A fun way to think about browsers, is as a standardized Virtual Machine (VM) that includes high-level APIs to do networking, sandboxed code execution and disk access. It runs on almost every platform, behaves similarly everywhere, and is always kept backwards compatible.

An important part of this machinery is networking. In the browser, there are 8 (or so) different ways to access the network:

  • By navigating to a new page
  • Through <script>, <link>, and other tags.
  • Using preload headers.
  • Using new XMLHTTPRequest(), window.fetch(), and navigator.sendBeacon().
  • Using (dynamic) import().
  • Using the Server Sent Events API.
  • Using the WebSocket API.
  • Using WebRTC.

In this section we'll go over some of the most common networking protocols, and show how to use these efficiently.

HTTP Requests

Most of the browser's networking happens through HTTP requests. HTTP is a data protocol on top of a TCP stream. Knowing that HTTP is based on TCP is useful, because it means that the browser will try its very best to make sure a request goes through. If it fails, it usually means that something is very wrong, like when the internet connection has dropped entirely.

You might be seeing the word HTTP/2 getting thrown around regularly. You don't need to worry too much about this, as HTTP/2 doesn't change any existing browser APIs. You can think of it as a "more efficient version HTTP". For now, don't worry about things like HTTP/2 push either - they're mostly details for servers, which we won't be covering here.

The browser has several ways of performing HTTP requests.

  • Navigating to a new page: performs an HTTP request whenever you type in a url in your browser. The server then replies with some index.html file which contains more links to assets and other pages.
  • Through <script>, <link> and other tags: When an index.html page is loaded, the browser will read out all <link> and <script> tags in the document's head. This will trigger requests for more resources, which in turn can request even more resources. There's also the <a> and <img> tags, which act similarly.
  • Using preload headers. When a browser page is loaded, the server can set headers for additional resources that should be loaded. The browser then proceeds to request those.
  • Using new XMLHTTPRequest() and window.fetch(): In order to make dynamic requests to say, a JSON API, you can use these APIs. XMLHTTPRequest is the classic way of performing requests, but these days there's the more modern window.fetch() API. Regardless of which API you use, they produce similar results.
  • Using navigator.sendBeacon(): Sometimes you want to perform an HTTP Request, but want to allow more important requests to be prioritized first, such as with analytics data. The sendBeacon() API allows for exactly this: it allows you to create an HTTP POST request, but schedules it to occur in the background, even if the page is closed before the request had a chance to complete.
  • Using (dynamic) import: Scripts can request other scripts, by using the import syntax. This can either be dynamic or static - but in both cases it makes an HTTP request to require JavaScript.


There are many ways to create an HTTP request, but the most common one these days is using fetch(). In Choo you might want to trigger a request based on some other event coming through the EventEmitter. Let's look at a brief example of how to trigger a request based on a Choo event:

note: we recommend going through the stores docs first. We assume you'll know how Choo's event emitter model works for the next few sections.

var choo = require('choo')

var app = choo(), emitter) => {
  state.tweets = []
  emitter.on('fetch-tweets', (username) => {   // 1.
    window.fetch(`/${username}/tweets`)        // 2.
      .then((res) => res.json())               // 3.
      .then((data) => {
        state.tweets.concat(data)              // 4.
      .catch((err) => {
        emitter.emit('error', err)             // 5.
  1. We create a listener for the 'fetch-tweets' event.
  2. When the listener is called, we create a new fetch() call to a JSON endpoint. The url is dynamically created based on the username that's passed.
  3. We know the response will be JSON, so we try and convert the binary blob to a valid JavaScript Object.
  4. Now that we have an object, we add the new tweets to our existing list of tweets. Once the new data is set, we emit the 'render' event to trigger a DOM update.
  5. If any of the steps above failed for any reason, we emit the 'error' event. If this was a real-world project, this is where we'd also make sure we had a good user-facing error, and would report the error to our analytics server. Good error handling is a very imporant aspect of production applications.

And that's about it. Depending on the application's requirements, you can switch up the arguments that are passed to fetch(). Perhaps you might even want to create an abstraction to reduce boilerplate. Whichever way you go: the abstraction is going to be similar.

Server Sent Events

So far we've only covered doing request-response (REQRES) using HTTP. You send a request, and you always expect a response. If you don't get a response, it's considered an error.

But REQRES isn't the only type of connection out there. Another common pattern is that of publisher-subscriber (PUBSUB). This pattern has a 'publisher' on one side, and a 'subscriber' on the other. For example: you might have a server that knows about events, and a client that listens to them. In the browser you can achieve PUBSUB using the Server Sent Events (SSE) API.

It's also useful to know that Server Sent Events are supported by HTTP/2. This means that unlike some other messaging protocols, Server Sent Events seamlessly integrate with HTTP/2's socket multiplexing (which means things become more efficient).

Let's create a small SSE client that connects to an /sse url. Whenever an event comes in, it'll emit an 'sse:message' event.

