Millions of developers use React every day to build fast, interactive user interfaces. But only a small percentage truly understands how React works behind the scenes how it updates the screen, manages components, handles state, performs diffing, and delivers such smooth performance.
Most beginners learn React by memorizing "state, props, and hooks."
But here's the truth:
React becomes 10× easier to use once you understand what's happening internally.
React is not magic.
It is a sophisticated system built on powerful concepts such as:
Virtual DOM
Reconciliation
Fiber Architecture
Component rendering phases
Scheduling
Hooks lifecycle
Commit and Render phases
This blog breaks down each of these concepts clearly, without complex jargon, helping you understand React like a senior engineer.
React's entire workflow can be explained in three major steps:
Trigger → Something changes (state or props)
Render → React prepares the changes
Commit → React updates the actual UI
Every React update a click, a form input, a data fetch, a rerender follows this pipeline.
Here is the flow:
State / Props Change → Virtual DOM Render → Diffing → Reconciliation → Real DOM Update
Let's break each step in a simple, human-friendly way.
Before React, updating the DOM manually was slow because:
Browsers needed to recalculate styles
Reflow the layout
Repaint the screen
React solved this by creating a Virtual DOM a lightweight, in-memory copy of the browser DOM.
React creates an internal tree-like representation of the UI.
Every update modifies the Virtual DOM first.
Then React compares old and new Virtual DOM snapshots.
By avoiding full page reloads and updating only what is necessary, React achieves:
Faster performance
Smoother UI interactions
Efficient rendering workflows
The Virtual DOM is not a real browser object it's a conceptual model that helps React calculate updates quickly.
When React updates, it compares:
Old Virtual DOM Tree
vs
New Virtual DOM Tree
This comparison is powered by React's diffing algorithm, optimized around two assumptions:
Different element types produce different trees
Elements with the same key are considered the same
If an element type changes (div → span), React destroys and rebuilds that node.
If the element type is the same, React compares attributes and children.
Keys help React identify which items moved, stayed, or were removed.
Imagine a list of three items:
Item A
Item B
Item C
If you shuffle them but keep the same keys, React instantly knows:
"These are the same items, just rearranged."
This reduces re-renders dramatically.
After diffing, React decides:
Which components should re-render
Which nodes should update
Which elements should be preserved
Which parts should not be touched
Reconciliation ensures that React updates the UI with minimal cost.
It removes outdated elements
It adds new nodes
It updates changed attributes
It ignores nodes that did not change
This selective update system is why React feels fast even for large applications.
React Fiber is the new internal architecture introduced in React 16.
Think of Fiber as React's "brain upgrade" that enables:
Prioritized rendering
Smooth animations
Non-blocking UI updates
Better scheduling
Faster reconciliation
Fiber allows React to break work into small units and process them efficiently.
Before Fiber, React used stack-based recursion, which had a major limitation:
Once React started rendering, it couldn't stop until it finished.
This meant:
Animations lagged
Slow renders froze the UI
Heavy components blocked interactions
Fiber solved this by implementing:
React breaks rendering work into small chunks called fibers.
Each fiber represents a unit of work:
A component
A subtree
An update
A callback
React can pause work, prioritize urgent tasks (like typing or clicking), and resume when the browser is free.
React rendering is divided into:
This phase is:
Pure
Interruptible
Can be paused
Can be restarted
No DOM updates here
React builds the Virtual DOM here and prepares a list of changes.
This phase:
Is synchronous
Cannot be interrupted
Updates the real DOM
Runs layout effects
Changes finally appear on the user's screen.
Beginners wonder:
"Why does setState not update the state immediately?"
The answer lies in how React batches updates.
React groups multiple updates together
Instead of updating one by one, React waits, groups the updates, and processes them more efficiently.
This batching reduces unnecessary re-renders.
Every component has a fiber node.
