What is the difference between nativeElement.click() and event handler's click?

Question

I have been trying to perform a test for angular 2 application that clicks a submit button which calls a function. I usually use two methods to perform the same.

element.nativeElement.click()

and

element.triggerEventHandler('click',null);

I thought both these methods were same, until I came across this situation where triggering event handler method does not work.

element = fixture.debugElement.query(By.css('.dropList li'));
element.triggerEventHandler('click',null);  //Click event works here
fixture.detectChanges();
let button = fixture.debugElement.query(By.css('button'));
//button.triggerEventHandler('click',null);    //Does not call function
button.nativeElement.click();    //Calls function
fixture.detectChanges();

Template for your reference:

<form (ngSubmit)="printSelection()">
   <div class="dropList">
     <ul>
        <li *ngFor="let element of data.elements" (click)="selectElement(element)"> </li>
    </ul>
   </div>
   <button type="submit">Submit</button>
</form>

So, are there any difference between these two approaches or do you think I may have gone wrong somewhere in my code?

Answer 1

element.nativeElement.click()

is native method to simulate a mouse click on an element. It's bubbling and it behaves in the same way as if we click on the element.

debugElement.triggerEventHandler(eventName, eventObj)

is angular built-in method that just calls all listeners for given eventName on current debug element:

triggerEventHandler(eventName: string, eventObj: any) {
  this.listeners.forEach((listener) => {
    if (listener.name == eventName) {
      listener.callback(eventObj);
    }
  });
}

Listeners are added when DebugRenderer2 runs listen method:

listen(
    target: 'document'|'windows'|'body'|any, eventName: string,
    callback: (event: any) => boolean): () => void {
  if (typeof target !== 'string') {
    const debugEl = getDebugNode(target);
    if (debugEl) {
      debugEl.listeners.push(new EventListener(eventName, callback));
    }
  }

  return this.delegate.listen(target, eventName, callback);
}

It happens when we apply event binding to element like

• (click)="handler()"
• @HostListener('click')
• host: '{ '(mouseenter): 'handler()' }
• renderer.listen

Let's say we have the following template:

<div (click)="test()">
  <div class="innerDiv">
     {{title}}
  </div>
</div>

And our test will look like:

de = fixture.debugElement.query(By.css('.innerDiv'));

de.nativeElement.click(); // event will be bubbled and test handler will be called

de.triggerEventHandler('click', null); // there is not handlers for click event 
                                       // that angular added to this element 
                                       // so test method won't be called

Then let's look at your template. There is no handlers for button so triggerEventHandler won't have any effect. On the other hand button.nativeElement.click(); will fire submit because button has type submit and its standart behaviour of button on click event.