title: Removing EventListener in ngOnDestroy
body: <p>I have the following implementation of a directive. How to removeEventListener in this case:</p>
  
  <pre><code>import { Directive, ElementRef, OnDestroy } from "@angular/core";
  
  @Directive({
      selector: "[Enter]"
  })
  export class Enter implements OnDestroy{
      constructor(el: ElementRef) {
          let enter = function(event){
              if(event.keyCode === 13){
                  el.nativeElement.click();
              }
          }
          document.addEventListener('keyup', enter , false);
      }
  
      ngOnDestroy(){
          document.removeEventListener('keyup', enter, false); //this line doesn't work because I can't access enter variable here!
      }
  }
  </code></pre>
  
  <p>I know I can simply use a global variable here and can access it. But I don't want to store the state of instance in the global variable.</p>