Leporello.js – interactive functional programming IDE for JavaScript
Hi! Leporello.js is an interactive functional programming environment designed for pure functional subset of JavaScript. It executes code instantly as you type and displays results next to it. Leporello.js also features an omnipresent debugger. Just position your cursor on any line or select any expression, and immediately see its value. Leporello.js visualizes a dynamic call tree of your program. Thanks to the data immutability in functional programming, it allows you to navigate the call tree both forward and backward, offering a time-travel-like experience. Leporello.js offers the ability…
In plain words
Leporello.js is an interactive development environment for functional JavaScript that executes code instantly as you type and displays results alongside it. It includes a debugger accessible by positioning the cursor on any line to see values immediately, and visualizes the program's call tree. Users can navigate forward and backward through execution, similar to time-travel debugging. The tool supports interactive HTML5 application development while preserving application state, and records and replays IO traces during subsequent executions.
written from the facts on this page · September 2026
From the sources
In the maker’s words, at launch
Hi! Leporello.js is an interactive functional programming environment designed for pure functional subset of JavaScript. It executes code instantly as you type and displays results next to it. Leporello.js also features an omnipresent debugger. Just position your cursor on any line or select any expression, and immediately see its value. Leporello.js visualizes a dynamic call tree of your program. Thanks to the data immutability in functional programming, it allows you to navigate the call tree both forward and backward, offering a time-travel-like experience. Leporello.js offers the ability to develop HTML5 applications interactively, enabling you to update your code without losing the application's state. It records an IO trace of your program, which is then transparently replayed during subsequent program executions. This allows you to instantly reexecute your code after making small tweaks, thereby tightening your feedback loop. Furthermore, Leporello.js can serve as an interactive notebook. You have the flexibility to utilize any JavaScript libraries to visualize your data directly within your code. For a more detailed walkthrough, please watch the product video. Currently, Leporello.js is available as a free online application that you can try right in your browser. My goal is to build the Leporello.js standalone Electron app and a VSCode plugin, both with TypeScript support. Additionally, I plan to add Node.js support (currently, Leporello.js is only for HTML5 apps). In the VSCode plugin, Leporello.js will sit on top of the built-in TypeScript/JavaScript mode, utilizing its code analysis information to enhance the default VSCode experience with unique Leporello.js features. I am building Leporello.js as a single independent developer. Leporello.js is funded solely by donations. Support me on Github Sponsors [0] and be the first to gain access to the Leporello.js Visual Studio Code plugin with TypeScript support. I'll be delighted to answer any questions you may have. [0] https://github.com/sponsors/leporello-js
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