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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →A React component is a reusable piece of user interface that React can render. It is commonly written as a JavaScript function that returns JSX, and components can be combined to build anything from a button to a full page.
What a React component looks like
Here is a small function component:
function Greeting({ name }) {
return <h1>Hello, {name}!</h1>;
}
export default function App() {
return <Greeting name="Sam" />;
}Greeting is a component. Its function returns JSX describing a heading, and App renders it using the JSX tag <Greeting name="Sam" />. The parent supplies name; the child reads that value and displays it. Component names begin with a capital letter, while lowercase JSX names such as <h1> represent built-in HTML elements. See React’s Your First Component guide.
How components, JSX, and the browser fit together
A component is the named unit you define and reuse; JSX is the syntax it can return to describe what should appear. JSX resembles HTML, but it is a JavaScript syntax extension, not HTML itself. JavaScript expressions can be placed inside JSX with curly braces, as {name} does in the example. React uses the component and its JSX to render the interface. React-based frameworks may also use components to generate HTML.
Components can be nested: one component can render another, and a page can be assembled from smaller pieces such as a navigation bar, profile card, and list. React components can supply all of an app’s interface or only selected interactive parts of a page; a project does not have to follow one universal app structure. React introduces components as reusable UI elements in its official tutorial.
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Props and state: inputs versus changing memory
Props come from a parent
Props are values a parent passes to a component, often through JSX attributes. In <Greeting name="Sam" />, name is a prop. The function parameter { name } uses JavaScript destructuring to read it directly. Props can contain strings, objects, arrays, functions, or JSX—not just text. A component reads its props; it should not mutate them. React’s guide to passing props explains this parent-to-child flow.
State is information the component remembers
State is component-specific memory for information that changes over time, such as a counter value after a click. Function components use the useState Hook to read a state value and request an update. As a practical distinction, use props for inputs supplied from outside the component and state for values the component needs to track as its interaction changes. React covers state and interaction in Adding Interactivity.
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Good habits when writing components
- Define components at the top level. Keep a child component outside its parent component’s function. If it needs parent data, pass that data as props rather than defining the child inside the parent.
- Render components through JSX. Write
<Greeting />rather than callingGreeting()as an ordinary function; React recommends using JSX so it can manage component rendering. - Return valid UI. A function component returns JSX or another valid React node. When a multiline JSX expression follows
return, wrap it in parentheses to avoid JavaScript’s automatic semicolon insertion ending the return early. - Follow Hook rules. Call Hooks such as
useStateat the top level of a function component, not conditionally or inside a nested function. See React’s Rules of React.
Function components and class components
React still supports class components, so you may encounter them in older code. However, React recommends defining components as functions rather than classes for new code. Beginners should therefore learn function components first; the React Component reference documents the class API and its status.
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