domcurl: curl + JavaScript

Paul Kinlan

For a long time I’ve been thinking about what the future of the web looks like when we go past what we know as the traditional web browser. I called this The Headless Web and I wanted to answer was “What if everything was powered by ‘The Web’, but you never saw a browser?“. Specifically I believe that if you have access to a full browser, but no visible to “chrome” then there is a huge opportunity for a new set of services.

Using the browser as a service is an incredible opportunity. It allows us to take the declarative HTML and combine it with the developer defined procedural execution of JavaScript and run deep analysis on the content.

Running a browser on the server will allow us to more easily build services which parse data that is generated dynamically, it will allow us to more easily us run our own logic against the logic in a page (form fill as an example) and I believe that it will open up the ability to more effectively run actions against data embedded on the page.

It’s taken a while, but I think we are getting there.

I’m enamored by Puppeteer. Puppeteer is a JavaScript library that sits on top of the Chrome Dev Tools protocol and it allows you to automate and script the Chrome browser.

My day-to-day work involves a lot of debugging web servers and ensuring. Like many developers I use curl to make requests to a web server and check the response. It’s an amazing utility, however in today’s world many developers are building sites that are built using a lot of JavaScript and this makes it impossible to inspect the complete response.

I decided to create a cUrl-like utility for fetching a resource and running the JavaScript on the page called domcurl.

domcurl is a small NodeJS application that uses Puppeteer and can be installed by issuing the following command: npm i domcurl. Like the curl command you can issue a simple domcurl [url] to fetch the resource and run the JS on the page.

It doesn’t replicate all of curl, but it is quite featureful with the following features.

  • Specify a url to fetch. i.e, domcurl [url]
  • Inspect the response headers with -v. i.e, domcurl -v [url]
  • Set cookies with -b i.e, domcurl [url] -b "test=hello;; HttpOnly;" -b "hello=world;; HttpOnly;"
  • Add custom headers using the -H argument.
  • Manually set the STDOUT with -o and STDERR with --stderr

I’m finding it quite useful eventhough it can’t stream the results like curl can because it has to wait for the CSS and JS to be downloaded and executed.

I also took the liberty to add a couple of extra features that are specific to JavaScript and Chrome.

  • Output a Chrome Dev Tools trace file (including screens hots.) domcurl --url --trace test.json
  • Include it as a JavaScript module if you have the need to integrate it into any of your existing applications.
const {domcurl} = require('domcurl');
domcurl(``, {});

Whilst this tool is more of a demo than a full service, I think The Headless Web is maturing and tools like Puppeteer and others are going to help us realize the continued power of the web. We just need to build for it.

Paul Kinlan

Trying to make the web and developers better.

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