdom, union-type, Ramda and Flyd.
  • File Uploader – Implements a file-upload component, i.e. it renders a list of chosen files, shows their upload progress, and allows users to cancel uploads. It demonstrates asynchronous side effects and a realistic stand-alone reuseable component or 'widget'.
  • Hot module reloading – Demonstrates how simple it is to use webpack's hot module replacement to achieve live code editing without browser refreshes.
  • Modal – Demonstrates a technique for implementing modals.
  • Nesting – A simple application that demonstrates three level nesting of components. This is intended to show how the action-routing scales.


The architecture is implementation independent. It can be implemented with varied combinations of libraries. It only requires a virtual DOM library and a way to update JavaScript data structures without mutations. Having a nice representation of actions is also useful.

Virtual DOM libraries

The view layer in the architecture consists of pure functions that takes part of the applications state and returns a description of it's view. Such a description will typically be a virtual DOM representation that will later be rendered with a virtual DOM library. A number of options exists.

  • Snabbdom – A small modular and extensible virtual DOM library with splendid performance.
  • virtual-dom – A popular virtual DOM library.
  • React – Mature and widely used. It supports JSX which many people like. It is however a bulky library. It supports stateful components which should not be used together with the architecture.

Updating data structures

When handling actions inside update functions it is necessary to update ones state without mutating it. Libraries can provide a help with regards to this.

  • Ramda – Ramda provides a huge amount of functions for working with native JavaScript data structures in a purely functional way.

Representing actions


A functional frontend framework.







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