fastpen is described as 'Fastpen is an open source alternative to codepen that just stores the source code compressed, in the url' and is a Code Editor in the development category. There are more than 25 alternatives to fastpen, not only websites but also apps for a variety of platforms, including Self-Hosted, Android, Mac and Windows apps. The best fastpen alternative is Replit, which is free. Other great sites and apps similar to fastpen are jsFiddle, CodePen, JSPad and JS Bin.
Replit is a collaborative cloud development environment offering in-browser IDE features like AI assistance, real-time collaboration, Git support, hosting, deployment, syntax highlighting, plugin extensibility, ad-free UI, dark mode, and live code sharing.
jsFiddle lets one easily test snippets of JavaScript/CSS/HTML code on-the-fly, adding optional libraries like jQuery.
CodePen is a communal front-end development platform providing real-time HTML, CSS, and JavaScript previews. Users can build projects, test code, and enhance skills through challenges. Features include Presentation Mode for conferences and a vast active community for sharing and discovery.
JSPad is a fast and lightweight online code editor for writing, testing, and previewing HTML, CSS, and JavaScript snippets instantly.
JS Bin is a collaborative open source JavaScript debugging tool. Enter Javascript in one pane and HTML in the other and see the result; share using a short URL. Choose popular JS libs from a menu, so you don't have to remember the URL.
kodeWeave is a realtime coding playground for HTML, CSS and Javascript. Similar to JSFiddle and JSBin, but kodeWeave was made to work offline but also as a prototyping application to build applications for desktop operating systems while on mobile devices.
dabblet is an interactive playground for quickly testing snippets of CSS and HTML code. It uses -prefix-free, so that you won't have to add any prefixes in your CSS code. You can save your work in Github gists, embed it in other websites and share it with others.
Codewith.it is an online HTML, CSS and JavaScript code playground with realtime preview with many tools for developing web pages and advanced code analysis systems, with many assistance tools.
CodeSandbox is an online editor with a focus on creating, sharing and importing new React projects
Storm is an online web development environment which allows users to run HTML, CSS, Javascript code in real time in a secured environment. It supports sharing Scribble (Storm project) via a dedicated link and also allows users to use jQuery and React framework of JavaScript.
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