React.js is a JavaScript library that provides robust functionalities for building user interfaces.

One of React's powerful mechanisms is Reconciliation, which enables the creation of highly efficient and dynamic user interfaces.

React employs stack reconciliation and fiber reconciliation as its underlying algorithms. Due to its performance benefits, React primarily uses fiber reconciliation, as explained below.

How Reconciliation Works

At a core level, React operates with two trees:

1. Current Tree: The tree is already rendered in the browser.
2. Virtual Tree: A virtual copy of the current tree created by React, stored in memory.

The Update Process

React updates the tree in two phases:

1. Render Phase: Determines what changes are needed.
2. Commit Phase: Applies those changes to the DOM.

When React detects a change in the application state, it renders the updated content in the virtual tree off-screen (in memory). If the virtual tree looks correct with no errors, React commits these changes to the current tree, which is displayed in the browser.

Work Loops in Fiber Reconciliation

React manages updates using a process called work loops, driven by functions like beginWork(current, virtual/workInProgress, lane).

Here’s how it works:

1. Render Phase:
   • When React detects a change, it calls the beginWork function, passing the current and virtual trees as parameters.
   • React compares the components of the current tree and the virtual tree. If a difference is found, React marks a flag on the corresponding component for updating.
   • The process is recursive: React continues calling beginWork for child components until it reaches the leaf nodes of the tree.
   • Once all components are processed, React calls completeWork(current, virtual/workInProgress, lane), constructing a reverse tree in memory.
2. Commit Phase:
   • Once the render phase is complete, React starts the commit phase.
   • The FiberRootNode, which initially points to the current tree, is updated to point to the virtual tree (or work-in-progress tree).
   • This marks the end of the reconciliation process, and the updated UI becomes visible to the user.

Summary

Fiber reconciliation provides React with a highly efficient mechanism for updating the UI by breaking down the update process into manageable units of work. This ensures React can deliver smooth and responsive interfaces, even for complex applications.