This part supposes you have installed the cozy development environment from the cozy development introduction.

Create a client-side application for Cozy

Developing an app only requires writing a frontend (javascript code for the browser). Cozy provides an API to access and manage the user’s data. Moreover, it handles the authentification for you. That means you can focus on the main features of your app without learning a new technology. You can use your favorite framework to build your app (Angular, React or Backbone).

Let’s write a new contact application for Cozy!

The goal of this tutorial is to build a client-side app which lists all contact names from the Contact app. It will also allow users to create, update, and delete contact names.

To do so, we’ll proceed in different steps. We’ll start slowly by deploying a client-side “Hello World” app, and we’ll finish with a full contact app using vanillaJS. This tutorial is for everybody, from padawan to jedi, so don’t be scared of it. We’ll explain everything.

Make a client-side “Hello World Application”

Here we are, doing the traditional “Hello World” app that can make you want to start a new career.
We can now introduce the application files structure:

Packaging and deployment

The first thing to do is packaging the application, then making it available, then installing it in our Cozy.

Packaging the application

First start by creating a folder your app and run a git init in it. The manifest package.json is a file containing meta information about the application, namely:

  "name": "your-app-name",
  "cozy-type": "static",
  "cozy-displayName": "Your App's Name",
  "version": "1.0.0",
  "description": "Your app's description",
  "icon-path": "main_icon.png",
  "cozy-permissions": {
    "Contacts": "This app manages your contacts"
The fields you need

Also, an index.html file needs to be at the root of your repository with your ‘Hello World’ written in it.

<!doctype html>
<html lang="en">
  <meta charset="utf-8">
  <title>Your app !</title>
  <p>Hello World!<p>

Deploying the application

Once we’ve packaged our application and did a Hello World! in our index.html, we must make it available for installation.

To achieve this, we can create a Github repository, and push our work on it.

# We here consider we have a Github repository created
git add --all .
git commit -m "A relevant commit message"
git push origin master

You should now be able to install it from the “store” page on your cozy. Write down the repository’s URL. At the bottom of http://localhost:9104/#applications, we type the URL, and start the installation process.

Once it’s done, we can see our app in the applications list: http://localhost:9104/#apps/my-app-identifier.

If you managed to deploy it, congratulations! If not, don’t worry, we’re here to help you: the most easy way to contact us is by joining our irc channel (#cozycloud on Freenode server).

Install field

Pushing every change you make to github can rapidly grow tiring.

Fortunately, the cozy team has provided you a tool that makes it unnecessary!

In your terminal, simply go to your app directory, and run the cozy-dev proxify command.

cd myapp
cozy-dev proxify http://localhost:9104

This command uses up to two params:

  1. a URL (mandatory): we here use the local VM (instanciated with cozy-dev vm:start), but you can safely use any cozy available remotely. No more need to use a local VM for development, you can safely use your own Cozy (like
  2. a subdir (optional): if your app is complex, you probably won’t distribute the source directly, but use a builder (like Webpack) to that produces output assets in a dedicated subdir (like build). You can pass its path as a second argument to indicate the directory to use as root for the proxy
cd myapp
npm run build
cozy-dev proxify ./build

Source code

At the end of this step, your app should be similar to this.

Source code

To see the fully working and finished version, you can go on this github repo.

Second step: Get contacts

CouchDB is a NoSQL database, and can’t be requested with SQL. It has a specific way to allow applications to fetch collection of documents. To fetch a subset of this huge list, we must declare views. A view is a sort of powerful filter, defined using map/reduce functions. The map/reduce functions returns a list of key-value, whose key and value are defined by the view’s creator (us).

We’ll describe in details how map/reduce work and how we can use them, later in this tutorial. For now, we’ll introduce cozysdk-client.js helpers to help us creating basic views, which are probably what we will need in most cases:

<script src="./cozysdk-client.js" />

First of all, please add cozysdk-client.js in your repository and link it into your html page.

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

Let’s add a link to our own script file too.

Then we want to start doing things. Once our app is loaded, nothing easier: we can add an event listener to the browser event DOMContentLoaded. It will allow us

function updateContactList(){
   // let's code here

document.addEventListener("DOMContentLoaded", updateContactList);

We can now start coding in our function updateContactList. Our objectives here are simple: getting the list of contacts from your contact app, only with javascript. The goal is to show you how simple it is to send requests to cozy-data-system, without worrying about which framework to chose.

