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Dan Abramov
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800 lines
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<title>State and Lifecycle - React</title>
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<a class="edit-page-link" href="https://github.com/facebook/react/tree/master/docs/docs/state-and-lifecycle.md" target="_blank">Edit on GitHub</a>
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<h1>
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State and Lifecycle
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</h1>
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<div class="subHeader"></div>
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<p>Consider the ticking clock example from <a href="/react/docs/rendering-elements.html#updating-the-rendered-element">one of the previous sections</a>.</p>
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<p>So far we have only learned one way to update the UI.</p>
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<p>We call <code>ReactDOM.render()</code> to change the rendered output:</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>In this section, we will learn how to make the <code>Clock</code> component truly reusable and encapsulated. It will set up its own timer and update itself every second.</p>
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<p>We can start by encapsulating how the clock looks:</p>
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<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kd">function</span> <span class="nx">Clock</span><span class="p">(</span><span class="nx">props</span><span class="p">)</span> <span class="p">{</span>
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<span class="k">return</span> <span class="p">(</span>
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<span class="hll"> <span class="o"><</span><span class="nx">div</span><span class="o">></span>
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</span><span class="hll"> <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><span class="hll"> <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="nx">props</span><span class="p">.</span><span class="nx">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><span class="hll"> <span class="o"><</span><span class="err">/div></span>
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</span> <span class="p">);</span>
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<span class="p">}</span>
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<span class="kd">function</span> <span class="nx">tick</span><span class="p">()</span> <span class="p">{</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="hll"> <span class="o"><</span><span class="nx">Clock</span> <span class="nx">date</span><span class="o">=</span><span class="p">{</span><span class="k">new</span> <span class="nb">Date</span><span class="p">()}</span> <span class="o">/></span><span class="p">,</span>
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</span> <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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<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/dpdoYR?editors=0010">Try it on CodePen.</a></p>
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<p>However, it misses a crucial requirement: the fact that the <code>Clock</code> sets up a timer and updates the UI every second should be an implementation detail of the <code>Clock</code>.</p>
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<p>Ideally we want to write this once and have the <code>Clock</code> update itself:</p>
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<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="nx">ReactDOM</span><span class="p">.</span><span class="nx">render</span><span class="p">(</span>
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<span class="hll"> <span class="o"><</span><span class="nx">Clock</span> <span class="o">/></span><span class="p">,</span>
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</span> <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>To implement this, we need to add "state" to the <code>Clock</code> component.</p>
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<p>State is similar to props, but it is private and fully controlled by the component.</p>
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<p>We <a href="/react/docs/components-and-props.html#functional-and-class-components">mentioned before</a> that components defined as classes have some additional features. Local state is exactly that: a feature available only to classes.</p>
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<h2>Converting a Function to a Class</h2>
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<p>You can convert a functional component like <code>Clock</code> to a class in five steps:</p>
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<ol>
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<li><p>Create an <a href="https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Classes">ES6 class</a> with the same name that extends <code>React.Component</code>.</p></li>
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<li><p>Add a single empty method to it called <code>render()</code>.</p></li>
