Web · Intermediate
Build Your Own Virtual DOM
Build a tiny virtual DOM library with an h() function, a diff algorithm, and reconciliation that patches only the changed parts of the real DOM.
You build a miniature virtual DOM library, the core idea behind React and Vue, that represents UI as plain JavaScript objects and updates the real DOM efficiently. You implement an h() (hyperscript) function to create virtual nodes, a render step that mounts a virtual tree into real DOM elements, and a diff algorithm that compares an old tree to a new one and produces the minimal set of patches to apply. Reconciliation then walks the patches and mutates only the nodes that actually changed. It is worth building because it demystifies the abstraction every modern frontend framework rests on and teaches tree diffing, recursion, and DOM manipulation. It is resume-worthy because understanding why virtual DOM updates are efficient, and where they are not, sets you apart from engineers who only consume the framework.
What you build
- An h() function that builds virtual nodes from tag, props, and children
- A mount step that renders a virtual tree into real DOM nodes
- A diff algorithm comparing old and new virtual trees
- Patch application that updates only changed nodes, attributes, and text
- Keyed children diffing to handle list reordering
- Event listener and attribute reconciliation
- A small demo app re-rendering on state change
What it teaches
- Representing UI as a tree of plain objects
- Tree diffing and minimal-patch computation
- Reconciliation and targeted DOM mutation
- Keyed list reconciliation
- Why virtual DOM updates are efficient and their limits
- Recursion over nested tree structures
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Take it further
- Add a component abstraction with local state and re-render scheduling.
- Batch updates so multiple state changes produce one diff pass.
- Add lifecycle hooks for mount and unmount.
