Disable button when input is empty in Angular 2

To bind functions to events, you don't have to prefix them with on. Just place the event.

For example, if you want to handle the keydown (plunker demo):

<input type="password" [(ngModel)]="myPassword" (keydown)="checkPasswordEmpty($event)"/>

---

But in your specific case, since you already are using ngModel you are better off using (ngModelChange):

<input type="password" [(ngModel)]="myPassword" (ngModelChange)="checkPasswordEmpty()"/>

Because it will pick up the changes when the user pastes (via CTRL+V ormouse right click menu -> Paste) the password instead of typing it.

See plunker demo for using (ngModelChange) here.

In this case, I would leverage form validators. I would add the "required" validation on your input and use the global state of your form to disable / enable your button.

With the template-driven approach, there is no code to add in your component, only stuff in its template...

Here is sample below:

<form #formCtrl="ngForm">
  <input ngControl="passwordCtrl" required>
  <button [disabled]="!formCtrl.form.valid">Some button</button>
</form>