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<li><p>Move the body of the function into the <code>render()</code> method.</p></li>
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<li><p>Replace <code>props</code> with <code>this.props</code> in the <code>render()</code> body.</p></li>
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<li><p>Delete the remaining empty function declaration.</p></li>
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</ol>
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<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kr">class</span> <span class="nx">Clock</span> <span class="kr">extends</span> <span class="nx">React</span><span class="p">.</span><span class="nx">Component</span> <span class="p">{</span>
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<span class="nx">render</span><span class="p">()</span> <span class="p">{</span>
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<span class="k">return</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">this</span><span class="p">.</span><span class="nx">props</span><span class="p">.</span><span class="nx">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="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/zKRGpo?editors=0010">Try it on CodePen.</a></p>
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<p><code>Clock</code> is now defined as a class rather than a function.</p>
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<p>This lets us use additional features such as local state and lifecycle hooks.</p>
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<h2>Adding Local State to a Class</h2>
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<p>We will move the <code>date</code> from props to state in three steps:</p>
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<p>1) Replace <code>this.props.date</code> with <code>this.state.date</code> in the <code>render()</code> method:</p>
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<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kr">class</span> <span class="nx">Clock</span> <span class="kr">extends</span> <span class="nx">React</span><span class="p">.</span><span class="nx">Component</span> <span class="p">{</span>
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<span class="nx">render</span><span class="p">()</span> <span class="p">{</span>
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<span class="k">return</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="hll"> <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">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">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> <span class="o"><</span><span class="err">/div></span>
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<span class="p">);</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>2) Add a <a href="https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Classes#Constructor">class constructor</a> that assigns the initial <code>this.state</code>:</p>
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<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kr">class</span> <span class="nx">Clock</span> <span class="kr">extends</span> <span class="nx">React</span><span class="p">.</span><span class="nx">Component</span> <span class="p">{</span>
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<span class="nx">constructor</span><span class="p">(</span><span class="nx">props</span><span class="p">)</span> <span class="p">{</span>
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<span class="kr">super</span><span class="p">(</span><span class="nx">props</span><span class="p">);</span>
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<span class="hll"> <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">date</span><span class="o">:</span> <span class="k">new</span> <span class="nb">Date</span><span class="p">()};</span>
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</span> <span class="p">}</span>
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<span class="nx">render</span><span class="p">()</span> <span class="p">{</span>
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<span class="k">return</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>
|
|
<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">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">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="p">}</span>
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<span class="p">}</span>
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</code></pre></div>
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<p>Note how we pass <code>props</code> to the base constructor:</p>
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<div class="highlight"><pre><code class="language-js" data-lang="js"> <span class="nx">constructor</span><span class="p">(</span><span class="nx">props</span><span class="p">)</span> <span class="p">{</span>
|
|
<span class="hll"> <span class="kr">super</span><span class="p">(</span><span class="nx">props</span><span class="p">);</span>
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</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">date</span><span class="o">:</span> <span class="k">new</span> <span class="nb">Date</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>Class components should always call the base constructor with <code>props</code>.</p>
