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<h1>
Reusable Components
</h1>
<div class="subHeader"></div>
<p>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.</p>
<h2><a class="anchor" name="prop-validation"></a>Prop Validation <a class="hash-link" href="#prop-validation">#</a></h2>
<p>As your app grows it&#39;s helpful to ensure that your components are used correctly. We do this by allowing you to specify <code>propTypes</code>. <code>React.PropTypes</code> 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 <code>propTypes</code> is only checked in development mode. Here is an example documenting the different validators provided:</p>
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="nx">React</span><span class="p">.</span><span class="nx">createClass</span><span class="p">({</span>
<span class="nx">propTypes</span><span class="o">:</span> <span class="p">{</span>
<span class="c1">// You can declare that a prop is a specific JS primitive. By default, these</span>
<span class="c1">// are all optional.</span>
<span class="nx">optionalArray</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">array</span><span class="p">,</span>
<span class="nx">optionalBool</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">bool</span><span class="p">,</span>
<span class="nx">optionalFunc</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">func</span><span class="p">,</span>
<span class="nx">optionalNumber</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">number</span><span class="p">,</span>
<span class="nx">optionalObject</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">object</span><span class="p">,</span>
<span class="nx">optionalString</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">string</span><span class="p">,</span>
<span class="nx">optionalSymbol</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">symbol</span><span class="p">,</span>
<span class="c1">// Anything that can be rendered: numbers, strings, elements or an array</span>
<span class="c1">// (or fragment) containing these types.</span>
<span class="nx">optionalNode</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">node</span><span class="p">,</span>
<span class="c1">// A React element.</span>
<span class="nx">optionalElement</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">element</span><span class="p">,</span>
<span class="c1">// You can also declare that a prop is an instance of a class. This uses</span>
<span class="c1">// JS&#39;s instanceof operator.</span>
<span class="nx">optionalMessage</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">instanceOf</span><span class="p">(</span><span class="nx">Message</span><span class="p">),</span>
<span class="c1">// You can ensure that your prop is limited to specific values by treating</span>
<span class="c1">// it as an enum.</span>
<span class="nx">optionalEnum</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">oneOf</span><span class="p">([</span><span class="s1">&#39;News&#39;</span><span class="p">,</span> <span class="s1">&#39;Photos&#39;</span><span class="p">]),</span>
<span class="c1">// An object that could be one of many types</span>
<span class="nx">optionalUnion</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">oneOfType</span><span class="p">([</span>
<span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">string</span><span class="p">,</span>
<span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">number</span><span class="p">,</span>
<span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">instanceOf</span><span class="p">(</span><span class="nx">Message</span><span class="p">)</span>
<span class="p">]),</span>
<span class="c1">// An array of a certain type</span>
<span class="nx">optionalArrayOf</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">arrayOf</span><span class="p">(</span><span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">number</span><span class="p">),</span>
<span class="c1">// An object with property values of a certain type</span>
<span class="nx">optionalObjectOf</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">objectOf</span><span class="p">(</span><span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">number</span><span class="p">),</span>
<span class="c1">// An object taking on a particular shape</span>
<span class="nx">optionalObjectWithShape</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">shape</span><span class="p">({</span>
<span class="nx">color</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">string</span><span class="p">,</span>
<span class="nx">fontSize</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">number</span>
<span class="p">}),</span>
<span class="c1">// You can chain any of the above with `isRequired` to make sure a warning</span>
<span class="c1">// is shown if the prop isn&#39;t provided.</span>
