Almost Bare Bones WebR Starter App

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Let’s walk through how to set up a ~minimal HTML/JS/CS + WebR-powered “app” on a server you own. This will be vanilla JS (i.e. no React/Vue/npm/bundler) you can hack on at-will.

TL;DR: You can find the source to the app and track changes to it over on GitHub if you want to jump right in.

In the docs/ directory in the GH repo you’ll see an example of using this in GH Pages.Here it is live: Info on what you need to do for that is below.

If all went well, you should see the output of a call to WebR right here (it may take a few seconds):

Getting Your Server Set Up

I’ll try to keep updating this with newer WebR releases. Current version is 0.1.0 and you can grab that from:

System-Wide WebR

You should read this section in the official WebR documentation before continuing.

I’m using a server-wide /webr directory on my domain so I can use it on any page I serve.

WebR performance will suffer if it can’t use SharedArrayBuffers. So, I have these headers enabled on my /webr directory:

Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp

I use nginx, so that looks like:

location ^~ /webr {
  add_header "Cross-Origin-Opener-Policy" "same-origin";
  add_header "Cross-Origin-Embedder-Policy" "require-corp";


For good measure (and in case I move things around), I stick those headers on my any app dir that will use WebR. I don’t use them server-wide, though.

And They Call It a MIME. A MIME!

WebR is a JavaScript module, and you need to make sure that files with an mjs extension have a MIME type of text/javascript, or some browsers won’t be happy.

A typical way for webservers to know how to communicate this is via a mime.types file. That is not true for all webservers, and I’ll add steps for ones that use a different way to configure this. The entry should look like this:

text/javascript  mjs;

Testing The WebR Set Up

You should be able to hit that path on your webserver in your browser and see the WebR console app. If you do, you can continue. If not, leave an issue and I can try to help you debug it, but that’s on a best-effort basis for me.

Installing The App

We’ll dig into the app in a bit, but you probably want to see it working, so let’s install this ~minimal app.

My personal demo app is anchored off of /webr-app on my web server. Here’s how to replicate it:

# Go someplace safe
$ cd $TMPDIR

# Get the app bundle
# You can also use the GH release version, just delete the README after installing it.
$ curl -o webr-app.tgz

# Expand it
$ tar -xvzf webr-app.tgz
x ./webr-app/
x ./webr-app/modules/
x ./webr-app/modules/webr-app.js
x ./webr-app/modules/webr-helpers.js
x ./webr-app/css/
x ./webr-app/css/simple.min.css
x ./webr-app/css/app.css
x ./webr-app/main.js
x ./webr-app/index.html

# 🚨 to make sure I'm not pwning you!
# 🚨 Don't trust anything or anyone.

# Go to the webserver root

# Move the directory
$ mv $TMPDIR/webr-app .

# Delete the tarball (optional)
$ rm $TMPDIR/webr-app.tgz

Hit up that path on your web server and you should see what you saw on mine.

WebR-Powered App Structure

├── css                  # CSS (obvsly)
│   ├── app.css          # app-specific ones
│   └── simple.min.css   # more on this in a bit
├── index.html           # The main app page
├── main.js              # The main app JS
└── modules              # We use ES6 JS modules
    ├── webr-app.js      # Main app module
    └── webr-helpers.js  # Some WebR JS Helpers I wrote

Simple CSS

If you sub to my newsletter, you know I play with tons of tools and frameworks. Please use what you prefer.For folks who don’t normally do this type of stuff, I included a copy of Simple CSS b/c, well, it is simple to use. Please use this resource to get familiar with it if you do continue to use it.

JavaScript Modules

When I’m in “hack” mode (like I was for the first few days after WebR’s launch), I revert to old, bad habits. We will not replicate those here.

We’re using JavaScript Modules as the project structure. We aren’t “bundling” (slurping up all app support files into a single, minified file) since not every R person is a JS tooling expert. We’re also not using them as they really aren’t needed, and I like to keep things simple and as dependency-free as possible.

In index.html you’ll see this line:

<script type="module" src="./main.js"></script> 

This tells the browser to load that JS file as if it were a module. As you read (you did read the MDN link, above, right?), modules give us locally-scoped names/objects/features and protection from clobbering imported names.

Our main module contains all the crunchy goodness core functionality of our app, which does nothing more than:

  • loads WebR
  • Tells you how fast it loaded + instantiated
  • Yanks mtcars from the instantiated R session (mtcars was the third “thing” I typed into R, ever, so my brain defaults to it).
  • Makes an HTML table from it using D3.