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<p>3) Remove the <code>date</code> prop from the <code><Clock /></code> element:</p>
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<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="nx">ReactDOM</span><span class="p">.</span><span class="nx">render</span><span class="p">(</span>
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<span class="hll"> <span class="o"><</span><span class="nx">Clock</span> <span class="o">/></span><span class="p">,</span>
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</span> <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>We will later add the timer code back to the component itself.</p>
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<p>The result looks like this:</p>
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<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kr">class</span> <span class="nx">Clock</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="hll"> <span class="nx">constructor</span><span class="p">(</span><span class="nx">props</span><span class="p">)</span> <span class="p">{</span>
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</span><span class="hll"> <span class="kr">super</span><span class="p">(</span><span class="nx">props</span><span class="p">);</span>
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</span><span class="hll"> <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">date</span><span class="o">:</span> <span class="k">new</span> <span class="nb">Date</span><span class="p">()};</span>
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</span><span class="hll"> <span class="p">}</span>
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</span>
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<span class="nx">render</span><span class="p">()</span> <span class="p">{</span>
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<span class="k">return</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="hll"> <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">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">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> <span class="o"><</span><span class="err">/div></span>
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<span class="p">);</span>
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<span class="p">}</span>
|
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<span class="p">}</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="hll"> <span class="o"><</span><span class="nx">Clock</span> <span class="o">/></span><span class="p">,</span>
|
|
</span> <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>
|
|
<span class="p">);</span>
|
|
</code></pre></div>
|
|
<p><a href="http://codepen.io/gaearon/pen/KgQpJd?editors=0010">Try it on CodePen.</a></p>
|
|
|
|
<p>Next, we'll make the <code>Clock</code> set up its own timer and update itself every second.</p>
|
|
|
|
<h2>Adding Lifecycle Methods to a Class</h2>
|
|
|
|
<p>In applications with many components, it's very important to free up resources taken by the components when they are destroyed.</p>
|
|
|
|
<p>We want to <a href="https://developer.mozilla.org/en-US/docs/Web/API/WindowTimers/setInterval">set up a timer</a> whenever the <code>Clock</code> is rendered to the DOM for the first time. This is called "mounting" in React.</p>
|
|
|
|
<p>We also want to <a href="https://developer.mozilla.org/en-US/docs/Web/API/WindowTimers/clearInterval">clear that timer</a> whenever the DOM produced by the <code>Clock</code> is removed. This is called "unmounting" in React.</p>
|
|
|
|
<p>We can declare special methods on the component class to run some code when a component mounts and unmounts:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kr">class</span> <span class="nx">Clock</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">date</span><span class="o">:</span> <span class="k">new</span> <span class="nb">Date</span><span class="p">()};</span>
|
|
<span class="p">}</span>
|
|
|
|
<span class="hll"> <span class="nx">componentDidMount</span><span class="p">()</span> <span class="p">{</span>
|
|
</span><span class="hll">
|
|
</span><span class="hll"> <span class="p">}</span>
|
|
</span>
|
|
<span class="hll"> <span class="nx">componentWillUnmount</span><span class="p">()</span> <span class="p">{</span>
|
|
</span><span class="hll">
|
|
</span><span class="hll"> <span class="p">}</span>
|
|
</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"><</span><span class="nx">div</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>
|
|
<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">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">date</span><span class="p">.</span><span class="nx">toLocaleTimeString</span><span class="p">()}.</span><span class="o"><</span><span class="err">/h2></span>
|
|
<span class="o"><</span><span class="err">/div></span>
|
|
<span class="p">);</span>
|
|
<span class="p">}</span>
|
|
<span class="p">}</span>
|
|
</code></pre></div>
|
|
<p>These methods are called "lifecycle hooks".</p>
|
|
|
|
