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Dan Abramov
660 lines
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HTML
660 lines
44 KiB
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<a class="edit-page-link" href="https://github.com/facebook/react/tree/master/docs/docs/lists-and-keys.md" target="_blank">Edit on GitHub</a>
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<h1>
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Lists and Keys
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</h1>
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<div class="subHeader"></div>
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<p>First, let's review how you transform lists in JavaScript.</p>
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<p>Given the code below, we use the <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map"><code>map()</code></a> function to take an array of <code>numbers</code> and double their values. We assign the new array returned by <code>map()</code> to the variable <code>doubled</code> and log it:</p>
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<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kr">const</span> <span class="nx">numbers</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">];</span>
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<span class="hll"><span class="kr">const</span> <span class="nx">doubled</span> <span class="o">=</span> <span class="nx">numbers</span><span class="p">.</span><span class="nx">map</span><span class="p">((</span><span class="nx">number</span><span class="p">)</span> <span class="o">=></span> <span class="nx">number</span> <span class="o">*</span> <span class="mi">2</span><span class="p">);</span>
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</span><span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="nx">doubled</span><span class="p">);</span>
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</code></pre></div>
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<p>This code logs <code>[2, 4, 6, 8, 10]</code> to the console.</p>
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<p>In React, transforming arrays into lists of <a href="/react/docs/rendering-elements.html">elements</a> is nearly identical.</p>
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<h3>Rendering Multiple Components</h3>
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<p>You can build collections of elements and <a href="/react/docs/introducing-jsx.html#embedding-expressions-in-jsx">include them in JSX</a> using curly braces <code>{}</code>.</p>
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<p>Below, we loop through the <code>numbers</code> array using the JavaScript <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map"><code>map()</code></a> function. We return an <code><li></code> element for each item. Finally, we assign the resulting array of elements to <code>listItems</code>:</p>
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<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kr">const</span> <span class="nx">numbers</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">];</span>
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<span class="hll"><span class="kr">const</span> <span class="nx">listItems</span> <span class="o">=</span> <span class="nx">numbers</span><span class="p">.</span><span class="nx">map</span><span class="p">((</span><span class="nx">number</span><span class="p">)</span> <span class="o">=></span>
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</span><span class="hll"> <span class="o"><</span><span class="nx">li</span><span class="o">></span><span class="p">{</span><span class="nx">number</span><span class="p">}</span><span class="o"><</span><span class="err">/li></span>
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</span><span class="hll"><span class="p">);</span>
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</span></code></pre></div>
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<p>We include the entire <code>listItems</code> array inside a <code><ul></code> element, and <a href="/react/docs/rendering-elements.html#rendering-an-element-into-the-dom">render it to the DOM</a>:</p>
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<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><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">ul</span><span class="o">></span><span class="p">{</span><span class="nx">listItems</span><span class="p">}</span><span class="o"><</span><span class="err">/ul>,</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><a href="https://codepen.io/gaearon/pen/GjPyQr?editors=0011">Try it on CodePen.</a></p>
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<p>This code displays a bullet list of numbers between 1 and 5.</p>
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<h3>Basic List Component</h3>
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<p>Usually you would render lists inside a <a href="/react/docs/components-and-props.html">component</a>.</p>
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<p>We can refactor the previous example into a component that accepts an array of <code>numbers</code> and outputs an unordered list of elements.</p>
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<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kd">function</span> <span class="nx">NumberList</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">const</span> <span class="nx">numbers</span> <span class="o">=</span> <span class="nx">props</span><span class="p">.</span><span class="nx">numbers</span><span class="p">;</span>