<span class="nx">requiredFunc</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">func</span><span class="p">.</span><span class="nx">isRequired</span><span class="p">,</span>
<span class="c1">// A value of any data type</span>
<span class="nx">requiredAny</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">any</span><span class="p">.</span><span class="nx">isRequired</span><span class="p">,</span>
<span class="c1">// You can also specify a custom validator. It should return an Error</span>
<span class="c1">// object if the validation fails. Don&#39;t `console.warn` or throw, as this</span>
<span class="c1">// won&#39;t work inside `oneOfType`.</span>
<span class="nx">customProp</span><span class="o">:</span> <span class="kd">function</span><span class="p">(</span><span class="nx">props</span><span class="p">,</span> <span class="nx">propName</span><span class="p">,</span> <span class="nx">componentName</span><span class="p">)</span> <span class="p">{</span>
<span class="k">if</span> <span class="p">(</span><span class="o">!</span><span class="sr">/matchme/</span><span class="p">.</span><span class="nx">test</span><span class="p">(</span><span class="nx">props</span><span class="p">[</span><span class="nx">propName</span><span class="p">]))</span> <span class="p">{</span>
<span class="k">return</span> <span class="k">new</span> <span class="nb">Error</span><span class="p">(</span>
<span class="s1">&#39;Invalid prop `&#39;</span> <span class="o">+</span> <span class="nx">propName</span> <span class="o">+</span> <span class="s1">&#39;` supplied to&#39;</span> <span class="o">+</span>
<span class="s1">&#39; `&#39;</span> <span class="o">+</span> <span class="nx">componentName</span> <span class="o">+</span> <span class="s1">&#39;`. Validation failed.&#39;</span>
<span class="p">);</span>
<span class="p">}</span>
<span class="p">},</span>
<span class="c1">// You can also supply a custom validator to `arrayOf` and `objectOf`.</span>
<span class="c1">// It should return an Error object if the validation fails. The validator</span>
<span class="c1">// will be called for each key in the array or object. The first two</span>
<span class="c1">// arguments of the validator are the array or object itself, and the</span>
<span class="c1">// current item&#39;s key.</span>
<span class="nx">customArrayProp</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">arrayOf</span><span class="p">(</span><span class="kd">function</span><span class="p">(</span><span class="nx">propValue</span><span class="p">,</span> <span class="nx">key</span><span class="p">,</span> <span class="nx">componentName</span><span class="p">,</span> <span class="nx">location</span><span class="p">,</span> <span class="nx">propFullName</span><span class="p">)</span> <span class="p">{</span>
<span class="k">if</span> <span class="p">(</span><span class="o">!</span><span class="sr">/matchme/</span><span class="p">.</span><span class="nx">test</span><span class="p">(</span><span class="nx">propValue</span><span class="p">[</span><span class="nx">key</span><span class="p">]))</span> <span class="p">{</span>
<span class="k">return</span> <span class="k">new</span> <span class="nb">Error</span><span class="p">(</span>
<span class="s1">&#39;Invalid prop `&#39;</span> <span class="o">+</span> <span class="nx">propFullName</span> <span class="o">+</span> <span class="s1">&#39;` supplied to&#39;</span> <span class="o">+</span>
<span class="s1">&#39; `&#39;</span> <span class="o">+</span> <span class="nx">componentName</span> <span class="o">+</span> <span class="s1">&#39;`. Validation failed.&#39;</span>
<span class="p">);</span>
<span class="p">}</span>
<span class="p">})</span>
<span class="p">},</span>
<span class="cm">/* ... */</span>
<span class="p">});</span>
</code></pre></div><h3><a class="anchor" name="single-child"></a>Single Child <a class="hash-link" href="#single-child">#</a></h3>
<p>With <code>React.PropTypes.element</code> you can specify that only a single child can be passed to a component as children.</p>
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kd">var</span> <span class="nx">MyComponent</span> <span class="o">=</span> <span class="nx">React</span><span class="p">.</span><span class="nx">createClass</span><span class="p">({</span>
<span class="nx">propTypes</span><span class="o">:</span> <span class="p">{</span>
<span class="nx">children</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">element</span><span class="p">.</span><span class="nx">isRequired</span>
<span class="p">},</span>
<span class="nx">render</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="k">return</span> <span class="p">(</span>
<span class="o">&lt;</span><span class="nx">div</span><span class="o">&gt;</span>
<span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">props</span><span class="p">.</span><span class="nx">children</span><span class="p">}</span> <span class="c1">// This must be exactly one element or it will warn.</span>
<span class="o">&lt;</span><span class="err">/div&gt;</span>