<p>The <code>componentDidMount()</code> hook runs after the component output has been rendered to the DOM. This is a good place to set up a timer:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"> <span class="nx">componentDidMount</span><span class="p">()</span> <span class="p">{</span>
|
|
<span class="hll"> <span class="k">this</span><span class="p">.</span><span class="nx">timerID</span> <span class="o">=</span> <span class="nx">setInterval</span><span class="p">(</span>
|
|
</span><span class="hll"> <span class="p">()</span> <span class="o">=></span> <span class="k">this</span><span class="p">.</span><span class="nx">tick</span><span class="p">(),</span>
|
|
</span><span class="hll"> <span class="mi">1000</span>
|
|
</span><span class="hll"> <span class="p">);</span>
|
|
</span> <span class="p">}</span>
|
|
</code></pre></div>
|
|
<p>Note how we save the timer ID right on <code>this</code>.</p>
|
|
|
|
<p>While <code>this.props</code> is set up by React itself and <code>this.state</code> has a special meaning, you are free to add additional fields to the class manually if you need to store something that is not used for the visual output.</p>
|
|
|
|
<p>If you don't use something in <code>render()</code>, it shouldn't be in the state.</p>
|
|
|
|
<p>We will tear down the timer in the <code>componentWillUnmount()</code> lifecycle hook:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"> <span class="nx">componentWillUnmount</span><span class="p">()</span> <span class="p">{</span>
|
|
<span class="hll"> <span class="nx">clearInterval</span><span class="p">(</span><span class="k">this</span><span class="p">.</span><span class="nx">timerID</span><span class="p">);</span>
|
|
</span> <span class="p">}</span>
|
|
</code></pre></div>
|
|
<p>Finally, we will implement the <code>tick()</code> method that runs every second.</p>
|
|
|
|
<p>It will use <code>this.setState()</code> to schedule updates to the component local state:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kr">class</span> <span class="nx">Clock</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">date</span><span class="o">:</span> <span class="k">new</span> <span class="nb">Date</span><span class="p">()};</span>
|
|
<span class="p">}</span>
|
|
|
|
<span class="nx">componentDidMount</span><span class="p">()</span> <span class="p">{</span>
|
|
<span class="k">this</span><span class="p">.</span><span class="nx">timerID</span> <span class="o">=</span> <span class="nx">setInterval</span><span class="p">(</span>
|
|
<span class="p">()</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="mi">1000</span>
|
|
<span class="p">);</span>
|
|
<span class="p">}</span>
|
|
|
|
<span class="nx">componentWillUnmount</span><span class="p">()</span> <span class="p">{</span>
|
|
<span class="nx">clearInterval</span><span class="p">(</span><span class="k">this</span><span class="p">.</span><span class="nx">timerID</span><span class="p">);</span>
|
|
<span class="p">}</span>
|
|
|
|
<span class="hll"> <span class="nx">tick</span><span class="p">()</span> <span class="p">{</span>
|
|
</span><span class="hll"> <span class="k">this</span><span class="p">.</span><span class="nx">setState</span><span class="p">({</span>
|
|
</span><span class="hll"> <span class="nx">date</span><span class="o">:</span> <span class="k">new</span> <span class="nb">Date</span><span class="p">()</span>
|
|
</span><span class="hll"> <span class="p">});</span>
|
|
</span><span class="hll"> <span class="p">}</span>
|
|
</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"><</span><span class="nx">div</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>
|
|
<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">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">date</span><span class="p">.</span><span class="nx">toLocaleTimeString</span><span class="p">()}.</span><span class="o"><</span><span class="err">/h2></span>
|
|
<span class="o"><</span><span class="err">/div></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"><</span><span class="nx">Clock</span> <span class="o">/></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">'root'</span><span class="p">)</span>
|
|
<span class="p">);</span>
|
|
</code></pre></div>
|
|
<p><a href="http://codepen.io/gaearon/pen/amqdNA?editors=0010">Try it on CodePen.</a></p>
|
|
|
|
<p>Now the clock ticks every second.</p>
|
|
|
|
<p>Let's quickly recap what's going on and the order in which the methods are called:</p>
|
|
|
|
<p>1) When <code><Clock /></code> is passed to <code>ReactDOM.render()</code>, React calls the constructor of the <code>Clock</code> component. Since <code>Clock</code> needs to display the current time, it initializes <code>this.state</code> with an object including the current time. We will later update this state.</p>
|
|
|
|
<p>2) React then calls the <code>Clock</code> component's <code>render()</code> method. This is how React learns what should be displayed on the screen. React then updates the DOM to match the <code>Clock</code>'s render output.</p>
|
|
|
|
<p>3) When the <code>Clock</code> output is inserted in the DOM, React calls the <code>componentDidMount()</code> lifecycle hook. Inside it, the <code>Clock</code> component asks the browser to set up a timer to call <code>tick()</code> once a second.</p>
|
|
|
|
