title: What is the Underscore for in front of the arrow function (_=>) in the hero.service.ts Angular v5 Tour of Heroes Tutorial?
body: <p>I have either a typescript or javascript syntax issue.
  Can someone tell me what _ => this.log... means?</p>
  
  <p>I am used to seeing a name the parameter being passed into the arrow function there.</p>
  
  <p>Does it simply mean 'no parameter'?</p>
  
  <p>Ref: <a href="https://angular.io/tutorial/toh-pt6#add-heroserviceupdatehero" rel="noreferrer">https://angular.io/tutorial/toh-pt6#add-heroserviceupdatehero</a></p>
  
  <pre><code>    /** PUT: update the hero on the server */
  updateHero (hero: Hero): Observable&lt;any&gt; {
    return this.http.put(this.heroesUrl, hero, httpOptions).pipe(
      tap(_ =&gt; this.log(`updated hero id=${hero.id}`)),
      catchError(this.handleError&lt;any&gt;('updateHero'))
    );
  }
  </code></pre>