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Howto use Zenroom within the browser

This article is part of a series of blog posts about interacting with Zenroom inside the Javascript messy world. This is the second entry and at the end of the article you should be able to run a fancy hello world within your browser.

  1. Part one Zenroom in node.js
  2. Part two Zenroom in the browser
  3. Part three Zenroom in React
  4. Part four Zenroom in commonjs and ES6

🔬Let’s look better at WASM

So we saw before that the way we have to interact from Zenroom to JS is via transpilation of our C code into WebAssembly (WASM).

The output of this operation is a binary .wasm file and a .js file that we will call glue code. In this glue code there are mostly helper functions to interact with the components inside the wasm file, starting with where to retrieve them and so on.

Zenroom itself exposes two main family of functions to interact with other languages zenroom_exec and zencode_exec as documented here that are exactly the functions exported and available in our .wasm file.

Making the WASM working within the browser, months ago was a hard thing, but nowadays a huge effort has spent, and all the new major browsers support it natively.

 🕹️ Let’s get our hands dirty

Create a new directory, and add Zenroom as a dependency like

mkdir zenroom-web-test
cd zenroom-web-test
yarn init
yarn add zenroom

Now let’s create a new file index.html with the following content

<!DOCTYPE html>
                <script src="./node_modules/zenroom/dist/lib/zenroom.js"></script>

To see the output of our html file, since the .wasm file has to be serverd with the correct mimetype, we must run a small http server in localhost, the fastsest way I actually know is with the standard python library. So you want to go with

$ python3 -m http.server

This will serve your pages by default on the 8000 port so point your browser to http://localhost:8000

Back to the code, that simply print the emscripten module that you can now interact with like native calls. Documentation on how to interact is available on

You should have seen something like this in your browser inspector:

That is great now let’s instruct our Module to do something useful like generate a keypair as we seen in the previous episode.

Edit your index.html file with the following content:

<!DOCTYPE html>
    <script src="./node_modules/zenroom/dist/lib/zenroom.js"></script>
        const keygen_contract = `rule check version 1.0.0
Scenario 'simple': Create the keypair
Given that I am known as 'Puria'
When I create the keypair
Then print my data`
        const Wasm = Module()
        Wasm.exec_error = function(){}
        Wasm.exec_ok = function(){}
        Wasm.print = console.log

        Wasm.onRuntimeInitialized = async _ => {
        const zenroom = {
          zencode_exec: C.cwrap('zencode_exec', 'number',
                                     ['string', 'string', 'string', 'string', 'number']),
        zenroom.zencode_exec(keygen_contract, null, null, null, 0)


The expected result is something like

Cool let’s celebrate 🎉 🎉 and let’s go through the code, we instantiate an object Wasm from our module, then we added some mandatory facilities exec_error, exec_ok and print

So exec_error and exec_ok are callbacks defined in zenroom that are executed after something goes wrong or goes successfully after each zenroom call. By zenroom call we intend each time a Zenroom VM is created and run. In our case are empty functions as are not needed for the purpose of this exercise

Then obviously print is the way we want to see the output, the standard function console.log is pretty okay 😉 so we just use that in this case, but sometimes you want to instruct it to print maybe directly on the DOM, that is up to you.

Let’s go now to the next line that is fundamental the onRuntimeInitialized is a callback that is executed when everything Wasm-side is okay is something you want always put as a wrapper of your interaction with the wasm module.

Now we are able to define our cwrap call as mentioned previously from the emscripten documentation, and define our zencode_exec as defined in the Zenroom’s embed documentation.

Briefly we passed to the zencode_exec our contract without any of conf, keys and data and with and muted with 0 verbosity

That’s all folks let’s see you next time for the React part.

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