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<h1>
<a href="https://f3real.github.io/mutationobserver_mediaquery.html"
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Using MutationObserver and MediaQuery
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<span class="published">
<i class="fa fa-calendar padding-small"></i><time datetime="2019-03-06T10:01:00+01:00"> Wed 06 March 2019</time>
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<a href="https://f3real.github.io/tag/js.html">js</a> / <a href="https://f3real.github.io/tag/mutationobserver.html">MutationObserver</a> / <a href="https://f3real.github.io/tag/mediaquery.html">MediaQuery</a> </span>
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<p>While trying to make the blog more accessible for users especially for those who use the keyboard I hit an interesting problem. How to trigger JS code on CSS changes?</p>
<p>Usually, I only used events to react to mouse clicks. For example, if we wanted to create behavior similar to boostrap collapse we could add this JS code:</p>
<div class="highlight"><pre><span></span><code><span class="w"> </span><span class="nx">element</span><span class="p">.</span><span class="nx">addEventListener</span><span class="p">(</span><span class="s1">'click'</span><span class="p">,</span><span class="w"> </span><span class="p">(</span><span class="nx">ev</span><span class="p">)</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">//current target not necessarily element on which event occurred</span>
<span class="w"> </span><span class="kd">const</span><span class="w"> </span><span class="nx">elm</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nx">ev</span><span class="p">.</span><span class="nx">currentTarget</span><span class="p">;</span><span class="w"> </span>
<span class="w"> </span><span class="nx">ev</span><span class="p">.</span><span class="nx">preventDefault</span><span class="p">();</span>
<span class="w"> </span><span class="kd">const</span><span class="w"> </span><span class="nx">selector</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nx">elm</span><span class="p">.</span><span class="nx">getAttribute</span><span class="p">(</span><span class="s1">'href'</span><span class="p">).</span><span class="nx">replace</span><span class="p">(</span><span class="s1">'#'</span><span class="p">,</span><span class="s1">''</span><span class="p">);</span>
<span class="w"> </span><span class="nx">elm</span><span class="p">.</span><span class="nx">classList</span><span class="p">.</span><span class="nx">toggle</span><span class="p">(</span><span class="s1">'collapsed'</span><span class="p">);</span>
<span class="w"> </span><span class="nb">document</span><span class="p">.</span><span class="nx">getElementById</span><span class="p">(</span><span class="nx">selector</span><span class="p">).</span><span class="nx">classList</span><span class="p">.</span><span class="nx">toggle</span><span class="p">(</span><span class="s1">'show'</span><span class="p">);</span>
<span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="kc">false</span><span class="p">);</span>
</code></pre></div>
<p>But what happens if we want to react, for example, on class being added to an element? The solution is to use <code>MutationObserver</code>. We can set it up to react on attribute change (or only specific attribute), adding/removing child nodes or even setting it to observe the whole subtree. It is also reasonably efficient, the callback will be triggered only after DOM has finished changing.</p>
<p>Example of creating <code>MutationObserver</code> for all attributes on the element:</p>
<div class="highlight"><pre><span></span><code><span class="w"> </span><span class="kd">var</span><span class="w"> </span><span class="nx">mutationCallback</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="kd">function</span><span class="p">(</span><span class="nx">mutationsList</span><span class="p">,</span><span class="w"> </span><span class="nx">observer</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">for</span><span class="p">(</span><span class="kd">var</span><span class="w"> </span><span class="nx">mutation</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="nx">mutationsList</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="nx">mutation</span><span class="p">.</span><span class="nx">type</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="s1">'attributes'</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="s1">'Attribute changed'</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="p">};</span>
<span class="w"> </span><span class="kd">var</span><span class="w"> </span><span class="nx">observer</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="ow">new</span><span class="w"> </span><span class="nx">MutationObserver</span><span class="p">(</span><span class="nx">mutationCallback</span><span class="p">);</span>
<span class="w"> </span><span class="nx">observer</span><span class="p">.</span><span class="nx">observe</span><span class="p">(</span><span class="nx">element</span><span class="p">,</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="nx">attributes</span><span class="o">:</span><span class="w"> </span><span class="kc">true</span><span class="p">});</span>
</code></pre></div>
<p>Also, even with <code>MutationObserver</code> we can't write code that will trigger on CSS changes themselves (unless there is class being added/removed). Changes in CSSOM are not detectable with <code>MutationObserver</code>. CSS is usually fixed but <code>@media</code> queries can trigger change on width/height/orientation changes. The solution to this problem is to create an event handler for those changes using <code>onchange</code> property of the <code>MediaQueryList</code> interface.</p>
<p>For example to react on screen width changes, we can use:</p>
<div class="highlight"><pre><span></span><code><span class="w"> </span><span class="kd">const</span><span class="w"> </span><span class="nx">mediaQuery</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nb">window</span><span class="p">.</span><span class="nx">matchMedia</span><span class="p">(</span><span class="s1">'(min-width:768px)'</span><span class="p">);</span>
<span class="w"> </span><span class="nx">mediaQuery</span><span class="p">.</span><span class="nx">onchange</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nx">e</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="nx">e</span><span class="p">.</span><span class="nx">matches</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="s1">'Condition is true'</span><span class="p">)</span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="s1">'Condition is false'</span><span class="p">)</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="p">}</span>
</code></pre></div>
<p>So, in summary, these are some of the ways we can add handlers for different changes on our page.</p>
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