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<span class="hll"> <span class="kr">const</span> <span class="nx">listItems</span> <span class="o">=</span> <span class="nx">numbers</span><span class="p">.</span><span class="nx">map</span><span class="p">((</span><span class="nx">number</span><span class="p">)</span> <span class="o">=></span>
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</span><span class="hll"> <span class="o"><</span><span class="nx">li</span><span class="o">></span><span class="p">{</span><span class="nx">number</span><span class="p">}</span><span class="o"><</span><span class="err">/li></span>
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</span><span class="hll"> <span class="p">);</span>
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</span> <span class="k">return</span> <span class="p">(</span>
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<span class="hll"> <span class="o"><</span><span class="nx">ul</span><span class="o">></span><span class="p">{</span><span class="nx">listItems</span><span class="p">}</span><span class="o"><</span><span class="err">/ul></span>
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</span> <span class="p">);</span>
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<span class="p">}</span>
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<span class="kr">const</span> <span class="nx">numbers</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</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">NumberList</span> <span class="nx">numbers</span><span class="o">=</span><span class="p">{</span><span class="nx">numbers</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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</code></pre></div>
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<p>When you run this code, you'll be given a warning that a key should be provided for list items. A "key" is a special string attribute you need to include when creating lists of elements. We'll discuss why it's important in the next section.</p>
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<p>Let's assign a <code>key</code> to our list items inside <code>numbers.map()</code> and fix the missing key issue.</p>
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<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kd">function</span> <span class="nx">NumberList</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">const</span> <span class="nx">numbers</span> <span class="o">=</span> <span class="nx">props</span><span class="p">.</span><span class="nx">numbers</span><span class="p">;</span>
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<span class="kr">const</span> <span class="nx">listItems</span> <span class="o">=</span> <span class="nx">numbers</span><span class="p">.</span><span class="nx">map</span><span class="p">((</span><span class="nx">number</span><span class="p">)</span> <span class="o">=></span>
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<span class="hll"> <span class="o"><</span><span class="nx">li</span> <span class="nx">key</span><span class="o">=</span><span class="p">{</span><span class="nx">number</span><span class="p">.</span><span class="nx">toString</span><span class="p">()}</span><span class="o">></span>
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</span> <span class="p">{</span><span class="nx">number</span><span class="p">}</span>
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<span class="o"><</span><span class="err">/li></span>
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<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">ul</span><span class="o">></span><span class="p">{</span><span class="nx">listItems</span><span class="p">}</span><span class="o"><</span><span class="err">/ul></span>
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<span class="p">);</span>
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<span class="p">}</span>
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<span class="kr">const</span> <span class="nx">numbers</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</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="o"><</span><span class="nx">NumberList</span> <span class="nx">numbers</span><span class="o">=</span><span class="p">{</span><span class="nx">numbers</span><span class="p">}</span> <span class="o">/></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="https://codepen.io/gaearon/pen/jrXYRR?editors=0011">Try it on CodePen.</a></p>
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<h2>Keys</h2>
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<p>Keys help React identify which items have changed, are added, or are removed. Keys should be given to the elements inside the array to give the elements a stable identity:</p>
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<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kr">const</span> <span class="nx">numbers</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">];</span>
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<span class="kr">const</span> <span class="nx">listItems</span> <span class="o">=</span> <span class="nx">numbers</span><span class="p">.</span><span class="nx">map</span><span class="p">((</span><span class="nx">number</span><span class="p">)</span> <span class="o">=></span>
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<span class="hll"> <span class="o"><</span><span class="nx">li</span> <span class="nx">key</span><span class="o">=</span><span class="p">{</span><span class="nx">number</span><span class="p">.</span><span class="nx">toString</span><span class="p">()}</span><span class="o">></span>
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</span> <span class="p">{</span><span class="nx">number</span><span class="p">}</span>
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<span class="o"><</span><span class="err">/li></span>
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<span class="p">);</span>
