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Components let you split the UI into independent, reusable pieces, and think about each piece in isolation. This page provides an introduction to the idea of components. You can find a detailed component API reference here.

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Conceptually, components are like JavaScript functions. They accept arbitrary inputs (called “props”) and return React elements describing what should appear on the screen.

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Function and Class Components

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The simplest way to define a component is to write a JavaScript function:

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function Welcome(props) {\n  return <h1>Hello, {props.name}</h1>;\n}
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This function is a valid React component because it accepts a single “props” (which stands for properties) object argument with data and returns a React element. We call such components “function components” because they are literally JavaScript functions.

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You can also use an ES6 class to define a component:

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class Welcome extends React.Component {\n  render() {\n    return <h1>Hello, {this.props.name}</h1>;\n  }\n}
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The above two components are equivalent from React’s point of view.

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Function and Class components both have some additional features that we will discuss in the next sections.

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Rendering a Component

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Previously, we only encountered React elements that represent DOM tags:

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const element = <div />;
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However, elements can also represent user-defined components:

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const element = <Welcome name=\"Sara\" />;
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When React sees an element representing a user-defined component, it passes JSX attributes and children to this component as a single object. We call this object “props”.

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For example, this code renders “Hello, Sara” on the page:

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function Welcome(props) {  return <h1>Hello, {props.name}</h1>;\n}\n\nconst element = <Welcome name=\"Sara\" />;ReactDOM.render(\n  element,\n  document.getElementById('root')\n);
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Try it on CodePen

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Let’s recap what happens in this example:

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  1. We call ReactDOM.render() with the <Welcome name="Sara" /> element.
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  2. React calls the Welcome component with {name: 'Sara'} as the props.
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  3. Our Welcome component returns a <h1>Hello, Sara</h1> element as the result.
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  4. React DOM efficiently updates the DOM to match <h1>Hello, Sara</h1>.
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Note: Always start component names with a capital letter.

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React treats components starting with lowercase letters as DOM tags. For example, <div /> represents an HTML div tag, but <Welcome /> represents a component and requires Welcome to be in scope.

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To learn more about the reasoning behind this convention, please read JSX In Depth.

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Composing Components

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Components can refer to other components in their output. This lets us use the same component abstraction for any level of detail. A button, a form, a dialog, a screen: in React apps, all those are commonly expressed as components.

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For example, we can create an App component that renders Welcome many times:

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function Welcome(props) {\n  return <h1>Hello, {props.name}</h1>;\n}\n\nfunction App() {\n  return (\n    <div>\n      <Welcome name=\"Sara\" />      <Welcome name=\"Cahal\" />      <Welcome name=\"Edite\" />    </div>\n  );\n}\n\nReactDOM.render(\n  <App />,\n  document.getElementById('root')\n);
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Try it on CodePen

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Typically, new React apps have a single App component at the very top. However, if you integrate React into an existing app, you might start bottom-up with a small component like Button and gradually work your way to the top of the view hierarchy.

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Extracting Components

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Don’t be afraid to split components into smaller components.

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For example, consider this Comment component:

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function Comment(props) {\n  return (\n    <div className=\"Comment\">\n      <div className=\"UserInfo\">\n        <img className=\"Avatar\"\n          src={props.author.avatarUrl}\n          alt={props.author.name}\n        />\n        <div className=\"UserInfo-name\">\n          {props.author.name}\n        </div>\n      </div>\n      <div className=\"Comment-text\">\n        {props.text}\n      </div>\n      <div className=\"Comment-date\">\n        {formatDate(props.date)}\n      </div>\n    </div>\n  );\n}
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Try it on CodePen

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It accepts author (an object), text (a string), and date (a date) as props, and describes a comment on a social media website.

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This component can be tricky to change because of all the nesting, and it is also hard to reuse individual parts of it. Let’s extract a few components from it.

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First, we will extract Avatar:

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function Avatar(props) {\n  return (\n    <img className=\"Avatar\"      src={props.user.avatarUrl}      alt={props.user.name}    />  );\n}
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The Avatar doesn’t need to know that it is being rendered inside a Comment. This is why we have given its prop a more generic name: user rather than author.

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We recommend naming props from the component’s own point of view rather than the context in which it is being used.

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We can now simplify Comment a tiny bit:

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function Comment(props) {\n  return (\n    <div className=\"Comment\">\n      <div className=\"UserInfo\">\n        <Avatar user={props.author} />        <div className=\"UserInfo-name\">\n          {props.author.name}\n        </div>\n      </div>\n      <div className=\"Comment-text\">\n        {props.text}\n      </div>\n      <div className=\"Comment-date\">\n        {formatDate(props.date)}\n      </div>\n    </div>\n  );\n}
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Next, we will extract a UserInfo component that renders an Avatar next to the user’s name:

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function UserInfo(props) {\n  return (\n    <div className=\"UserInfo\">      <Avatar user={props.user} />      <div className=\"UserInfo-name\">        {props.user.name}      </div>    </div>  );\n}
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This lets us simplify Comment even further:

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function Comment(props) {\n  return (\n    <div className=\"Comment\">\n      <UserInfo user={props.author} />      <div className=\"Comment-text\">\n        {props.text}\n      </div>\n      <div className=\"Comment-date\">\n        {formatDate(props.date)}\n      </div>\n    </div>\n  );\n}
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Try it on CodePen

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Extracting components might seem like grunt work at first, but having a palette of reusable components pays off in larger apps. A good rule of thumb is that if a part of your UI is used several times (Button, Panel, Avatar), or is complex enough on its own (App, FeedStory, Comment), it is a good candidate to be extracted to a separate component.

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Props are Read-Only

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Whether you declare a component as a function or a class, it must never modify its own props. Consider this sum function:

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function sum(a, b) {\n  return a + b;\n}
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Such functions are called “pure” because they do not attempt to change their inputs, and always return the same result for the same inputs.

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In contrast, this function is impure because it changes its own input:

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function withdraw(account, amount) {\n  account.total -= amount;\n}
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React is pretty flexible but it has a single strict rule:

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All React components must act like pure functions with respect to their props.

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Of course, application UIs are dynamic and change over time. In the next section, we will introduce a new concept of “state”. State allows React components to change their output over time in response to user actions, network responses, and anything else, without violating this rule.

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