What does Fiber Architecture do in the context of the lifecycle?

ReactMiddle~3 min read

What React Fiber is

React Fiber is a new implementation of React's internal mechanism that is responsible for how and when component rendering happens.

In other words: Fiber is React's "new engine", which manages the entire process:

calling components,
updating the Virtual DOM,
scheduling work,
and running lifecycle methods (componentDidMount, useEffect, and so on).

React moved to Fiber starting with React 16, to make rendering stepwise, pausable, and priority-driven.

Before Fiber (React 15 and earlier)

React had synchronous rendering: when an update happened, React walked the entire tree until it finished.
Large trees blocked the main thread → the page "froze" during rendering.
All lifecycle methods (componentWillMount, componentDidMount, componentDidUpdate) were called all at once, with no way to optimize priority.

After Fiber (React 16+)

Fiber turned React into a cooperative task scheduler. Rendering is now a staged, priority-driven process, where React can:

Split work into small chunks (fiber nodes)
Pause and resume rendering
Interrupt less important work (for example, an animation or user input takes priority)
Precisely control when lifecycle methods run

What a Fiber Node is

Every component (function, class, or host element) is represented by an internal object - a Fiber.

A Fiber is a node in a linked list that stores:

a reference to the component type (function, class, div, and so on),
the current state and props,
references to its parent, children, and siblings,
information about the priority of the work (in which order to run it),
effects (what needs to be done in the commit phase).

This lets React know exactly where it stopped, if it needs to interrupt work and then resume it.

How Fiber Architecture relates to the lifecycle

React now splits the lifecycle into 2 phases:

Phase	What it does	Methods it calls
Render phase	Computes what needs to change	`shouldComponentUpdate`, `render`
Commit phase	Applies the changes to the DOM	`componentDidMount`, `componentDidUpdate`, `useLayoutEffect`, `useEffect`

How Fiber implements this:

Render Phase (reconciliation) Fiber walks the tree of Fiber nodes and computes which changes are needed.

This phase can be paused, resumed, or discarded.
Here React can "split" the work and schedule it by priority.
For example, the user clicked → React may defer rendering less important parts of the UI. The following are called:
shouldComponentUpdate
render() (for classes)
the body of function components The DOM is not updated yet during this phase.

Commit Phase (applying changes) Once Fiber finishes the calculations, it commits the changes:

updates the DOM,
calls the post-render hooks. The following are called:
componentDidMount
componentDidUpdate
componentWillUnmount
useLayoutEffect
useEffect This phase is always synchronous - React cannot interrupt it.

An example, visually

javascript

Fiber: start of Render Phase
    ↓
analyzing changes → diff Virtual DOM
    ↓
can be interrupted or paused
    ↓
Fiber: Commit Phase
    ↓
DOM update + lifecycle calls

Why Fiber matters for the lifecycle

Before Fiber	After Fiber
All components updated one after another, with no stopping	Rendering can be paused and resumed
Methods were called strictly in sequence	Methods run at different priorities
React did not understand priorities	React knows which updates matter more
Rendering blocked the UI	The UI stays responsive during rendering

Example: how this affects effects

javascript

useEffect(() => {
  console.log('effect');
  return () => console.log('cleanup');
}, [value]);

In the render phase React calls the component and decides whether an update is needed.
In the commit phase React applies the changes and calls useEffect.
If the update is deferred (low priority), the effect does not fire until React reaches commit.
Fiber makes sure that cleanup is always called strictly before a new effect.

Concurrent Rendering (React 18+)

Thanks to Fiber, React now supports concurrent rendering:

parts of the UI can be rendered "in the background";
the user does not see "freezes";
lifecycle methods are called only once the changes are committed.

Summary

Fiber Architecture is the mechanism that made React "smart": it controls the timing, priority, and order of lifecycle execution.

Phase	What it does	Methods
Render phase	Computes changes (can be paused)	`render`, `shouldComponentUpdate`, the function component's body
Commit phase	Applies changes (always synchronous)	`componentDidMount`, `componentDidUpdate`, `useEffect`, `cleanup`

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