Build a Mini UI Framework - The Magic, Demystified

You've used a frontend framework - maybe you've read our React, Vue, or Svelte guides - and somewhere in your head there's still a box labeled "magic." How does changing count update the screen? What is a virtual DOM, physically? What does "tracking dependencies" actually track?

This project empties the box. In about 120 lines of plain JavaScript, run right here in your browser, you'll build the load-bearing machinery of a modern UI framework: a reactivity system that knows who read what (Vue's engine, Svelte's engine), an effect and computed layer on top of it, a virtual DOM with a render function (React's core idea), and a diff that finds the minimal set of changes. At the end you'll wire them into one working micro-framework and map each piece onto the real frameworks' names for it.

Nothing here is a toy in the pejorative sense - these are the actual algorithms, minus the production hardening. After this project, framework documentation reads like a description of code you've written.

What you need

Comfortable JavaScript: objects, functions as values, arrays, map/filter. Having read one of the frontend from-zero guides helps you connect the dots but isn't required - this project also works as a prequel that makes those guides land harder.

How to read this

Every phase builds and runs real code in the page. Run every block - and do the exercises before peeking at the next block, because each phase's machinery becomes the next phase's raw material.

The phases

1. Reactive Objects - a Proxy that notices reads and writes: the atom of all reactivity.
2. Effects and Computed - automatic dependency tracking, the trick every framework shares.
3. The Virtual DOM - UI as cheap description objects, and a render function.
4. The Diff - comparing two descriptions to find the minimal change - and why keys exist.
5. Wiring It Together - state → render → diff as one loop, and the map from your 120 lines to React, Vue, Svelte, and Angular.