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<h1>If-Else в JSX</h1>
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<div class="subHeader"></div>
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<p>Некоторые конструкции, такие как <code>if-else</code>, нельзя использовать внутри JSX, так как JSX в результате преобразуется в вызов функции JS, как показано в примере:</p>
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<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="c1">// JSX:</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">div</span> <span class="nx">id</span><span class="o">=</span><span class="s2">"msg"</span><span class="o">></span><span class="nx">Hello</span> <span class="nx">World</span><span class="o">!<</span><span class="err">/div>, mountNode);</span>
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<span class="c1">// Преобразованный в JS:</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="nx">React</span><span class="p">.</span><span class="nx">createElement</span><span class="p">(</span><span class="s2">"div"</span><span class="p">,</span> <span class="p">{</span><span class="nx">id</span><span class="o">:</span><span class="s2">"msg"</span><span class="p">},</span> <span class="s2">"Hello World!"</span><span class="p">),</span> <span class="nx">mountNode</span><span class="p">);</span>
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</code></pre></div>
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<p>Это означает что оператор <code>if</code> нельзя встроить таким образом:</p>
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<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="c1">// JSX:</span>
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<span class="o"><</span><span class="nx">div</span> <span class="nx">id</span><span class="o">=</span><span class="p">{</span><span class="k">if</span> <span class="p">(</span><span class="nx">condition</span><span class="p">)</span> <span class="p">{</span> <span class="s1">'msg'</span> <span class="p">}}</span><span class="o">></span><span class="nx">Hello</span> <span class="nx">World</span><span class="o">!<</span><span class="err">/div></span>
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<span class="c1">// Преобразованный в JS:</span>
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<span class="nx">React</span><span class="p">.</span><span class="nx">createElement</span><span class="p">(</span><span class="s2">"div"</span><span class="p">,</span> <span class="p">{</span><span class="nx">id</span><span class="o">:</span> <span class="k">if</span> <span class="p">(</span><span class="nx">condition</span><span class="p">)</span> <span class="p">{</span> <span class="s1">'msg'</span> <span class="p">}},</span> <span class="s2">"Hello World!"</span><span class="p">);</span>
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</code></pre></div>
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<p>В результате преобразования получается неверный JS код. В таких случаях используется тернарный условный оператор:</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><span class="o"><</span><span class="nx">div</span> <span class="nx">id</span><span class="o">=</span><span class="p">{</span><span class="nx">condition</span> <span class="o">?</span> <span class="s1">'msg'</span> <span class="o">:</span> <span class="kc">null</span><span class="p">}</span><span class="o">></span><span class="nx">Hello</span> <span class="nx">World</span><span class="o">!<</span><span class="err">/div>, mountNode);</span>
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</code></pre></div>
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<p>Если тернарного оператора недостаточно, вы можете вынести оператор <code>if</code>, определяющий выбор компонентов, вне JSX:</p>
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<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="kd">var</span> <span class="nx">loginButton</span><span class="p">;</span>
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<span class="k">if</span> <span class="p">(</span><span class="nx">loggedIn</span><span class="p">)</span> <span class="p">{</span>
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<span class="nx">loginButton</span> <span class="o">=</span> <span class="o"><</span><span class="nx">LogoutButton</span> <span class="o">/></span><span class="p">;</span>
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<span class="p">}</span> <span class="k">else</span> <span class="p">{</span>
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<span class="nx">loginButton</span> <span class="o">=</span> <span class="o"><</span><span class="nx">LoginButton</span> <span class="o">/></span><span class="p">;</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">nav</span><span class="o">></span>
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<span class="o"><</span><span class="nx">Home</span> <span class="o">/></span>
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<span class="p">{</span><span class="nx">loginButton</span><span class="p">}</span>
|
|
<span class="o"><</span><span class="err">/nav></span>
|
|
<span class="p">);</span>
|
|
</code></pre></div>
|
|
<p>Также вы можете обернуть код в <a href="https://en.wikipedia.org/wiki/Immediately-invoked_function_expression">немедленно вызываемую функцию</a> и расположить её <em>внутри</em> JSX.</p>
|
|
<div class="highlight"><pre><code class="language-js" data-lang="js"><span class="k">return</span> <span class="p">(</span>
|
|
<span class="o"><</span><span class="nx">section</span><span class="o">></span>
|
|
<span class="o"><</span><span class="nx">h1</span><span class="o">></span><span class="nx">Color</span><span class="o"><</span><span class="err">/h1></span>
|
|
<span class="o"><</span><span class="nx">h3</span><span class="o">></span><span class="nx">Name</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="k">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">color</span> <span class="o">||</span> <span class="s2">"white"</span><span class="p">}</span><span class="o"><</span><span class="err">/p></span>
|
|
<span class="o"><</span><span class="nx">h3</span><span class="o">></span><span class="nx">Hex</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="o">=></span> <span class="p">{</span>
|
|
<span class="k">switch</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">color</span><span class="p">)</span> <span class="p">{</span>
|
|
<span class="k">case</span> <span class="s2">"red"</span><span class="o">:</span> <span class="k">return</span> <span class="s2">"#FF0000"</span><span class="p">;</span>
|
|
<span class="k">case</span> <span class="s2">"green"</span><span class="o">:</span> <span class="k">return</span> <span class="s2">"#00FF00"</span><span class="p">;</span>
|
|
<span class="k">case</span> <span class="s2">"blue"</span><span class="o">:</span> <span class="k">return</span> <span class="s2">"#0000FF"</span><span class="p">;</span>
|
|
<span class="k">default</span><span class="o">:</span> <span class="k">return</span> <span class="s2">"#FFFFFF"</span><span class="p">;</span>
|
|
<span class="p">}</span>
|
|
<span class="p">})()}</span>
|
|
<span class="o"><</span><span class="err">/p></span>
|
|
<span class="o"><</span><span class="err">/section></span>
|
|
<span class="p">);</span>
|
|
</code></pre></div>
|
|
<blockquote>
|
|
<p>Примечание:</p>
|
|
|
|
<p>В приведенном выше примере, используется <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions">функция-стрелка</a> из ES6 для связи со значением <code>this</code>.</p>
|
|
</blockquote>
|
|
|
|
<p>Вы можете попробовать этот синтаксис внутри <a href="https://babeljs.io/repl/">Babel REPL</a>.</p>
|
|
|
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|
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