Understanding React Rendering Without the Buzzwords

React Rendering and Re-rendering: Essential Information

Understanding how rendering and re-rendering work in React is fundamental for developers aiming to optimize application performance. This article outlines the triggers for re-rendering, the impact of re-renders, and strategies to manage them effectively.

When React renders a component, it executes the component's function to determine its appearance on the screen. For instance, given the component:

function Greeting() { return <h1>Hello, world!</h1>; } React renders it by calling Greeting() and inserting <h1>Hello, world!</h1> into the DOM.

State Changes: Alterations in a component's state using useState . Prop Changes: Updates to data passed from the parent component. Parent Re-renders: When a parent component re-renders, its children may follow suit.

function Counter() { const [count, setCount] = React.useState(0);

return ( <div> <p>You clicked {count} times</p> <button onClick={() => setCount(count + 1)}>Click me</button> </div> ); } Each button click updates the state, triggering React to call Counter() again for the latest UI.

function Greeting({ name }) { return <h1>Hello, {name}!</h1>; }

function App() { const [name, setName] = React.useState("Alice");

return ( <div> <Greeting name={name} /> <button onClick={() => setName("Bob")}>Change Name</button> </div> ); } Changing the name from "Alice" to "Bob" causes Greeting to re-render due to the name prop change.

Even when a child's props remain unchanged, it may re-render if its parent does.

function Child() { console.log("Child rendered"); return <p>Child</p>; }

function Parent() { const [count, setCount] = React.useState(0);

return ( <div> <Child /> <button onClick={() => setCount(count + 1)}>Click</button> </div> ); } The Child component logs “Child rendered” with each button click, even without props or state, due to its parent re-rendering.

While React's rendering is efficient, unnecessary re-renders in large applications can slow down performance and cause visual disruptions. Understanding how to optimize renders is crucial for maintaining application efficiency.

Wrap components with React.memo() to ensure they only re-render when their props change.

const Greeting = React.memo(function Greeting({ name }) { console.log("Greeting rendered"); return <h1>Hello, {name}</h1>; }); This approach is suitable for pure components with consistent prop values.

Use useCallback to Avoid Changing Functions

React treats new functions as new props. Use useCallback to maintain function identity across renders.

const handleClick = React.useCallback(() => { console.log("Clicked"); }, []);

For expensive calculations within components, wrap them in useMemo to run only when dependencies change.

const result = React.useMemo(() => { return slowFunction(input); }, [input]);

When Should You Care About Re-renders?

Optimization is generally unnecessary unless experiencing performance issues such as visual glitches or slow UI in applications with extensive item lists or requiring high-performance precision.

“React will re-render the entire app!”

Incorrect. React optimizes by re-rendering only necessary components and updating only the changed DOM elements.

Re-rendering is fundamental to React's operation. Focus on eliminating only the redundant re-renders impacting performance.

Prioritize clarity and functionality before engaging in optimization. Build, measure, then refine if performance issues arise.

React renders components to generate updated UI. State, prop, or parent changes trigger re-renders. Avoid unnecessary re-renders in large applications. Utilize React.memo , useCallback , and useMemo appropriately. Optimize only when application performance demands it.

Based on reporting by hackernoon.com.