But wait a minute… What is the data-system?

The Data System is the data layer of Cozy Cloud. Technically, it is a wrapper for CouchDB that manages authorization and authentification of applications willing to access user’s data. CouchDB is an open-source NoSQL database document-oriented. If you are familiar with SQL database such as MySQL, you will find it different in various ways: * There is no concept of “table” * The database is one huge list of typeless JSON documents

The Data System introduces the concept of document type. Each document has a document type. Each application can declare or reuse document types. It can be Contact, an Event, a File, a Message, a Todo, etc. It’s a coherent data assembly that we define the schema (or use the one defined by others). There are many existing document types, we’ve documented the main ones here. The document type is automatically stored in the CouchDB document by the Data System. From our developper point of view, it’s as if we were using SQL tables.

The Data System offers a REST API, which means one must use HTTP requests to communicate with it. In order to facilitate this communication, we’ve built a module to provide developers a programatic API: please, meet cozy-browser-sdk.

So what we want to do here is getting all contacts and display them into an html array. To do so, we will use the cozy-browser-sdk, and more acurately its functions defineView and a queryView. Nothing more.

The permissions

  "name": "my-app-identifier",
  "displayName": "My Application Name",
  "description": "My application's punchline",
  "version": "1.0.0",
  "cozy-permissions": {
    "contact": {
      "description": "Why do my application needs to access this document type"

cozy-permission is the list of document types (docTypes) our application will be granted access to. We must add a description to inform the user why our application needs to access this document type. It’s the first step to make transparent what our application does, and build trust with our future users. Authentication and authorization is managed by the Data System, and cozysdk-client automatically adds the information needed to authenticate itself.

What are a docTypes or document types?

doctypes are type of document and are how data are structured inside Cozy. You can see them as SQL tables. There are already plenty of doctypes you can reuse. So during the installation of an application, the user is prompted to allow or deny access to various types of data (“doctypes”), so they can decide whether they trust the application or not. It also opens the opportunity to use multiple data sources. Do you want to have contacts in your contact application? Well, you can. It’s all up to you!

One important thing to understand about Cozy, is that the platform owns the data, not the application. Applications are just granted permissions by the user to access and manipulate them.

Do the request with docType

For security reasons, you have to add a specific docType to access data from an other app in each requests. It will either define a new docType from the data-system, or identify itself to an existing one. So for example if you want to define a request to get all the contacts, you’ll have to add Contacts as a docType name.

Add permissions

You also need to make sure that you added permissions in package.json like so:

"cozy-permissions": {
    "Contact": {
        "description": "To easily find your contact when talking about someone."

description is going to be showned when a user installs your application. So I advise you to be persuasive and explain why it needs to access your Contact data.

This is just an example but if you want to work with Emails for example, you’ll just need to add Message as a cozy-permission and name it as a docType in the define function.


As mentioned in the Data System introduction, the data-system uses CouchDB as a database engine. CouchDB is a NoSQL database, and can’t be requested with SQL. It has a specific way to allow applications to fetch collection of documents.

The principle is simple: CouchDB is a huge list of documents. To fetch a subset of this huge list, we must declare views. A view is a sort of powerful filter, defined using map/reduce functions. The map/reduce functions returns a list of key-value, whose key and value are defined by the view’s creator (us).

function updateContactList(){
  cozysdk.defineView('Contact', 'all', 'function(doc) { emit(doc.n); }', function(err, res) {
    if (err != null) return alert(err);
    cozysdk.queryView('Contact', 'all', {}, function(err, res) {
      if (err != null) return alert(err);
      var contacts = JSON.parse("" + res);
      /* contacts == [
        {id:"323274828329", key:"Jane;Willson"},
        {id:"323274827428", key:"John;Smith"}


In our case, defineView asks us to use the Map/Reduce method and defines a document from their original structure into a new key/value pair. You can then choose to map only a specific field of a document. Here, for example, is a function that can Map the name field of a contact document.

function(doc) {
    if (doc.n) {

The call to the emit function is when the mapping takes place. The emit function accepts two arguments: a key and a value. Both arguments are optional and will default to null if omitted. As it’s helpful to know which document the mapped data came from, the id of the mapped document is also automatically included.

Here you can easily customize your request and use it whenever you wish in your app.