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</code></pre></div>
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<p>The best way to pick a key is to use a string that uniquely identifies a list item among its siblings. Most often you would use IDs from your data as keys:</p>
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<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kr">const</span> <span class="nx">todoItems</span> <span class="o">=</span> <span class="nx">todos</span><span class="p">.</span><span class="nx">map</span><span class="p">((</span><span class="nx">todo</span><span class="p">)</span> <span class="o">=></span>
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<span class="hll"> <span class="o"><</span><span class="nx">li</span> <span class="nx">key</span><span class="o">=</span><span class="p">{</span><span class="nx">todo</span><span class="p">.</span><span class="nx">id</span><span class="p">}</span><span class="o">></span>
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</span> <span class="p">{</span><span class="nx">todo</span><span class="p">.</span><span class="nx">text</span><span class="p">}</span>
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<span class="o"><</span><span class="err">/li></span>
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<span class="p">);</span>
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</code></pre></div>
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<p>When you don't have stable IDs for rendered items, you may use the item index as a key as a last resort:</p>
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<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kr">const</span> <span class="nx">todoItems</span> <span class="o">=</span> <span class="nx">todos</span><span class="p">.</span><span class="nx">map</span><span class="p">((</span><span class="nx">todo</span><span class="p">,</span> <span class="nx">index</span><span class="p">)</span> <span class="o">=></span>
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<span class="hll"> <span class="c1">// Only do this if items have no stable IDs</span>
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</span><span class="hll"> <span class="o"><</span><span class="nx">li</span> <span class="nx">key</span><span class="o">=</span><span class="p">{</span><span class="nx">index</span><span class="p">}</span><span class="o">></span>
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</span> <span class="p">{</span><span class="nx">todo</span><span class="p">.</span><span class="nx">text</span><span class="p">}</span>
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<span class="o"><</span><span class="err">/li></span>
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<span class="p">);</span>
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</code></pre></div>
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<p>We don't recommend using indexes for keys if the items can reorder, as that would be slow. You may read an <a href="/react/docs/reconciliation.html#recursing-on-children">in-depth explanation about why keys are necessary</a> if you're interested.</p>
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<h3>Extracting Components with Keys</h3>
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<p>Keys only make sense in the context of the surrounding array.</p>
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<p>For example, if you <a href="/react/docs/components-and-props.html#extracting-components">extract</a> a <code>ListItem</code> component, you should keep the key on the <code><ListItem /></code> elements in the array rather than on the root <code><li></code> element in the <code>ListItem</code> itself.</p>
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<p><strong>Example: Incorrect Key Usage</strong></p>
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<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kd">function</span> <span class="nx">ListItem</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">const</span> <span class="nx">value</span> <span class="o">=</span> <span class="nx">props</span><span class="p">.</span><span class="nx">value</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="c1">// Wrong! There is no need to specify the key here:</span>
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</span><span class="hll"> <span class="o"><</span><span class="nx">li</span> <span class="nx">key</span><span class="o">=</span><span class="p">{</span><span class="nx">value</span><span class="p">.</span><span class="nx">toString</span><span class="p">()}</span><span class="o">></span>
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</span> <span class="p">{</span><span class="nx">value</span><span class="p">}</span>
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<span class="o"><</span><span class="err">/li></span>
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<span class="p">);</span>
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<span class="p">}</span>
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<span class="kd">function</span> <span class="nx">NumberList</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">const</span> <span class="nx">numbers</span> <span class="o">=</span> <span class="nx">props</span><span class="p">.</span><span class="nx">numbers</span><span class="p">;</span>
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<span class="kr">const</span> <span class="nx">listItems</span> <span class="o">=</span> <span class="nx">numbers</span><span class="p">.</span><span class="nx">map</span><span class="p">((</span><span class="nx">number</span><span class="p">)</span> <span class="o">=></span>
