<p dir="auto"><strong>Do you want to request a <em>feature</em> or report a <em>bug</em>?</strong><br>
Bug</p>
<p dir="auto"><strong>What is the current behavior?</strong><br>
When you have an unexpected prop in a component but that prop is not meant to be generated on the HTML DOM so that is set to {null}, React triggers this warning:</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="Warning: Unknown prop `activeClassName` on &lt;h1&gt; tag. Remove this prop from the element. For details, see https://fb.me/react-unknown-prop
    in h1 (created by HelloWorldComponent)
    in HelloWorldComponent"><pre class="notranslate"><code class="notranslate">Warning: Unknown prop `activeClassName` on &lt;h1&gt; tag. Remove this prop from the element. For details, see https://fb.me/react-unknown-prop
    in h1 (created by HelloWorldComponent)
    in HelloWorldComponent
</code></pre></div>
<p dir="auto">The actual use case is to have a reusable component called <code class="notranslate">&lt;Anchor&gt;</code> and it'd be either <code class="notranslate">&lt;Link activeClassName="'is-active" /&gt;</code> or <code class="notranslate">&lt;a activeClassName={null} /&gt;</code>. <a href="https://github.com/worona/worona-dashboard/blob/dev/client/packages/bulma-dashboard-theme-worona/src/components/Header/MenuItem.jsx#L25">Here's our actual code.</a><br>
In the HTML output, there's no any activeClassName attribute for the second case, so there shouldn't be any react warning.</p>
<p dir="auto"><strong>Example reproducing it</strong><br>
I've reproduced the issue in a simple example here:<br>
<a href="https://jsbin.com/bozuwuq/edit?html,js,console,output" rel="nofollow">https://jsbin.com/bozuwuq/edit?html,js,console,output</a></p>
<p dir="auto"><strong>What is the expected behavior?</strong><br>
Don't warn when a prop is defined to {null} and, therefore, never generated in the DOM.</p>
<p dir="auto"><strong>Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?</strong><br>
React version: 15.3.0<br>
OS: Ubuntu and Mac OS X. Chromium and Google Chrome.</p>
<p dir="auto"><code class="notranslate">Warning: Unknown prop ... on ... tag. Remove this prop from the element.</code></p>
<p dir="auto">These are emitted even if the property value is <code class="notranslate">undefined</code>. Should <code class="notranslate">undefined</code> perhaps be treated as not existing perhaps? That's how it is intended AFAIK.</p>