After operating under the radar for a while, we're now publicly launching the code evaluation API that allows anyone to execute code in whatever language from anywhere on the internet. Here is the story so far: and the underlying tech is a project that grew out of my frustration from the process of setting up machines to learn programming. I believed that one of the main turn offs for people wanting to learn how to code was setting up the development environment. That's when I thought that putting a REPL on the web was the best way to get started with programming.

In 2011, and after working on the problem for almost a year we were able to cross-compile numerous language interpreters to JavaScript and we also hand coded some. We open sourced everything along the way and little did we know that our work would help accelerate a revolution in online programming education.

Our open source code evaluation infrastructure was used by companies like Codeacademy, Udacity, Bloc, and many others to deliver an in-browser coding experience. Unfortunately, the way we were doing things was pushing the envelope for what browsers could do at the time and they would break our code very often. Coupled with the fact that users had to download megabytes worth of JavaScript before being able to do anything made it unreliable for websites serving millions of users all over the world. Since then, I stopped using it in production in favor for a server-based system and to keep myself honest I also deprecated the open source project.

After the move, I still got emails from people asking for support on those projects, and I felt guilty for not being able to help. At the same time was growing and with it my wallet was shrinking. One of the benefits of doing client-side code evaluation was that the cost was practically zero. I considered putting ads on the site but I couldn't do this to our users. Especially because many of them happen to be students learning as part of classrooms:

Yesterday we visited a high school in Mountain View where the entire school were learning how to code. Hundreds of...

Posted by Amjad Masad on  Saturday, October 24, 2015

After getting so many emails from people asking for support I decided to offer our code evaluation infrastructure as a service. Since then we had a few customers use us -- enough to cover's cost. More than anything, I'm happy that our customers are deriving a lot of value from the service and in many cases they're building things in line with our mission of democratizing programming. For example, Carnegie Mellon University is using us as part of their online open learning initiative. Flatiron school and Trinket are using us to build their online education platforms. Oneinterview and Airety are using us to deliver online interviewing services. And of course uses the same API for the main site, and the embeds.

I'm planning to do a technical write up about the tech and architecture behind the service but the basic idea is that you connect to our servers and then send us programs to execute. It's fast, reliable, and scalable. Try out at And check out the API page for more information. Here is what an API call looks like for a "Hello World" in Ruby:

var repl = new ReplitClient('', 80, 'ruby', token);
repl.connect().then(() => repl.evaluate(
  'puts "hello world"',
  { stdout: out => console.log(out) }

What started as something that I thought of as a necessary annoyance to keep the site running turned out to be something that I'm actually proud of. This wouldn't have been possible without the work from Haya and Faris where they continue to build awesome new features and products on top of