<span class="p">);</span>
<span class="p">}</span>
<span class="p">});</span>
</code></pre></div><h2><a class="anchor" name="default-prop-values"></a>Default Prop Values <a class="hash-link" href="#default-prop-values">#</a></h2>
<p>React lets you define default values for your <code>props</code> in a very declarative way:</p>
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kd">var</span> <span class="nx">ComponentWithDefaultProps</span> <span class="o">=</span> <span class="nx">React</span><span class="p">.</span><span class="nx">createClass</span><span class="p">({</span>
<span class="nx">getDefaultProps</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="k">return</span> <span class="p">{</span>
<span class="nx">value</span><span class="o">:</span> <span class="s1">&#39;default value&#39;</span>
<span class="p">};</span>
<span class="p">}</span>
<span class="cm">/* ... */</span>
<span class="p">});</span>
</code></pre></div>
<p>The result of <code>getDefaultProps()</code> will be cached and used to ensure that <code>this.props.value</code> 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.</p>
<h2><a class="anchor" name="transferring-props-a-shortcut"></a>Transferring Props: A Shortcut <a class="hash-link" href="#transferring-props-a-shortcut">#</a></h2>
<p>A common type of React component is one that extends a basic HTML element in a simple way. Often you&#39;ll want to copy any HTML attributes passed to your component to the underlying HTML element. To save typing, you can use the JSX <em>spread</em> syntax to achieve this:</p>
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kd">var</span> <span class="nx">CheckLink</span> <span class="o">=</span> <span class="nx">React</span><span class="p">.</span><span class="nx">createClass</span><span class="p">({</span>
<span class="nx">render</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="c1">// This takes any props passed to CheckLink and copies them to &lt;a&gt;</span>
<span class="k">return</span> <span class="o">&lt;</span><span class="nx">a</span> <span class="p">{...</span><span class="k">this</span><span class="p">.</span><span class="nx">props</span><span class="p">}</span><span class="o">&gt;</span><span class="p">{</span><span class="s1">&#39;√ &#39;</span><span class="p">}{</span><span class="k">this</span><span class="p">.</span><span class="nx">props</span><span class="p">.</span><span class="nx">children</span><span class="p">}</span><span class="o">&lt;</span><span class="err">/a&gt;;</span>
<span class="p">}</span>
<span class="p">});</span>
<span class="nx">ReactDOM</span><span class="p">.</span><span class="nx">render</span><span class="p">(</span>
<span class="o">&lt;</span><span class="nx">CheckLink</span> <span class="nx">href</span><span class="o">=</span><span class="s2">&quot;/checked.html&quot;</span><span class="o">&gt;</span>
<span class="nx">Click</span> <span class="nx">here</span><span class="o">!</span>
<span class="o">&lt;</span><span class="err">/CheckLink&gt;,</span>
<span class="nb">document</span><span class="p">.</span><span class="nx">getElementById</span><span class="p">(</span><span class="s1">&#39;example&#39;</span><span class="p">)</span>
<span class="p">);</span>
</code></pre></div><h2><a class="anchor" name="mixins"></a>Mixins <a class="hash-link" href="#mixins">#</a></h2>
<p>Components are the best way to reuse code in React, but sometimes very different components may share some common functionality. These are sometimes called <a href="https://en.wikipedia.org/wiki/Cross-cutting_concern">cross-cutting concerns</a>. React provides <code>mixins</code> to solve this problem.</p>
<p>One common use case is a component wanting to update itself on a time interval. It&#39;s easy to use <code>setInterval()</code>, but it&#39;s important to cancel your interval when you don&#39;t need it anymore to save memory. React provides <a href="/react/docs/working-with-the-browser.html#component-lifecycle">lifecycle methods</a> that let you know when a component is about to be created or destroyed. Let&#39;s create a simple mixin that uses these methods to provide an easy <code>setInterval()</code> function that will automatically get cleaned up when your component is destroyed.</p>
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kd">var</span> <span class="nx">SetIntervalMixin</span> <span class="o">=</span> <span class="p">{</span>
<span class="nx">componentWillMount</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="k">this</span><span class="p">.</span><span class="nx">intervals</span> <span class="o">=</span> <span class="p">[];</span>
<span class="p">},</span>
<span class="nx">setInterval</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="k">this</span><span class="p">.</span><span class="nx">intervals</span><span class="p">.</span><span class="nx">push</span><span class="p">(</span><span class="nx">setInterval</span><span class="p">.</span><span class="nx">apply</span><span class="p">(</span><span class="kc">null</span><span class="p">,</span> <span class="nx">arguments</span><span class="p">));</span>
