The Role of Fiber in React Rendering (Part 2): Buffers, Hooks, Lanes, and the Commit

In Part 1 , we opened up React's Fiber engine and looked at the basics: what a Fiber node is, how the linked list tree is wired with child / sibling / return pointers, how the two-phase walk ( beginWork down, completeWork up) processes every component, and how reconciliation diffs old Fibers against new elements. Now we go deeper. This part covers the systems that make Fiber actually powerful : double buffering, effect flags, priority lanes, where your hooks live, and the three sub-phases of the commit. By the end, you'll understand exactly why React can pause rendering mid-tree and come back later without breaking anything. Double Buffering: Current vs. Work-In-Progress Here's a clever trick React borrows from game rendering: double buffering . At any given moment, React maintains two Fiber trees: The current tree is what's actually on screen right now The workInProgress tree is the next version React is building Each Fiber in one tree has an alternate pointer to its counterpart in th