<p>4) Every second the browser calls the <code>tick()</code> method. Inside it, the <code>Clock</code> component schedules a UI update by calling <code>setState()</code> with an object containing the current time. Thanks to the <code>setState()</code> call, React knows the state has changed, and calls <code>render()</code> method again to learn what should be on the screen. This time, <code>this.state.date</code> in the <code>render()</code> method will be different, and so the render output will include the updated time. React updates the DOM accordingly.</p>
|
|
|
|
<p>5) If the <code>Clock</code> component is ever removed from the DOM, React calls the <code>componentWillUnmount()</code> lifecycle hook so the timer is stopped.</p>
|
|
|
|
<h2>Using State Correctly</h2>
|
|
|
|
<p>There are three things you should know about <code>setState()</code>.</p>
|
|
|
|
<h3>Do Not Modify State Directly</h3>
|
|
|
|
<p>For example, this will not re-render a component:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="c1">// Wrong</span>
|
|
<span class="k">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">comment</span> <span class="o">=</span> <span class="s1">'Hello'</span><span class="p">;</span>
|
|
</code></pre></div>
|
|
<p>Instead, use <code>setState()</code>:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="c1">// Correct</span>
|
|
<span class="k">this</span><span class="p">.</span><span class="nx">setState</span><span class="p">({</span><span class="nx">comment</span><span class="o">:</span> <span class="s1">'Hello'</span><span class="p">});</span>
|
|
</code></pre></div>
|
|
<p>The only place where you can assign <code>this.state</code> is the constructor.</p>
|
|
|
|
<h3>State Updates May Be Asynchronous</h3>
|
|
|
|
<p>React may batch multiple <code>setState()</code> calls into a single update for performance.</p>
|
|
|
|
<p>Because <code>this.props</code> and <code>this.state</code> may be updated asynchronously, you should not rely on their values for calculating the next state.</p>
|
|
|
|
<p>For example, this code may fail to update the counter:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="c1">// Wrong</span>
|
|
<span class="k">this</span><span class="p">.</span><span class="nx">setState</span><span class="p">({</span>
|
|
<span class="nx">counter</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">counter</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">increment</span><span class="p">,</span>
|
|
<span class="p">});</span>
|
|
</code></pre></div>
|
|
<p>To fix it, use a second form of <code>setState()</code> that accepts a function rather than an object. That function will receive the previous state as the first argument, and the props at the time the update is applied as the second argument:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="c1">// Correct</span>
|
|
<span class="k">this</span><span class="p">.</span><span class="nx">setState</span><span class="p">((</span><span class="nx">prevState</span><span class="p">,</span> <span class="nx">props</span><span class="p">)</span> <span class="o">=></span> <span class="p">({</span>
|
|
<span class="nx">counter</span><span class="o">:</span> <span class="nx">prevState</span><span class="p">.</span><span class="nx">counter</span> <span class="o">+</span> <span class="nx">props</span><span class="p">.</span><span class="nx">increment</span>
|
|
<span class="p">}));</span>
|
|
</code></pre></div>
|
|
<p>We used an <a href="https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Functions/Arrow_functions">arrow function</a> above, but it also works with regular functions:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="c1">// Correct</span>
|
|
<span class="k">this</span><span class="p">.</span><span class="nx">setState</span><span class="p">(</span><span class="kd">function</span><span class="p">(</span><span class="nx">prevState</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="p">{</span>
|
|
<span class="nx">counter</span><span class="o">:</span> <span class="nx">prevState</span><span class="p">.</span><span class="nx">counter</span> <span class="o">+</span> <span class="nx">props</span><span class="p">.</span><span class="nx">increment</span>
|
|
<span class="p">};</span>
|
|
<span class="p">});</span>
|
|
</code></pre></div>
|
|
<h3>State Updates are Merged</h3>
|
|
|
|
<p>When you call <code>setState()</code>, React merges the object you provide into the current state.</p>
|
|
|
|
<p>For example, your state may contain several independent variables:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"> <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="hll"> <span class="nx">posts</span><span class="o">:</span> <span class="p">[],</span>
|
|
</span><span class="hll"> <span class="nx">comments</span><span class="o">:</span> <span class="p">[]</span>
|
|
</span> <span class="p">};</span>
|
|
<span class="p">}</span>
|
|
</code></pre></div>
|
|
<p>Then you can update them independently with separate <code>setState()</code> calls:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"> <span class="nx">componentDidMount</span><span class="p">()</span> <span class="p">{</span>
|
|
<span class="nx">fetchPosts</span><span class="p">().</span><span class="nx">then</span><span class="p">(</span><span class="nx">response</span> <span class="o">=></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="hll"> <span class="nx">posts</span><span class="o">:</span> <span class="nx">response</span><span class="p">.</span><span class="nx">posts</span>