<span class="p">},</span>
<span class="nx">componentWillUnmount</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="k">this</span><span class="p">.</span><span class="nx">intervals</span><span class="p">.</span><span class="nx">forEach</span><span class="p">(</span><span class="nx">clearInterval</span><span class="p">);</span>
<span class="p">}</span>
<span class="p">};</span>
<span class="kd">var</span> <span class="nx">TickTock</span> <span class="o">=</span> <span class="nx">React</span><span class="p">.</span><span class="nx">createClass</span><span class="p">({</span>
<span class="nx">mixins</span><span class="o">:</span> <span class="p">[</span><span class="nx">SetIntervalMixin</span><span class="p">],</span> <span class="c1">// Use the mixin</span>
<span class="nx">getInitialState</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="k">return</span> <span class="p">{</span><span class="nx">seconds</span><span class="o">:</span> <span class="mi">0</span><span class="p">};</span>
<span class="p">},</span>
<span class="nx">componentDidMount</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="k">this</span><span class="p">.</span><span class="nx">setInterval</span><span class="p">(</span><span class="k">this</span><span class="p">.</span><span class="nx">tick</span><span class="p">,</span> <span class="mi">1000</span><span class="p">);</span> <span class="c1">// Call a method on the mixin</span>
<span class="p">},</span>
<span class="nx">tick</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="k">this</span><span class="p">.</span><span class="nx">setState</span><span class="p">({</span><span class="nx">seconds</span><span class="o">:</span> <span class="k">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">seconds</span> <span class="o">+</span> <span class="mi">1</span><span class="p">});</span>
<span class="p">},</span>
<span class="nx">render</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
<span class="k">return</span> <span class="p">(</span>
<span class="o">&lt;</span><span class="nx">p</span><span class="o">&gt;</span>
<span class="nx">React</span> <span class="nx">has</span> <span class="nx">been</span> <span class="nx">running</span> <span class="k">for</span> <span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">seconds</span><span class="p">}</span> <span class="nx">seconds</span><span class="p">.</span>
<span class="o">&lt;</span><span class="err">/p&gt;</span>
<span class="p">);</span>
<span class="p">}</span>
<span class="p">});</span>
<span class="nx">ReactDOM</span><span class="p">.</span><span class="nx">render</span><span class="p">(</span>
<span class="o">&lt;</span><span class="nx">TickTock</span> <span class="o">/&gt;</span><span class="p">,</span>
<span class="nb">document</span><span class="p">.</span><span class="nx">getElementById</span><span class="p">(</span><span class="s1">&#39;example&#39;</span><span class="p">)</span>
<span class="p">);</span>
</code></pre></div>
<p>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.</p>
<h2><a class="anchor" name="es6-classes"></a>ES6 Classes <a class="hash-link" href="#es6-classes">#</a></h2>
<p>You may also define your React classes as a plain JavaScript class. For example using ES6 class syntax:</p>
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kr">class</span> <span class="nx">HelloMessage</span> <span class="kr">extends</span> <span class="nx">React</span><span class="p">.</span><span class="nx">Component</span> <span class="p">{</span>
<span class="nx">render</span><span class="p">()</span> <span class="p">{</span>
<span class="k">return</span> <span class="o">&lt;</span><span class="nx">div</span><span class="o">&gt;</span><span class="nx">Hello</span> <span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">props</span><span class="p">.</span><span class="nx">name</span><span class="p">}</span><span class="o">&lt;</span><span class="err">/div&gt;;</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="nx">ReactDOM</span><span class="p">.</span><span class="nx">render</span><span class="p">(</span><span class="o">&lt;</span><span class="nx">HelloMessage</span> <span class="nx">name</span><span class="o">=</span><span class="s2">&quot;Sebastian&quot;</span> <span class="o">/&gt;</span><span class="p">,</span> <span class="nx">mountNode</span><span class="p">);</span>
</code></pre></div>
<p>The API is similar to <code>React.createClass</code> with the exception of <code>getInitialState</code>. Instead of providing a separate <code>getInitialState</code> method, you set up your own <code>state</code> property in the constructor. Just like the return value of <code>getInitialState</code>, the value you assign to <code>this.state</code> will be used as the initial state for your component.</p>