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<span class="hll"> <span class="c1">// Wrong! The key should have been specified here:</span>
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</span><span class="hll"> <span class="o"><</span><span class="nx">ListItem</span> <span class="nx">value</span><span class="o">=</span><span class="p">{</span><span class="nx">number</span><span class="p">}</span> <span class="o">/></span>
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</span> <span class="p">);</span>
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<span class="k">return</span> <span class="p">(</span>
|
|
<span class="o"><</span><span class="nx">ul</span><span class="o">></span>
|
|
<span class="p">{</span><span class="nx">listItems</span><span class="p">}</span>
|
|
<span class="o"><</span><span class="err">/ul></span>
|
|
<span class="p">);</span>
|
|
<span class="p">}</span>
|
|
|
|
<span class="kr">const</span> <span class="nx">numbers</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</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">NumberList</span> <span class="nx">numbers</span><span class="o">=</span><span class="p">{</span><span class="nx">numbers</span><span class="p">}</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><strong>Example: Correct Key Usage</strong></p>
|
|
<div class="highlight"><pre><code class="language-javascript" data-lang="javascript"><span class="kd">function</span> <span class="nx">ListItem</span><span class="p">(</span><span class="nx">props</span><span class="p">)</span> <span class="p">{</span>
|
|
<span class="hll"> <span class="c1">// Correct! There is no need to specify the key here:</span>
|
|
</span><span class="hll"> <span class="k">return</span> <span class="o"><</span><span class="nx">li</span><span class="o">></span><span class="p">{</span><span class="nx">props</span><span class="p">.</span><span class="nx">value</span><span class="p">}</span><span class="o"><</span><span class="err">/li>;</span>
|
|
</span><span class="p">}</span>
|
|
|
|
<span class="kd">function</span> <span class="nx">NumberList</span><span class="p">(</span><span class="nx">props</span><span class="p">)</span> <span class="p">{</span>
|
|
<span class="kr">const</span> <span class="nx">numbers</span> <span class="o">=</span> <span class="nx">props</span><span class="p">.</span><span class="nx">numbers</span><span class="p">;</span>
|
|
<span class="kr">const</span> <span class="nx">listItems</span> <span class="o">=</span> <span class="nx">numbers</span><span class="p">.</span><span class="nx">map</span><span class="p">((</span><span class="nx">number</span><span class="p">)</span> <span class="o">=></span>
|
|
<span class="hll"> <span class="c1">// Correct! Key should be specified inside the array.</span>
|
|
</span><span class="hll"> <span class="o"><</span><span class="nx">ListItem</span> <span class="nx">key</span><span class="o">=</span><span class="p">{</span><span class="nx">number</span><span class="p">.</span><span class="nx">toString</span><span class="p">()}</span>
|
|
</span> <span class="nx">value</span><span class="o">=</span><span class="p">{</span><span class="nx">number</span><span class="p">}</span> <span class="o">/></span>
|
|
<span class="p">);</span>
|
|
<span class="k">return</span> <span class="p">(</span>
|
|
<span class="o"><</span><span class="nx">ul</span><span class="o">></span>
|
|
<span class="p">{</span><span class="nx">listItems</span><span class="p">}</span>
|
|
<span class="o"><</span><span class="err">/ul></span>
|
|
<span class="p">);</span>
|
|
<span class="p">}</span>
|
|
|
|
<span class="kr">const</span> <span class="nx">numbers</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</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">NumberList</span> <span class="nx">numbers</span><span class="o">=</span><span class="p">{</span><span class="nx">numbers</span><span class="p">}</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="https://codepen.io/gaearon/pen/ZXeOGM?editors=0010">Try it on CodePen.</a></p>
|
|
|
|
<p>A good rule of thumb is that elements inside the <code>map()</code> call need keys.</p>
|
|
|
|
<h3>Keys Must Only Be Unique Among Siblings</h3>
|
|
|
|
<p>Keys used within arrays should be unique among their siblings. However they don't need to be globally unique. We can use the same keys when we produce two different arrays:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kd">function</span> <span class="nx">Blog</span><span class="p">(</span><span class="nx">props</span><span class="p">)</span> <span class="p">{</span>
|
|
<span class="hll"> <span class="kr">const</span> <span class="nx">sidebar</span> <span class="o">=</span> <span class="p">(</span>
|
|
</span> <span class="o"><</span><span class="nx">ul</span><span class="o">></span>
|
|
<span class="p">{</span><span class="nx">props</span><span class="p">.</span><span class="nx">posts</span><span class="p">.</span><span class="nx">map</span><span class="p">((</span><span class="nx">post</span><span class="p">)</span> <span class="o">=></span>
|
|
<span class="hll"> <span class="o"><</span><span class="nx">li</span> <span class="nx">key</span><span class="o">=</span><span class="p">{</span><span class="nx">post</span><span class="p">.</span><span class="nx">id</span><span class="p">}</span><span class="o">></span>
|
|
</span> <span class="p">{</span><span class="nx">post</span><span class="p">.</span><span class="nx">title</span><span class="p">}</span>
|
|
<span class="o"><</span><span class="err">/li></span>
|
|
<span class="p">)}</span>
|
|
<span class="o"><</span><span class="err">/ul></span>
|
|
<span class="p">);</span>
|
|
<span class="hll"> <span class="kr">const</span> <span class="nx">content</span> <span class="o">=</span> <span class="nx">props</span><span class="p">.</span><span class="nx">posts</span><span class="p">.</span><span class="nx">map</span><span class="p">((</span><span class="nx">post</span><span class="p">)</span> <span class="o">=></span>
