Rendering and Reactivity
When you change data in a frontend application, the UI should update automatically. But directly manipulating the DOM is slow — browsers weren't designed for constant updates. Frameworks solve this problem with clever strategies that minimize actual DOM changes while keeping your UI perfectly synchronized with your data.
The Problem Frameworks Solve
Imagine a todo list with 100 items. If you check off one item, should the browser rebuild all 100? That would be wasteful. Ideally, only the single checkbox that changed should update.
Frameworks track which data affects which parts of the UI, then update only what's necessary. They do this through two main approaches: virtual DOM and fine-grained reactivity.
Virtual DOM (React's Approach)
React pioneered the virtual DOM — a lightweight JavaScript representation of the actual DOM. When data changes, React:
- Creates a new virtual DOM tree
- Compares it to the previous virtual DOM (called "diffing")
- Calculates the minimum changes needed
- Applies only those changes to the real DOM
Data changes → New virtual DOM → Diff with old → Patch real DOM
This approach is predictable and works well for complex UIs. The tradeoff is that React re-renders entire component subtrees, then figures out what actually changed.
Fine-Grained Reactivity (Vue, Svelte)
Vue and Svelte take a different approach. They track exactly which UI pieces depend on which data. When data changes, only the specific elements using that data update — no diffing required.
Track dependencies → Data changes → Update only affected elements
Svelte goes further by doing this work at compile time. Your Svelte code compiles to efficient JavaScript that directly updates the DOM, with no runtime framework overhead.
Optimizing Render Performance
Regardless of framework, certain patterns help performance:
Use keys for lists. When rendering arrays, keys help the framework identify which items changed, moved, or were removed.
Memoize expensive computations. React's useMemo and Vue's computed cache results so they're not recalculated unnecessarily.
Avoid unnecessary re-renders. In React, React.memo prevents re-rendering when props haven't changed.
Lazy load components. Don't load code for components the user hasn't seen yet.
Understanding how your framework updates the UI helps you write faster applications and debug performance issues when they arise.