title: ESLint warns about unused function parameter type
body: <p>Without bypassing the warning <code>no-unused-vars</code>, how can I fix this code for ESLint to not complain about <code>_</code>?</p>
  <p>I thought renaming the variable to <code>_</code> would solve the issue.</p>
  <pre class="lang-js prettyprint-override"><code>type ById = (_: any) =&gt; string
  
  export const normalize = (byId: ById) =&gt; (acc: Record&lt;string, unknown&gt;, doc: { data: () =&gt; unknown }) =&gt; ({
    ...acc,
    [byId(doc)]: doc.data(),
  })
  </code></pre>
  <pre><code>2:14  error  '_' is defined but never used       no-unused-vars
  </code></pre>