Then, from the callback response, you will be able to use the queryView function that will query a request defined by defineView(docType, name, request, callback). So in this case, I will ask queryView to call the request defined with all by the defineView function. The response is an id, a key and a value.

If you want to add params in queryView, you will be able to enable users to fine tune what they want to get.

So for example params could look like this:

{key: 'bob'}

A simple example

Imagine you have a Cozy database with contact records and you want a view of those records using the name of each user as keys. If so, you can easily do the following:

cozysdk.defineView("Contact", "lastName", function(doc) {
    if (doc.n) {
        emit(doc.n, doc);

If you query this function (queryView(docType, name, params)), you will have a result like this:

   { key: "Clarke", value: { n: "Clarke", ... } },
   { key: "Kelly",  value: { n: "Kelly",  ... } },
   { key: "Smith",  value: { n: "Smith",  ... } },


function render(contacts){
  var HTML = ''
  for (var i = 0; i < contacts.length; i++) {
    HTML += '<tr data-id="' + contacts[i].id + '">' +
          +   '<td><label>' + contacts[i].key + '</label></td>'
          + '</tr>';
  document.querySelector('.contact-list').innerHTML = HTML;

For simplicity, we re-render the whole array every time it changes. So we create a string of HTML with rows and cells, and use innerHTML to display it in our application.

Source code

You can find the source code for this step here.

Third step: Create, destroy or update a contact

So, displaying the contacts from the cozy is cool, but can our app change them ? Yes We Can !

Follow along on github

Contact creation

<input class="send" placeholder="Add a new contact">
document.querySelector('.send').addEventListener('change', onSendChanged);

So we add a field to our html page and an event handler. when the user writes something in the field, the function onSendChanged is called and we are able to create a contact with its name.

function onSendChanged(){
  var contactName = document.querySelector('.send').value.trim();
  var values = {n: contactName};
  cozysdk.create('Contact', values, function(err, res) {
    if (err != null) return alert(err);
    // == "8239283928326"
    document.querySelector('.send').value = '';

In order to do that, we are going to get the value from the field and we’ll pass it to the cozy SDK’s create function, along with the type of document we want to create.

When the contact has been created, the callback is called and we can get the contact id. At this point, we could try to be smart and inject a row into the table, or we could go the easy way and just update the whole list again.

Contact create, update & delete

Using cozydb to build a CRUD

In this section, we will learn how to define a new document type in our application, and build a CRUD (a set of basic operations: Create, Update, Delete) for it.


cozysdk.create('Contact', contact, function(err, res) {
  if (err != null) {
    return alert(err);
  } else {
    document.querySelector('.send').value = '';

The function create(docType, attributes, callback) creates a new object with the data from the [contact]( object.

When we try again to create a new contact from the browser, we notice the result is not just the payload we sent, but the document from the database itself. Most importantly, it has an ’_id’ field, which is a unique identifier for the document, that we can reuse for later access, update, or deletion.


First things first, let’s add some columns and controls to our render function. You can see it done here.

To capture the user interaction with these buttons, we are going to use something called event bubling. It means we can add event listeners to the whole table and check the event to see if they match our criteria. This is done in the attachEventHandler function.

So now, when the user changes the value of our name fields, the onUpdatePressed function is called. However we still need to retrieve the id of the contact. For this, we use the getIDFromElement, the way to do it is a recursive function: the function will not stop going back to itself until it finds a tag with data-id.

So we first check the <input>, no data-id, we go up, the <td>, still nope, then the <tr data-id="id of the modified contact"> and we got it !

For update, we pass the id and the changes we want to make to the cozysdk.updateAttributes.


The next handy method is destroy(docType, id, callback), which requires an ID as parameter, that we get from `getIDFromElement( Straightforward, isn’t it?

For delete, we pass the id to cozysdk.destroy

In both case the callback is called when the action is finished and we can update the list again!

Going further

I think my role is complete. You now have the technical tool to develop client-side apps on Cozy. If you want to use a framework, you can experiment with using it and the cozysdk and let us know how it goes.

The challenge now will be to understand and meet the needs of a random Cozy user. What new service can you offer this user, in order to simplify the managing of his or her data? You now know how to synchronise data from the different applications, so you’ll need to go further and imagine what you can do with this knowledge.

When your application is going to be able to change the life of all the Cozy users, you can add it on cozy-registry by making a pull request.

To conclude we would like to give you a link to a full music player application: cozy-music. That way you can find inspiration by looking at this app.