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<meta charset="utf-8">
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<title>Rendering Elements - React</title>
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<a class="edit-page-link" href="https://github.com/facebook/react/tree/master/docs/docs/rendering-elements.md" target="_blank">Edit on GitHub</a>
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<h1>
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Rendering Elements
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</h1>
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<div class="subHeader"></div>
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<p>Elements are the smallest building blocks of React apps.</p>
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<p>An element describes what you want to see on the screen:</p>
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<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kr">const</span> <span class="nx">element</span> <span class="o">=</span> <span class="o"><</span><span class="nx">h1</span><span class="o">></span><span class="nx">Hello</span><span class="p">,</span> <span class="nx">world</span><span class="o"><</span><span class="err">/h1>;</span>
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</code></pre></div>
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<p>Unlike browser DOM elements, React elements are plain objects, and are cheap to create. React DOM takes care of updating the DOM to match the React elements.</p>
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<blockquote>
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<p><strong>Note:</strong></p>
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<p>One might confuse elements with a more widely known concept of "components". We will introduce components in the <a href="/react/docs/components-and-props.html">next section</a>. Elements are what components are "made of", and we encourage you to read this section before jumping ahead.</p>
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</blockquote>
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<h2>Rendering an Element into the DOM</h2>
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<p>Let's say there is a <code><div></code> somewhere in your HTML file:</p>
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<div class="highlight"><pre><code class="language-html" data-lang="html"><span class="nt"><div</span> <span class="na">id=</span><span class="s">"root"</span><span class="nt">></div></span>
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</code></pre></div>
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<p>We call this a "root" DOM node because everything inside it will be managed by React DOM.</p>
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<p>Applications built with just React usually have a single root DOM node. If you are integrating React into an existing app, you may have as many isolated root DOM nodes as you like.</p>
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<p>To render a React element into a root DOM node, pass both to <code>ReactDOM.render()</code>:</p>
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<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kr">const</span> <span class="nx">element</span> <span class="o">=</span> <span class="o"><</span><span class="nx">h1</span><span class="o">></span><span class="nx">Hello</span><span class="p">,</span> <span class="nx">world</span><span class="o"><</span><span class="err">/h1>;</span>
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<span class="nx">ReactDOM</span><span class="p">.</span><span class="nx">render</span><span class="p">(</span>
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<span class="nx">element</span><span class="p">,</span>
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<span class="nb">document</span><span class="p">.</span><span class="nx">getElementById</span><span class="p">(</span><span class="s1">'root'</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><a href="http://codepen.io/gaearon/pen/rrpgNB?editors=1010">Try it on CodePen.</a></p>
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<p>It displays "Hello World" on the page.</p>
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<h2>Updating the Rendered Element</h2>
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<p>React elements are <a href="https://en.wikipedia.org/wiki/Immutable_object">immutable</a>. Once you create an element, you can't change its children or attributes. An element is like a single frame in a movie: it represents the UI at a certain point in time.</p>
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<p>With our knowledge so far, the only way to update the UI is to create a new element, and pass it to <code>ReactDOM.render()</code>.</p>
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<p>Consider this ticking clock example:</p>
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<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kd">function</span> <span class="nx">tick</span><span class="p">()</span> <span class="p">{</span>
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<span class="kr">const</span> <span class="nx">element</span> <span class="o">=</span> <span class="p">(</span>
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<span class="o"><</span><span class="nx">div</span><span class="o">></span>
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<span class="o"><</span><span class="nx">h1</span><span class="o">></span><span class="nx">Hello</span><span class="p">,</span> <span class="nx">world</span><span class="o">!<</span><span class="err">/h1></span>
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<span class="o"><</span><span class="nx">h2</span><span class="o">></span><span class="nx">It</span> <span class="nx">is</span> <span class="p">{</span><span class="k">new</span> <span class="nb">Date</span><span class="p">().</span><span class="nx">toLocaleTimeString</span><span class="p">()}.</span><span class="o"><</span><span class="err">/h2></span>
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<span class="o"><</span><span class="err">/div></span>
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<span class="p">);</span>
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<span class="hll"> <span class="nx">ReactDOM</span><span class="p">.</span><span class="nx">render</span><span class="p">(</span>
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</span><span class="hll"> <span class="nx">element</span><span class="p">,</span>
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</span><span class="hll"> <span class="nb">document</span><span class="p">.</span><span class="nx">getElementById</span><span class="p">(</span><span class="s1">'root'</span><span class="p">)</span>
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</span><span class="hll"> <span class="p">);</span>
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</span><span class="p">}</span>
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<span class="nx">setInterval</span><span class="p">(</span><span class="nx">tick</span><span class="p">,</span> <span class="mi">1000</span><span class="p">);</span>
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</code></pre></div>
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<p><a href="http://codepen.io/gaearon/pen/gwoJZk?editors=0010">Try it on CodePen.</a></p>
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<p>It calls <code>ReactDOM.render()</code> every second from a <a href="https://developer.mozilla.org/en-US/docs/Web/API/WindowTimers/setInterval"><code>setInterval()</code></a> callback.</p>
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<blockquote>
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<p><strong>Note:</strong></p>
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<p>In practice, most React apps only call <code>ReactDOM.render()</code> once. In the next sections we will learn how such code gets encapsulated into <a href="/react/docs/state-and-lifecycle.html">stateful components</a>.</p>
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<p>We recommend that you don't skip topics because they build on each other.</p>
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</blockquote>
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<h2>React Only Updates What's Necessary</h2>
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<p>React DOM compares the element and its children to the previous one, and only applies the DOM updates necessary to bring the DOM to the desired state.</p>
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<p>You can verify by inspecting the <a href="http://codepen.io/gaearon/pen/gwoJZk?editors=0010">last example</a> with the browser tools:</p>
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<p><img src="/react/img/docs/granular-dom-updates.gif" alt="DOM inspector showing granular updates"></p>
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<p>Even though we create an element describing the whole UI tree on every tick, only the text node whose contents has changed gets updated by React DOM.</p>
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<p>In our experience, thinking about how the UI should look at any given moment rather than how to change it over time eliminates a whole class of bugs.</p>
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