If you're interested in a premade solution for Server Sent Events in Choo, check out the choo-sse package.

var choo = require('choo')

var app = choo(), emitter) => {
  emitter.on('DOMContentLoaded', () => {                             // 1.
    var source = new window.EventSource('/sse')                      // 2.
    source.addEventListener('open', () => emitter.emit('sse:open'))  // 3.
    window.addEventListener('beforeunload', () => source.close())    // 4.
    source.addEventListener('message', (event) => {                  // 5.
      try {
        var data = JSON.parse(
      } catch (e) {
        return emitter.emit('sse:error', e)                          // 6.
      emitter.emit('sse:message', data)                              // 7.

    source.addEventListener('error', (event) => {                    // 8.
      if ( === window.EventSource.CLOSED) {   // 9.
      } else if ( === window.EventSource.CONNECTING) {
        emitter.emit('sse:reconnect')                                // 10.
      } else {
        emitter.emit('sse:error', event)                             // 11.
  1. Before we can create an SSE listener in the browser, we need to check if we're indeed running in the browser. The best way to achieve this is to listen to Choo's 'DOMContentLoaded' event first. It's never triggered in Node, and has the nice side effect of improving time till interactive in the browser.
  2. Once the DOM is loaded, we create a new EventSource instance. We pass it a URL to connect to. It makes a regular HTTP request, but keeps the connection open so the server can send multiple chunks of data to the client as is needed. You can mostly think of it as a fancy HTTP request.
  3. Once the connection is opened, we emit the 'sse:open' event. There might be parts of the UI that are interested in this information.
  4. When we close the page, we want all connections to be cleaned up nicely. In order to do that, we listen to the browser's window.onBeforeUnload event, and close the connection before closing the page. This prevents errors from showing up in our logs.
  5. Here's the part that we came for: listening for events. doesn't require any particular formatting, but it's recommended to use JSON, just like with HTTP APIs. So we should try and parse it to JSON before passing it to the rest of our application.
  6. If for some reason a parsing error occurs, we should emit an 'sse:error' event.
  7. If everything has gone well, we can expose the event to the rest of our app using the 'sse:message' event.
  8. Connection errors can occur. The connection can be closed from the server, it can be reconnecting or some other unknown error might have occured.
  9. If the connection was closed cleanly, we emit the 'sse:closed' connection, and close the whole connection.
  10. If the connection is trying to reconnect, we emit the 'sse:reconnect' event.
  11. If an actual error occured, we emit the 'sse:error' event.


Server Sent Events allow you to create a PUBSUB channel that sends data from one side to the other. But what if we want to send and receive events on both sides? This is where WebSockets (WS) come in.

You can think of WebSockets as two PUBSUB channels. The browser and server can both emit and receive events. This is useful if you are dealing with live data on both the client and server - for example with a chat server, or a document editor.

At the moment WebSockets don't integrate with HTTP/2, so in order to use them both the server and client will need to negotiate a new handshake and establish a new connection.

When using websockets, you'll see that WebSocket urls usually start with either ws:// or wss://. The difference between these two is that wss:// is secure, where ws:// is not. If possible, try and use wss://.

Let's take a look at how to write a store that allows us to send and receive messages from a websocket!

If you're interested in a premade solution for WebSockets in Choo, check out the choo-websocket package.

var choo = require('choo')
var app = choo(), emitter) => {
  emitter.on('DOMContentLoaded', () => {                       // 1.
    var socket = new window.WebSocket('wss://localhost:8080')  // 2.
    state.websocket = { open: false }                          // 3.

    socket.addEventListener('open', (event) => {               // 4.
      emitter.on('ws:send', (data) => socket.send(data))       // 5. = true                              // 6.
      emitter.emit('ws:open')                                  // 7.

    socket.addEventListener('message', (event) => {            // 8.
      emitter.emit('ws:message', event)

    socket.addEventListener('close', (event) => {              // 9. = false

    socket.addEventListener('error', (err) => {              // 10.
      emitter.emit('ws:error', err)
  1. Before anything, we want to make sure that the DOM is available. If the page is still loading, there's a good chance that the network is still saturated - so it makes a lot of sense.
  2. Now that the DOM is ready, we want to open up a new socket. We point it to our local server, using a secure connection. We get a websocket instance back which we name socket.
  3. When we first receive access to a websocket instance, the connection itself will not be ready yet. Parts of our user interface might want to reflect this, so until the socket is open, we set to false.
  4. Before we can start sending events, we need to wait for the socket to finish initializing. This is done by listening for the 'open' event.
  5. Once the websocket is ready, we can start listening for 'ws:send' events. This is then used to call socket.send() and pass any data down. It's important to start listening for events before we emit the 'ws:open' event. Otherwise we might lose events if they were sent in the same tick as 'ws:open'.
  6. We can now tell the system that our websocket is ready to be used, so we update our state before that.
  7. Now that we're reading to start listening for events, we can emit 'ws:open'.
  8. Whenever we receive a 'message' event, we emit the 'ws:message' event.
  9. Whenever the connection closes, we set to false and we emit the 'ws:close' event.
  10. Whenever we receive an 'error' event, we emit the 'ws:error' event.

Wrapping Up

And that's it! We hope you've now read enough to get started with network protocols in the browser! There's much more to explore, and as the web evolves so will the protocols. But we're confident that interfacing Choo with the browser's networking protocols will always remain straightforward and convenient. Happy (network) hacking!


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By Yosh & friends