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Paul O’Shannessy
232 lines
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HTML
232 lines
15 KiB
HTML
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<title>React | Component Data</title>
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React
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<section class="content wrap documentationContent">
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<h3>Getting started</h3>
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<li><a href="/react/docs/getting-started.html">Getting Started</a></li>
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<h3>Reference</h3>
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<ul>
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<li><a href="/react/docs/syntax.html">JSX Syntax</a></li>
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<li><a href="/react/docs/component-basics.html">Component Basics</a></li>
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<li><a href="/react/docs/component-data.html" class="active">Component Data</a></li>
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<li><a href="/react/docs/component-lifecycle.html">Component Lifecycle</a></li>
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<li><a href="/react/docs/event-handling.html">Event Handling</a></li>
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</ul>
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</div>
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</div>
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<div class="inner-content">
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<h1>Component Data</h1>
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<div class="subHeader">How is data passed into a component?</div>
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<h2>Props</h2>
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<p>Components use data to determine what should be rendered. For example:</p>
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<div class="highlight"><pre><code class="javascript language-javascript" data-lang="javascript"><span class="kd">var</span> <span class="nx">LikeLink</span> <span class="o">=</span> <span class="nx">React</span><span class="p">.</span><span class="nx">createClass</span><span class="p">({</span>
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<span class="nx">render</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
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<span class="kd">var</span> <span class="nx">text</span> <span class="o">=</span> <span class="k">this</span><span class="p">.</span><span class="nx">props</span><span class="p">.</span><span class="nx">liked</span> <span class="o">?</span> <span class="s1">'Liked'</span> <span class="o">:</span> <span class="s1">'Like'</span><span class="p">;</span>
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<span class="k">return</span> <span class="o"><</span><span class="nx">a</span><span class="o">></span><span class="p">{</span><span class="nx">text</span><span class="p">}</span><span class="o"><</span><span class="err">/a>;</span>
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<span class="p">}</span>
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<span class="p">});</span>
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<span class="kd">var</span> <span class="nx">myLikeLink</span> <span class="o">=</span> <span class="o"><</span><span class="nx">LikeLink</span> <span class="nx">liked</span><span class="o">=</span><span class="p">{</span><span class="kc">false</span><span class="p">}</span> <span class="o">/></span><span class="p">;</span>
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</code></pre></div>
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<p>In this example, <code>LikeLink</code> takes <code>liked</code> as boolean data. This type of data
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that is passed in is called a "prop". Examples of props on DOM components
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include <code>className</code> and <code>onClick</code>.</p>
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<p>Whenever a component's props change, its <code>render()</code> function will be
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re-evaluated and the DOM will be updated. React will ensure that the DOM is
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always kept up-to-date.</p>
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<h2>State</h2>
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<p>Let's build a small <code>LikeApp</code> application that makes use of the <code><LikeLink></code>
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component from above. It should start off unliked and we should be able to like
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it by clicking the link:</p>
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<div class="highlight"><pre><code class="javascript language-javascript" data-lang="javascript"><span class="kd">var</span> <span class="nx">LikeApp</span> <span class="o">=</span> <span class="nx">React</span><span class="p">.</span><span class="nx">createClass</span><span class="p">({</span>
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<span class="nx">render</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
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<span class="kd">var</span> <span class="nx">isClicked</span> <span class="o">=</span> <span class="kc">false</span><span class="p">;</span>
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<span class="k">return</span> <span class="o"><</span><span class="nx">LikeLink</span> <span class="nx">liked</span><span class="o">=</span><span class="p">{</span><span class="nx">isClicked</span><span class="p">}</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">handleClick</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">/></span><span class="p">;</span>
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<span class="p">},</span>
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<span class="nx">handleClick</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
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<span class="c1">// Somehow update `isClicked`.</span>
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<span class="p">}</span>
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<span class="p">});</span>
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</code></pre></div>
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<p>This renders a <code><LikeLink></code> with a click listener. However, it is not clear how
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<code>handleClick</code> should update <code>isClicked</code> to true. <code>LikeApp</code> needs a way to store
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<strong>state</strong> about whether or not it has been clicked.</p>
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<h3>State vs. Props</h3>
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<p>State is data that is managed <em>internally</em> by a composite component. Like props,
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the <code>render()</code> function will be re-evaluated whenever state changes. Props and
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state differ in that:</p>
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<ul>
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<li>Props are passed in from the creator.</li>
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<li>State is private to and managed by the component.</li>
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</ul>
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<h3>Managing State</h3>
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<p>Let's update our <code>LikeApp</code> component using state:</p>
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<div class="highlight"><pre><code class="javascript language-javascript" data-lang="javascript"><span class="kd">var</span> <span class="nx">LikeApp</span> <span class="o">=</span> <span class="nx">React</span><span class="p">.</span><span class="nx">createClass</span><span class="p">({</span>
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<span class="hll"> <span class="nx">getInitialState</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
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</span><span class="hll"> <span class="k">return</span> <span class="p">{</span><span class="nx">isClicked</span><span class="o">:</span> <span class="kc">false</span><span class="p">};</span>
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</span><span class="hll"> <span class="p">},</span>
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</span> <span class="nx">render</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
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<span class="hll"> <span class="kd">var</span> <span class="nx">isClicked</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">isClicked</span><span class="p">;</span>