|
|
</span><span class="hll"> <span class="o"><</span><span class="nx">div</span> <span class="nx">key</span><span class="o">=</span><span class="p">{</span><span class="nx">post</span><span class="p">.</span><span class="nx">id</span><span class="p">}</span><span class="o">></span>
|
|
</span> <span class="o"><</span><span class="nx">h3</span><span class="o">></span><span class="p">{</span><span class="nx">post</span><span class="p">.</span><span class="nx">title</span><span class="p">}</span><span class="o"><</span><span class="err">/h3></span>
|
|
<span class="o"><</span><span class="nx">p</span><span class="o">></span><span class="p">{</span><span class="nx">post</span><span class="p">.</span><span class="nx">content</span><span class="p">}</span><span class="o"><</span><span class="err">/p></span>
|
|
<span class="o"><</span><span class="err">/div></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="hll"> <span class="p">{</span><span class="nx">sidebar</span><span class="p">}</span>
|
|
</span> <span class="o"><</span><span class="nx">hr</span> <span class="o">/></span>
|
|
<span class="hll"> <span class="p">{</span><span class="nx">content</span><span class="p">}</span>
|
|
</span> <span class="o"><</span><span class="err">/div></span>
|
|
<span class="p">);</span>
|
|
<span class="p">}</span>
|
|
|
|
<span class="kr">const</span> <span class="nx">posts</span> <span class="o">=</span> <span class="p">[</span>
|
|
<span class="p">{</span><span class="nx">id</span><span class="o">:</span> <span class="mi">1</span><span class="p">,</span> <span class="nx">title</span><span class="o">:</span> <span class="s1">'Hello World'</span><span class="p">,</span> <span class="nx">content</span><span class="o">:</span> <span class="s1">'Welcome to learning React!'</span><span class="p">},</span>
|
|
<span class="p">{</span><span class="nx">id</span><span class="o">:</span> <span class="mi">2</span><span class="p">,</span> <span class="nx">title</span><span class="o">:</span> <span class="s1">'Installation'</span><span class="p">,</span> <span class="nx">content</span><span class="o">:</span> <span class="s1">'You can install React from npm.'</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">Blog</span> <span class="nx">posts</span><span class="o">=</span><span class="p">{</span><span class="nx">posts</span><span class="p">}</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="https://codepen.io/gaearon/pen/NRZYGN?editors=0010">Try it on CodePen.</a></p>
|
|
|
|
<p>Keys serve as a hint to React but they don't get passed to your components. If you need the same value in your component, pass it explicitly as a prop with a different name:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kr">const</span> <span class="nx">content</span> <span class="o">=</span> <span class="nx">posts</span><span class="p">.</span><span class="nx">map</span><span class="p">((</span><span class="nx">post</span><span class="p">)</span> <span class="o">=></span>
|
|
<span class="o"><</span><span class="nx">Post</span>
|
|
<span class="hll"> <span class="nx">key</span><span class="o">=</span><span class="p">{</span><span class="nx">post</span><span class="p">.</span><span class="nx">id</span><span class="p">}</span>
|
|
</span><span class="hll"> <span class="nx">id</span><span class="o">=</span><span class="p">{</span><span class="nx">post</span><span class="p">.</span><span class="nx">id</span><span class="p">}</span>
|
|
</span> <span class="nx">title</span><span class="o">=</span><span class="p">{</span><span class="nx">post</span><span class="p">.</span><span class="nx">title</span><span class="p">}</span> <span class="o">/></span>
|
|
<span class="p">);</span>
|
|
</code></pre></div>
|
|
<p>With the example above, the <code>Post</code> component can read <code>props.id</code>, but not <code>props.key</code>.</p>
|
|
|
|
<h3>Embedding map() in JSX</h3>
|
|
|
|
<p>In the examples above we declared a separate <code>listItems</code> variable and included it in JSX:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kd">function</span> <span class="nx">NumberList</span><span class="p">(</span><span class="nx">props</span><span class="p">)</span> <span class="p">{</span>
|
|
<span class="kr">const</span> <span class="nx">numbers</span> <span class="o">=</span> <span class="nx">props</span><span class="p">.</span><span class="nx">numbers</span><span class="p">;</span>
|
|
<span class="hll"> <span class="kr">const</span> <span class="nx">listItems</span> <span class="o">=</span> <span class="nx">numbers</span><span class="p">.</span><span class="nx">map</span><span class="p">((</span><span class="nx">number</span><span class="p">)</span> <span class="o">=></span>
|
|
</span><span class="hll"> <span class="o"><</span><span class="nx">ListItem</span> <span class="nx">key</span><span class="o">=</span><span class="p">{</span><span class="nx">number</span><span class="p">.</span><span class="nx">toString</span><span class="p">()}</span>
|
|
</span><span class="hll"> <span class="nx">value</span><span class="o">=</span><span class="p">{</span><span class="nx">number</span><span class="p">}</span> <span class="o">/></span>
|
|
</span><span class="hll"> <span class="p">);</span>
|
|
</span> <span class="k">return</span> <span class="p">(</span>
|
|
<span class="o"><</span><span class="nx">ul</span><span class="o">></span>
|
|
<span class="p">{</span><span class="nx">listItems</span><span class="p">}</span>
|
|
<span class="o"><</span><span class="err">/ul></span>
|
|
<span class="p">);</span>
|
|
<span class="p">}</span>
|
|
</code></pre></div>
|
|
