title: What kind of function is this?
body: <p>Recently I saw this functon. I mean specifically this line/syntax:</p>
  <pre><code>export const selectItemBySpecificId = (id: string):any =&gt; createSelector(
  </code></pre>
  <p>What kind of function is this?</p>
  <p>I know functions like:
  function doSomething() { ... } // function delaration statement</p>
  <p>let doSomething = function()  { .. } // function expresion</p>
  <p><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions" rel="nofollow noreferrer">https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions</a></p>
  <p>Full example (it's a NgRx Selector with parameter):</p>
  <pre><code>export const selectItemBySpecificId = (id: string):any =&gt; createSelector(
      selectAllItems,
      (data: any) =&gt; {
          const result = data.filter((item: IItem) =&gt; iem.id === id);
          if (result.length &gt;= 1) {
              return result[0];
          }
          return null;
      }
  );
  </code></pre>