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</span> <span class="k">return</span> <span class="o"><</span><span class="nx">LikeLink</span> <span class="nx">liked</span><span class="o">=</span><span class="p">{</span><span class="nx">isClicked</span><span class="p">}</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">handleClick</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">/></span><span class="p">;</span>
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<span class="p">},</span>
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<span class="nx">handleClick</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
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<span class="hll"> <span class="k">this</span><span class="p">.</span><span class="nx">setState</span><span class="p">({</span><span class="nx">isClicked</span><span class="o">:</span> <span class="kc">true</span><span class="p">});</span>
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</span> <span class="p">}</span>
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<span class="p">});</span>
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</code></pre></div>
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<p>There's a lot going on here, so let's work our way from top to bottom:</p>
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<ul>
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<li><code>getInitialState()</code> describes what state data looks like when the component
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is created.</li>
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<li>In <code>render()</code>, state data can be accessed via <code>this.state</code>.</li>
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<li>When the link is clicked, we update state using <code>setState()</code>.</li>
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</ul>
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<p>Now when we click the link, the <code><LikeLink></code> will get updated, right? Wrong.</p>
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<h2>Transferring Props</h2>
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<p>If you have been following carefully, you may have noticed that although we pass
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a click handler into <code><LikeLink></code> as a prop, <code>LikeLink</code> does not do anything
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with <code>this.props.onClick</code>! Let's fix that.</p>
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<div class="highlight"><pre><code class="javascript language-javascript" data-lang="javascript"><span class="kd">var</span> <span class="nx">LikeLink</span> <span class="o">=</span> <span class="nx">React</span><span class="p">.</span><span class="nx">createClass</span><span class="p">({</span>
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<span class="nx">render</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
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<span class="kd">var</span> <span class="nx">text</span> <span class="o">=</span> <span class="k">this</span><span class="p">.</span><span class="nx">props</span><span class="p">.</span><span class="nx">liked</span> <span class="o">?</span> <span class="s1">'Liked'</span> <span class="o">:</span> <span class="s1">'Like'</span><span class="p">;</span>
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<span class="hll"> <span class="k">return</span> <span class="o"><</span><span class="nx">a</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">props</span><span class="p">.</span><span class="nx">onClick</span><span class="p">}</span><span class="o">></span><span class="p">{</span><span class="nx">text</span><span class="p">}</span><span class="o"><</span><span class="err">/a>;</span>
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</span> <span class="p">}</span>
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<span class="p">});</span>
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</code></pre></div>
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<p>Although this works, realize that this would quickly become tedious if we wanted
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to also transfer <code>href</code>, <code>title</code>, <code>target</code>, and other events from <code>this</code> to the
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rendered <code><a></code>. React provides a convenience method, <code>transferPropsTo()</code>, for
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transferring props:</p>
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<div class="highlight"><pre><code class="javascript language-javascript" data-lang="javascript"><span class="kd">var</span> <span class="nx">LikeLink</span> <span class="o">=</span> <span class="nx">React</span><span class="p">.</span><span class="nx">createClass</span><span class="p">({</span>
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<span class="nx">render</span><span class="o">:</span> <span class="kd">function</span><span class="p">()</span> <span class="p">{</span>
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<span class="kd">var</span> <span class="nx">text</span> <span class="o">=</span> <span class="k">this</span><span class="p">.</span><span class="nx">props</span><span class="p">.</span><span class="nx">liked</span> <span class="o">?</span> <span class="s1">'Liked'</span> <span class="o">:</span> <span class="s1">'Like'</span><span class="p">;</span>
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<span class="hll"> <span class="k">return</span> <span class="k">this</span><span class="p">.</span><span class="nx">transferPropsTo</span><span class="p">(</span><span class="o"><</span><span class="nx">a</span><span class="o">></span><span class="p">{</span><span class="nx">text</span><span class="p">}</span><span class="o"><</span><span class="err">/a>);</span>
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</span> <span class="p">}</span>
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<span class="p">});</span>
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</code></pre></div>
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<p>This will transfer all props from <code>this</code> to the specified component (including
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<code>onClick</code>).</p>
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<h2>Summary</h2>
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<p>Now we are done. <code>LikeApp</code> renders an unliked link which, when clicked, will:</p>
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<ol>
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<li>Update the internal state of <code>LikeApp</code>.</li>
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<li>Change the props passed into <code>LikeLink</code>.</li>
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<li>Change the return value of <code>render()</code>.</li>
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<li>Trigger an update to the DOM.</li>
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</ol>
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<p>It's worth noting that React will handle new return values of <code>render()</code> by
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making the mininal set of mutations necessary to bring the DOM up-to-date. In
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this case, only the <code>textContent</code> of the rendered link will be mutated.</p>
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<p>In summary:</p>
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<ul>
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<li>Props are passed in whereas state is managed internally by a component.</li>
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<li>Never mutate <code>this.props</code> or <code>this.state</code>. You should pass props into other
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components and mutate state using <code>setState()</code>.</li>
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<li>State is private. Never read <code>state</code> or call <code>setState()</code> on
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anything but <code>this</code>.</li>
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<li>Whenever props or state changes, <code>render()</code> will be re-evaluated and the DOM
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updated. Also, <code>render()</code> should not depend on anything besides <code>this.props</code>
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and <code>this.state</code>.</li>
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</ul>
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<div class="docs-prevnext">
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<a class="docs-prev" href="/react/docs/component-basics.html">← Prev</a>
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<a class="docs-next" href="/react/docs/component-lifecycle.html">Next →</a>
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<div class="left">A Facebook & Instagram collaboration.</div>
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