<p>JSX allows <a href="/react/docs/introducing-jsx.html#embedding-expressions-in-jsx">embedding any expressions</a> in curly braces so we could inline the <code>map()</code> result:</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kd">function</span> <span class="nx">NumberList</span><span class="p">(</span><span class="nx">props</span><span class="p">)</span> <span class="p">{</span>
|
|
<span class="kr">const</span> <span class="nx">numbers</span> <span class="o">=</span> <span class="nx">props</span><span class="p">.</span><span class="nx">numbers</span><span class="p">;</span>
|
|
<span class="k">return</span> <span class="p">(</span>
|
|
<span class="o"><</span><span class="nx">ul</span><span class="o">></span>
|
|
<span class="hll"> <span class="p">{</span><span class="nx">numbers</span><span class="p">.</span><span class="nx">map</span><span class="p">((</span><span class="nx">number</span><span class="p">)</span> <span class="o">=></span>
|
|
</span><span class="hll"> <span class="o"><</span><span class="nx">ListItem</span> <span class="nx">key</span><span class="o">=</span><span class="p">{</span><span class="nx">number</span><span class="p">.</span><span class="nx">toString</span><span class="p">()}</span>
|
|
</span><span class="hll"> <span class="nx">value</span><span class="o">=</span><span class="p">{</span><span class="nx">number</span><span class="p">}</span> <span class="o">/></span>
|
|
</span><span class="hll"> <span class="p">)}</span>
|
|
</span> <span class="o"><</span><span class="err">/ul></span>
|
|
<span class="p">);</span>
|
|
<span class="p">}</span>
|
|
</code></pre></div>
|
|
<p><a href="https://codepen.io/gaearon/pen/BLvYrB?editors=0010">Try it on CodePen.</a></p>
|
|
|
|
<p>Sometimes this results in clearer code, but this style can also be abused. Like in JavaScript, it is up to you to decide whether it is worth extracting a variable for readability. Keep in mind that if the <code>map()</code> body is too nested, it might be a good time to <a href="/react/docs/components-and-props.html#extracting-components">extract a component</a>.</p>
|
|
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<div class="docs-prevnext">
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<a class="docs-prev" href="/react/docs/conditional-rendering.html">← Prev</a>
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<a class="docs-next" href="/react/docs/forms.html">Next →</a>
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</div>
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</div>
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<div class="nav-docs">
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<!-- Docs Nav -->
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<div class="nav-docs-section">
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<h3>Quick Start</h3>
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<ul>
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<li>
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<a href="/react/docs/installation.html">Installation</a>
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<a href="/react/docs/hello-world.html">Hello World</a>
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<a href="/react/docs/introducing-jsx.html">Introducing JSX</a>
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<a href="/react/docs/rendering-elements.html">Rendering Elements</a>
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<a href="/react/docs/state-and-lifecycle.html">State and Lifecycle</a>
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<a href="/react/docs/handling-events.html">Handling Events</a>
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<a href="/react/docs/refs-and-the-dom.html">Refs and the DOM</a>
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<li>
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<a href="/react/docs/uncontrolled-components.html">Uncontrolled Components</a>
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</li>
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<a href="/react/docs/optimizing-performance.html">Optimizing Performance</a>
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<a href="/react/docs/react-without-jsx.html">React Without JSX</a>
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</li>
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<li>
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<a href="/react/docs/reconciliation.html">Reconciliation</a>
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<a href="/react/docs/context.html">Context</a>
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</li>
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<li>
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<a href="/react/docs/portals.html">Portals</a>
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</li>
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<li>
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<a href="/react/docs/web-components.html">Web Components</a>
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<li>
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<a href="/react/docs/higher-order-components.html">Higher-Order Components</a>
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</li>
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<li>
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<a href="/react/docs/integrating-with-other-libraries.html">Integrating with Other Libraries</a>
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<a href="/react/docs/react-api.html">React</a>
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<ul>
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<li>
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<a href="/react/docs/react-component.html">React.Component</a>
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</li>
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