<p>Another difference is that <code>propTypes</code> and <code>defaultProps</code> are defined as properties on the constructor instead of in the class body.</p>
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kr">export</span> <span class="kr">class</span> <span class="nx">Counter</span> <span class="kr">extends</span> <span class="nx">React</span><span class="p">.</span><span class="nx">Component</span> <span class="p">{</span>
<span class="nx">constructor</span><span class="p">(</span><span class="nx">props</span><span class="p">)</span> <span class="p">{</span>
<span class="kr">super</span><span class="p">(</span><span class="nx">props</span><span class="p">);</span>
<span class="k">this</span><span class="p">.</span><span class="nx">state</span> <span class="o">=</span> <span class="p">{</span><span class="nx">count</span><span class="o">:</span> <span class="nx">props</span><span class="p">.</span><span class="nx">initialCount</span><span class="p">};</span>
<span class="k">this</span><span class="p">.</span><span class="nx">tick</span> <span class="o">=</span> <span class="k">this</span><span class="p">.</span><span class="nx">tick</span><span class="p">.</span><span class="nx">bind</span><span class="p">(</span><span class="k">this</span><span class="p">);</span>
<span class="p">}</span>
<span class="nx">tick</span><span class="p">()</span> <span class="p">{</span>
<span class="k">this</span><span class="p">.</span><span class="nx">setState</span><span class="p">({</span><span class="nx">count</span><span class="o">:</span> <span class="k">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">count</span> <span class="o">+</span> <span class="mi">1</span><span class="p">});</span>
<span class="p">}</span>
<span class="nx">render</span><span class="p">()</span> <span class="p">{</span>
<span class="k">return</span> <span class="p">(</span>
<span class="o">&lt;</span><span class="nx">div</span> <span class="nx">onClick</span><span class="o">=</span><span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">tick</span><span class="p">}</span><span class="o">&gt;</span>
<span class="nx">Clicks</span><span class="o">:</span> <span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">count</span><span class="p">}</span>
<span class="o">&lt;</span><span class="err">/div&gt;</span>
<span class="p">);</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="nx">Counter</span><span class="p">.</span><span class="nx">propTypes</span> <span class="o">=</span> <span class="p">{</span> <span class="nx">initialCount</span><span class="o">:</span> <span class="nx">React</span><span class="p">.</span><span class="nx">PropTypes</span><span class="p">.</span><span class="nx">number</span> <span class="p">};</span>
<span class="nx">Counter</span><span class="p">.</span><span class="nx">defaultProps</span> <span class="o">=</span> <span class="p">{</span> <span class="nx">initialCount</span><span class="o">:</span> <span class="mi">0</span> <span class="p">};</span>
</code></pre></div><h3><a class="anchor" name="no-autobinding"></a>No Autobinding <a class="hash-link" href="#no-autobinding">#</a></h3>
<p>Methods follow the same semantics as regular ES6 classes, meaning that they don&#39;t automatically bind <code>this</code> to the instance. You&#39;ll have to explicitly use <code>.bind(this)</code> or <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions">arrow functions</a> <code>=&gt;</code>:</p>
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="c1">// You can use bind() to preserve `this`</span>
<span class="o">&lt;</span><span class="nx">div</span> <span class="nx">onClick</span><span class="o">=</span><span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">tick</span><span class="p">.</span><span class="nx">bind</span><span class="p">(</span><span class="k">this</span><span class="p">)}</span><span class="o">&gt;</span>
<span class="c1">// Or you can use arrow functions</span>
<span class="o">&lt;</span><span class="nx">div</span> <span class="nx">onClick</span><span class="o">=</span><span class="p">{()</span> <span class="o">=&gt;</span> <span class="k">this</span><span class="p">.</span><span class="nx">tick</span><span class="p">()}</span><span class="o">&gt;</span>
</code></pre></div>
<p>We recommend that you bind your event handlers in the constructor so they are only bound once for every instance:</p>
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="nx">constructor</span><span class="p">(</span><span class="nx">props</span><span class="p">)</span> <span class="p">{</span>
<span class="kr">super</span><span class="p">(</span><span class="nx">props</span><span class="p">);</span>
<span class="k">this</span><span class="p">.</span><span class="nx">state</span> <span class="o">=</span> <span class="p">{</span><span class="nx">count</span><span class="o">:</span> <span class="nx">props</span><span class="p">.</span><span class="nx">initialCount</span><span class="p">};</span>