It’s small enough to include here:

import { format } from "";
import * as HelpR from './modules/webr-helpers.js'; // WebR-specific helpers
// import * as App from './modules/webr-app.js'; // our app's functions, if it had some

console.time('Execution Time'); // keeps on tickin'
const timerStart =;

import { WebR } from '/webr/webr.mjs'; // service workers == full path starting with /

globalThis.webR = new WebR({
    WEBR_URL: "/webr/", # our system-wide WebR
    SW_URL: "/webr/"    # what ^^ said
await globalThis.webR.init(); 

// WebR is ready to use. So, brag about it!

const timerEnd =;
console.timeEnd('Execution Time');

document.getElementById('loading').innerText = `WebR Loaded! (${format(",.2r")((timerEnd - timerStart) / 1000)} seconds)`;

const mtcars = await HelpR.getDataFrame(globalThis.webR, "mtcars");
HelpR.simpleDataFrameTable("#tbl", mtcars);

globalThis is a special JS object that lets you shove stuff into the global JS environment. Not 100% needed, but if you want to use the same WebR context in in other app module blocks, this is how you’d do it.

Let’s focus on the last three lines.

const mtcars = await HelpR.getDataFrame(globalThis.webR, "mtcars");

This uses a helper function I made to get a data frame object from R in a way more compatible for most JS and JS libraries than the default JS object WebR’s toJs() function converts all R objects to.


This makes a nice table in the browser’s Developer Tools console. I did this so I could have you open up the console to see it, but I also want you to inspect the contents of the object (just type mtcars and hit enter/return) to see this nice format.

We pass in a WebR context we know will work, and then any R code that will evaluate and return a data frame. It is all on you (for the moment) to ensure the code runs and that it returns a data frame.

The last line:

HelpR.simpleDataFrameTable("#tbl", mtcars);

calls another helper function to make the table.


I may eventually blather eloquently and completely about what’s in modules/webr-helpers.js. For now, let me focus on just a couple things, especially since it’s got some sweet JSDoc comments.

First off, let’s talk more about those comments.

I use VS Code for ~60% of my daily ops, and used it for this project. If you open up the project root in VS Code and select/hover over simpleDataFrameTable in that last line, you’ll get some sweet lookin’formatted help. VS Code is wired up for this (other editors/IDEs are too), so I encourage you to make liberal use of JSDoc comments in your own functions/modules.

Now, let’s peek behind the curtain of getDataFrame:

export async function getDataFrame(ctx, rEvalCode) {
    let result = await ctx.evalR(`${rEvalCode}`);
    let output = await result.toJs();
    return (Promise.resolve(webRDataFrameToJS(output)));

The export tells the JS environment that that function is available if imported properly. Without the export the function is local to the module.

let result = await ctx.evalR(`${rEvalCode}`);

A proper app would use JS try/catch potential errors. There’s an example of that in the fancy React app code over at WebR’s site. We just throw caution to the wind and evaluate whatever we’re given. In theory, we should have R ensure it’s a data frame which we kind of can’t do on the JS side since the next line:

let output = await result.toJs();

will show the type as a list (b/c data.frames are lists).

I’ll likely add some more helpers to a more standalone helper module, but I suspect that corporate R will beat me to that, so I will likely also not invest too much time on it, at least externally.

Await! Await! Do Tell Me (about await)!

Before we can talk about the last line:

return (Promise.resolve(webRDataFrameToJS(output)));

let’s briefly talk about async ops in JS.

The JavaScript environment in your browser is single-threaded. async-hronous ops let pass of code to threads to avoid blocking page operations. These get executed “whenever”, so all you get is a vapid and shallow promise to of code execution and potentially giving you something back.

We explicitly use await for when we really need the code to run and, in this case, give us something back. We can keep chaining async function calls, but — if we need to make sure the code runs and/or we get data back — we will eventually need to keep our promise to do so; hence, Promise.resolve.

Serving WebR From GitHub Pages

The docs/ directory in the repo shows a working version on GH pages.

main.js needs a few tweaks:

// This will use Posit's CDN

import('').then( // this wraps the main app code
    async ({ WebR }) => {

        globalThis.webR = new WebR({
            SW_URL: "/webr-app/"            // 👈🏼 needs to be your GHP main path
        await globalThis.webR.init();

        const timerEnd =;
        console.timeEnd('Execution Time');

        document.getElementById('loading').innerText = `WebR Loaded! (${format(",.2r")((timerEnd - timerStart) / 1000)} seconds)`;

        const mtcars = await HelpR.getDataFrame(globalThis.webR, "mtcars");
        HelpR.simpleDataFrameTable("#tbl", mtcars);


Moar To Come

Please hit up this terribly coded dashboard app to see some fancier use. I’ll be converting that to modules and expanding git a bit.

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