Reusable Components

When designing interfaces, break down the common design elements (buttons, form fields, layout components, etc.) into reusable components with well-defined interfaces. That way, the next time you need to build some UI, you can write much less code. This means faster development time, fewer bugs, and fewer bytes down the wire.

Prop Validation

As your app grows it's helpful to ensure that your components are used correctly. We do this by allowing you to specify propTypes. React.PropTypes exports a range of validators that can be used to make sure the data you receive is valid. When an invalid value is provided for a prop, a warning will be shown in the JavaScript console. Note that for performance reasons propTypes is only checked in development mode. Here is an example documenting the different validators provided:

React.createClass({ propTypes: { // You can declare that a prop is a specific JS primitive. By default, these // are all optional. optionalArray: React.PropTypes.array, optionalBool: React.PropTypes.bool, optionalFunc: React.PropTypes.func, optionalNumber: React.PropTypes.number, optionalObject: React.PropTypes.object, optionalString: React.PropTypes.string,

// Anything that can be rendered: numbers, strings, elements or an array
// (or fragment) containing these types.
optionalNode: React.PropTypes.node,

// A React element.
optionalElement: React.PropTypes.element,

// You can also declare that a prop is an instance of a class. This uses
// JS's instanceof operator.
optionalMessage: React.PropTypes.instanceOf(Message),

// You can ensure that your prop is limited to specific values by treating
// it as an enum.
optionalEnum: React.PropTypes.oneOf(['News', 'Photos']),

// An object that could be one of many types
optionalUnion: React.PropTypes.oneOfType([
  React.PropTypes.string,
  React.PropTypes.number,
  React.PropTypes.instanceOf(Message)
]),

// An array of a certain type
optionalArrayOf: React.PropTypes.arrayOf(React.PropTypes.number),

// An object with property values of a certain type
optionalObjectOf: React.PropTypes.objectOf(React.PropTypes.number),

// An object taking on a particular shape
optionalObjectWithShape: React.PropTypes.shape({
  color: React.PropTypes.string,
  fontSize: React.PropTypes.number
}),

// You can chain any of the above with `isRequired` to make sure a warning
// is shown if the prop isn't provided.
requiredFunc: React.PropTypes.func.isRequired,

// A value of any data type
requiredAny: React.PropTypes.any.isRequired,

// You can also specify a custom validator. It should return an Error
// object if the validation fails. Don't `console.warn` or throw, as this
// won't work inside `oneOfType`.
customProp: function(props, propName, componentName) {
  if (!/matchme/.test(props[propName])) {
    return new Error('Validation failed!');
  }
}

}, / ... / });

Single Child

With React.PropTypes.element you can specify that only a single child can be passed to a component as children.

var MyComponent = React.createClass({ propTypes: { children: React.PropTypes.element.isRequired },

render: function() { return (

{this.props.children} // This must be exactly one element or it will warn.

); }

});

Default Prop Values

React lets you define default values for your props in a very declarative way:

var ComponentWithDefaultProps = React.createClass({ getDefaultProps: function() { return { value: 'default value' }; } / ... / }); The result of getDefaultProps() will be cached and used to ensure that this.props.value will have a value if it was not specified by the parent component. This allows you to safely just use your props without having to write repetitive and fragile code to handle that yourself.

Transferring Props: A Shortcut

A common type of React component is one that extends a basic HTML element in a simple way. Often you'll want to copy any HTML attributes passed to your component to the underlying HTML element. To save typing, you can use the JSX spread syntax to achieve this:

var CheckLink = React.createClass({ render: function() { // This takes any props passed to CheckLink and copies them to return {'√ '}{this.props.children}; } });

ReactDOM.render( Click here! , document.getElementById('example') );

Mixins

Components are the best way to reuse code in React, but sometimes very different components may share some common functionality. These are sometimes called cross-cutting concerns. React provides mixins to solve this problem.

One common use case is a component wanting to update itself on a time interval. It's easy to use setInterval(), but it's important to cancel your interval when you don't need it anymore to save memory. React provides lifecycle methods that let you know when a component is about to be created or destroyed. Let's create a simple mixin that uses these methods to provide an easy setInterval() function that will automatically get cleaned up when your component is destroyed.

React has been running for {this.state.seconds} seconds.

); } });

ReactDOM.render( , document.getElementById('example') ); A nice feature of mixins is that if a component is using multiple mixins and several mixins define the same lifecycle method (i.e. several mixins want to do some cleanup when the component is destroyed), all of the lifecycle methods are guaranteed to be called. Methods defined on mixins run in the order mixins were listed, followed by a method call on the component.

ES6 Classes

You may also define your React classes as a plain JavaScript class. For example using ES6 class syntax:

class HelloMessage extends React.Component { render() { return

Hello {this.props.name}

; } } ReactDOM.render(, mountNode); The API is similar to React.createClass with the exception of getInitialState. Instead of providing a separate getInitialState method, you set up your own state property in the constructor.

Another difference is that propTypes and defaultProps are defined as properties on the constructor instead of in the class body.

export class Counter extends React.Component { constructor(props) { super(props); this.state = {count: props.initialCount}; } tick() { this.setState({count: this.state.count + 1}); } render() { return (

Clicks: {this.state.count}

); } } Counter.propTypes = { initialCount: React.PropTypes.number }; Counter.defaultProps = { initialCount: 0 };

No Autobinding

Methods follow the same semantics as regular ES6 classes, meaning that they don't automatically bind this to the instance. You'll have to explicitly use .bind(this) or arrow functions =>.

No Mixins

Unfortunately ES6 launched without any mixin support. Therefore, there is no support for mixins when you use React with ES6 classes. Instead, we're working on making it easier to support such use cases without resorting to mixins.

Stateless Functions

You may also define your React classes as a plain JavaScript function. For example using the stateless function syntax:

function HelloMessage(props) { return

Hello {props.name}

; } ReactDOM.render(, mountNode); Or using the new ES6 arrow syntax:

var HelloMessage = (props) =>

Hello {props.name}

; ReactDOM.render(, mountNode); This simplified component API is intended for components that are pure functions of their props. These components must not retain internal state, do not have backing instances, and do not have the component lifecycle methods. They are pure functional transforms of their input, with zero boilerplate.

NOTE: Because stateless functions don't have a backing instance, you can't attach a ref to a stateless function component. Normally this isn't an issue, since stateless functions do not provide an imperative API. Without an imperative API, there isn't much you could do with an instance anyway. However, if a user wants to find the DOM node of a stateless function component, they must wrap the component in a stateful component (eg. ES6 class component) and attach the ref to the stateful wrapper component. In an ideal world, most of your components would be stateless functions because these stateless components can follow a faster code path within the React core. This is the recommended pattern, when possible.