<span class="k">this</span><span class="p">.</span><span class="nx">tick</span> <span class="o">=</span> <span class="k">this</span><span class="p">.</span><span class="nx">tick</span><span class="p">.</span><span class="nx">bind</span><span class="p">(</span><span class="k">this</span><span class="p">);</span>
<span class="p">}</span>
</code></pre></div>
<p>Now you can use <code>this.tick</code> directly as it was bound once in the constructor:</p>
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="c1">// It is already bound in the constructor</span>
<span class="o">&lt;</span><span class="nx">div</span> <span class="nx">onClick</span><span class="o">=</span><span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">tick</span><span class="p">}</span><span class="o">&gt;</span>
</code></pre></div>
<p>This is better for performance of your application, especially if you implement <a href="/react/docs/component-specs.html#updating-shouldcomponentupdate">shouldComponentUpdate()</a> with a <a href="/react/docs/shallow-compare.html">shallow comparison</a> in the child components.</p>
<h3><a class="anchor" name="no-mixins"></a>No Mixins <a class="hash-link" href="#no-mixins">#</a></h3>
<p>Unfortunately ES6 launched without any mixin support. Therefore, there is no support for mixins when you use React with ES6 classes. Instead, we&#39;re working on making it easier to support such use cases without resorting to mixins.</p>
<h2><a class="anchor" name="stateless-functions"></a>Stateless Functions <a class="hash-link" href="#stateless-functions">#</a></h2>
<p>You may also define your React classes as a plain JavaScript function. For example using the stateless function syntax:</p>
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kd">function</span> <span class="nx">HelloMessage</span><span class="p">(</span><span class="nx">props</span><span class="p">)</span> <span class="p">{</span>
<span class="k">return</span> <span class="o">&lt;</span><span class="nx">div</span><span class="o">&gt;</span><span class="nx">Hello</span> <span class="p">{</span><span class="nx">props</span><span class="p">.</span><span class="nx">name</span><span class="p">}</span><span class="o">&lt;</span><span class="err">/div&gt;;</span>
<span class="p">}</span>
<span class="nx">ReactDOM</span><span class="p">.</span><span class="nx">render</span><span class="p">(</span><span class="o">&lt;</span><span class="nx">HelloMessage</span> <span class="nx">name</span><span class="o">=</span><span class="s2">&quot;Sebastian&quot;</span> <span class="o">/&gt;</span><span class="p">,</span> <span class="nx">mountNode</span><span class="p">);</span>
</code></pre></div>
<p>Or using the new ES6 arrow syntax:</p>
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kr">const</span> <span class="nx">HelloMessage</span> <span class="o">=</span> <span class="p">(</span><span class="nx">props</span><span class="p">)</span> <span class="o">=&gt;</span> <span class="o">&lt;</span><span class="nx">div</span><span class="o">&gt;</span><span class="nx">Hello</span> <span class="p">{</span><span class="nx">props</span><span class="p">.</span><span class="nx">name</span><span class="p">}</span><span class="o">&lt;</span><span class="err">/div&gt;;</span>
<span class="nx">ReactDOM</span><span class="p">.</span><span class="nx">render</span><span class="p">(</span><span class="o">&lt;</span><span class="nx">HelloMessage</span> <span class="nx">name</span><span class="o">=</span><span class="s2">&quot;Sebastian&quot;</span> <span class="o">/&gt;</span><span class="p">,</span> <span class="nx">mountNode</span><span class="p">);</span>
</code></pre></div>
<p>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.
However, you may still specify <code>.propTypes</code> and <code>.defaultProps</code> by setting them as properties on the function, just as you would set them on an ES6 class.</p>
<blockquote>
<p>NOTE:</p>
<p>Because stateless functions don&#39;t have a backing instance, you can&#39;t attach a ref to a stateless function component. Normally this isn&#39;t an issue, since stateless functions do not provide an imperative API. Without an imperative API, there isn&#39;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.</p>
<p>NOTE:</p>
<p>In React v0.14, stateless functional components were not permitted to return <code>null</code> or <code>false</code> (a workaround is to return a <code>&lt;noscript /&gt;</code> instead). This was fixed in React v15, and stateless functional components are now permitted to return <code>null</code>.</p>
</blockquote>
<p>In an ideal world, most of your components would be stateless functions because in the future we’ll also be able to make performance optimizations specific to these components by avoiding unnecessary checks and memory allocations. This is the recommended pattern, when possible.</p>
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