React Lifecycle in 3 Minutes

Mastering the React lifecycle is the first step to debugging like a pro 😉.

Here’s an easy 3-minute guide to help you understand how everything works.

📚 Download my FREE 101 React Tips And Tricks Book for a head start.

What Is the React Component Lifecycle?

A React component is like a human being 😅.

Just as we're born, evolve, and eventually die, React components follow a lifecycle: they're born, they update, and then they die.

This process is known as the component lifecycle, which has three main phases:

• Mounting (aka Birth): React creates the component and adds it to the DOM.

• Updating (aka Evolving): The component changes in response to state changes.

• Unmounting (aka Dying): React removes the component and cleans up resources.

Each phase includes specific steps React performs to optimize rendering and manage resources.

React Component Lifecycle Analogy With Human Lifecycle

Why Do You Need To Understand React Component Lifecycle?

Understanding the React lifecycle helps you:

• Write better effects with useEffect and useLayoutEffect.

• Avoid inefficiencies, like setting state inside useEffect.

• Prevent memory leaks by cleaning up.

• Optimize performance and avoid unnecessary re-renders.

The Three Phases of the React Component Lifecycle

Timeline of a React Component With Hooks. Source: https://julesblom.com/writing/react-hook-component-timeline

1. Mounting: When a Component Is First Rendered

Mounting occurs when React adds (mounts) a component to the screen.

What triggers mounting?

• When you call root.render(<MyComponent />)

• When React adds a component to the JSX tree and re-renders its parent to include the new component.

What happens during mounting?

1. Render: React calls the component function (or render method in class components) to create React elements. React also initializes hooks.

2. Insert DOM Nodes: React updates the real DOM with the new elements. This step happens during the commit phase when React applies changes to the DOM.

3. Set DOM Refs: If you use useRef, React sets references to the DOM nodes.

4. Run useLayoutEffect: React runs the useLayoutEffect functions. Changes made here won't cause visible flickering since the change will occur before the user sees any visual changes (which happen during painting).

5. DOM Paint: The browser paints the updated UI.

6. Run useEffect: Finally, React runs the useEffect functions.

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2. Updating: When a Component Re-Renders

Updating happens when React re-renders a component to reflect changes.

What triggers updating?

• A state update

• An updated context value

• A parent component re-rendering (if the component wasn't memoized)

• Etc.

What happens during updating?

1. Re-render: React recalculates the component's output.

2. Reconciliation: React compares the new React tree with the previous one to determine changes.

3. Commit Changes: React updates the DOM with the changes.

4. Unset DOM Refs: React nullifies ref.current and calls non-stable ref callbacks with null if the referenced DOM node is removed or replaced during the update.

5. Cleanup useLayoutEffect: React runs cleanup functions from the previous useLayoutEffect.

6. Set DOM Refs: React sets new ref values for the updated DOM elements (like in the mounting phase).

7. Run useLayoutEffect: React calls useLayoutEffect hooks, like in the mounting phase (if its dependencies have changed according to Object.is).

8. Paint the DOM: The browser updates the UI to reflect changes.

9. Cleanup useEffect: Cleanup functions from the previous useEffect run to prevent memory leaks.

10. Run useEffect: React runs the updated useEffect function (if its dependencies have changed according to Object.is).

    ---

3. Unmounting: When React Removes a Component

Unmounting occurs when React removes a component from the DOM, freeing resources.

What triggers unmounting?

• The component is no longer part of the JSX tree

• React unmounts its parent.

• The component’s key changed (React will unmount the old instance and mount a new one.).

• Etc.

What happens during unmounting?

• Run Cleanup Functions: React runs cleanup functions defined in useEffect and useLayoutEffect.

• Unset DOM Refs: React nullifies ref.current and calls ref callbacks with null.

• Remove DOM Nodes: React removes the DOM nodes associated with the element.

Additional Resources To Go Further

If you want to go deeper into the React lifecycle, I recommend these amazing resources:

• React Docs

  • Render and Commit
  • Lifecycle of Reactive Effects

• Timeline of a React Component With Hooks ❤️

• Why React Re-Renders

• A (Mostly) Complete Guide to React Rendering Behavior - warning: this is a very long article 😅

Summary

Once you understand the React component lifecycle, you'll be ahead of 99% of junior frontend devs.

It's simpler than you think: only three phases — mounting, updating, and unmounting.

Start practicing these concepts with real-world examples:

• Use useLayoutEffect for DOM measurements.

• Always clean up effects when a component unmounts.

You've got this 💪.

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