Each fiber stores:
State queue
Props
Hooks list
Effects
Child fibers
Alternate fiber (previous tree)
When you call a state update:
React pushes an update into the state queue
During the render phase, React processes this queue
New state is calculated
Component is re-rendered
Understanding this helps beginners avoid common issues like:
stale closures
unnecessary renders
misunderstanding dependency arrays
Hooks like useState and useEffect look simple externally, but internally they rely on:
React stores hooks in a list associated with the fiber node.
Each new render resets the pointer to the first hook.
This is why:
Hooks must always appear in the same order.
React uses this order to match the right state/effect to the right hook call.
Here's how React handles effects:
React stores the effect in the fiber node
After the commit phase, React runs the effect
On re-render, React compares old dependencies with new
React decides whether to skip or re-run the effect
React cleans up the previous effect before running a new one
This ensures predictable behavior without repeated executions.
React uses multiple optimization strategies:
Memoization
React remembers previous values.
ShouldComponentUpdate (Class Components)
Helps React skip updates.
PureComponent
Auto-compares props.
React.memo (Function Components)
Prevents unnecessary re-renders.
Key Prop Optimization
Helps React track list items efficiently.
Together, these strategies keep React apps fast even at scale.
When you:
click a button,
type in an input,
scroll,
drag,
submit a form
React captures the event using its internal Synthetic Event System.
Instead of attaching event listeners everywhere:
React attaches a single listener at the root
React captures events and normalizes them
React distributes them internally to components
This results in:
Better performance
Consistent behavior across browsers
Reduced memory usage
React uses a priority system to decide what updates first:
Non-urgent updates
Background data loading
Re-renders
State updates
User interactions
Typing
Clicking
Animations
React Fiber handles this priority-based scheduling using:
Time slicing
Cooperative scheduling
Work loops
Expiration times
This is why React feels responsive.
Once React finishes diffing:
It knows exactly which nodes changed
It prepares a mutation list
It updates only the changed parts
The UI updates without unnecessary reflows.
Text content
Styles
Event listeners
Child components
Element attributes
Component order
React never replaces the whole DOM unless absolutely necessary.
Every component goes through phases:
React:
Initializes state
Builds fiber node
Creates Virtual DOM
Compares it with empty tree
Updates real DOM
React:
Processes state updates
Re-renders Virtual DOM
Reconciles differences
Updates UI
React:
Cleans up effects
Removes event listeners
Frees memory
Understanding lifecycle helps avoid memory leaks, unnecessary effects, and performance issues.
React can:
Pre-render HTML on the server
Send it to the browser
Hydrate it into an interactive app
This improves:
SEO
Performance
First contentful paint (FCP)
Tools like Next.js use these principles to build high-performance React apps.
Concurrent Rendering is React's latest evolution, allowing React to:
Pause renders
Prioritize urgent updates
Keep apps responsive
Render in the background
This is especially useful for:
Search bars
Large lists
Infinite scroll components
Data-heavy dashboards
React's upcoming features will rely heavily on this architecture.
Let's summarize React's behind-the-scenes process:
Click, input, fetch, state change.
Decides priority.
Generates new Virtual DOM.
Uses Fiber to break work into parts.
Can pause or resume.
Compares old and new Virtual DOM.
Decides what changes.
Updates the real DOM.
useEffect, layout effects.
This system makes React:
fast
stable
scalable
predictable
beginner-friendly
No. React uses batching and scheduling to update efficiently.
To calculate changes faster and avoid expensive real DOM updates.
A new architecture that helps React pause, resume, and prioritize rendering work.
Changes in state, props, or context.
They help React track items and avoid re-rendering the whole list.
Because React uses the call order to match hook data to components.
Render = preparing changes
Commit = updating the real DOM
Through diffing, reconciliation, Fiber, scheduling, and optimized DOM updates.
To master React JS with comprehensive understanding of its internal workings, consider enrolling in our specialized React JS Online Training program. For developers looking to build complete applications with React, we also offer comprehensive Full Stack Java Developer Training that covers React along with modern backend technologies.
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