|
|
</span> <span class="p">});</span>
|
|
<span class="p">});</span>
|
|
|
|
<span class="nx">fetchComments</span><span class="p">().</span><span class="nx">then</span><span class="p">(</span><span class="nx">response</span> <span class="o">=></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="hll"> <span class="nx">comments</span><span class="o">:</span> <span class="nx">response</span><span class="p">.</span><span class="nx">comments</span>
|
|
</span> <span class="p">});</span>
|
|
<span class="p">});</span>
|
|
<span class="p">}</span>
|
|
</code></pre></div>
|
|
<p>The merging is shallow, so <code>this.setState({comments})</code> leaves <code>this.state.posts</code> intact, but completely replaces <code>this.state.comments</code>.</p>
|
|
|
|
<h2>The Data Flows Down</h2>
|
|
|
|
<p>Neither parent nor child components can know if a certain component is stateful or stateless, and they shouldn't care whether it is defined as a function or a class.</p>
|
|
|
|
<p>This is why state is often called local or encapsulated. It is not accessible to any component other than the one that owns and sets it.</p>
|
|
|
|
<p>A component may choose to pass its state down as props to its child components:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"><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">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">date</span><span class="p">.</span><span class="nx">toLocaleTimeString</span><span class="p">()}.</span><span class="o"><</span><span class="err">/h2></span>
|
|
</code></pre></div>
|
|
<p>This also works for user-defined components:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="o"><</span><span class="nx">FormattedDate</span> <span class="nx">date</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">date</span><span class="p">}</span> <span class="o">/></span>
|
|
</code></pre></div>
|
|
<p>The <code>FormattedDate</code> component would receive the <code>date</code> in its props and wouldn't know whether it came from the <code>Clock</code>'s state, from the <code>Clock</code>'s props, or was typed by hand:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kd">function</span> <span class="nx">FormattedDate</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"><</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="nx">props</span><span class="p">.</span><span class="nx">date</span><span class="p">.</span><span class="nx">toLocaleTimeString</span><span class="p">()}.</span><span class="o"><</span><span class="err">/h2>;</span>
|
|
<span class="p">}</span>
|
|
</code></pre></div>
|
|
<p><a href="http://codepen.io/gaearon/pen/zKRqNB?editors=0010">Try it on CodePen.</a></p>
|
|
|
|
<p>This is commonly called a "top-down" or "unidirectional" data flow. Any state is always owned by some specific component, and any data or UI derived from that state can only affect components "below" them in the tree.</p>
|
|
|
|
<p>If you imagine a component tree as a waterfall of props, each component's state is like an additional water source that joins it at an arbitrary point but also flows down.</p>
|
|
|
|
<p>To show that all components are truly isolated, we can create an <code>App</code> component that renders three <code><Clock></code>s:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kd">function</span> <span class="nx">App</span><span class="p">()</span> <span class="p">{</span>
|
|
<span class="k">return</span> <span class="p">(</span>
|
|
<span class="o"><</span><span class="nx">div</span><span class="o">></span>
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<span class="hll"> <span class="o"><</span><span class="nx">Clock</span> <span class="o">/></span>
|
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</span><span class="hll"> <span class="o"><</span><span class="nx">Clock</span> <span class="o">/></span>
|
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</span><span class="hll"> <span class="o"><</span><span class="nx">Clock</span> <span class="o">/></span>
|
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</span> <span class="o"><</span><span class="err">/div></span>
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<span class="p">);</span>
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|
<span class="p">}</span>
|
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|
<span class="nx">ReactDOM</span><span class="p">.</span><span class="nx">render</span><span class="p">(</span>
|
|
<span class="o"><</span><span class="nx">App</span> <span class="o">/></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">'root'</span><span class="p">)</span>
|
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<span class="p">);</span>
|
|
</code></pre></div>
|
|
<p><a href="http://codepen.io/gaearon/pen/vXdGmd?editors=0010">Try it on CodePen.</a></p>
|
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<p>Each <code>Clock</code> sets up its own timer and updates independently.</p>
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<p>In React apps, whether a component is stateful or stateless is considered an implementation detail of the component that may change over time. You can use stateless components inside stateful components, and vice versa.</p>
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