Validate AlertController input field

Ionic Framework ionic-v3
1

Please let me know what is the meaning of User in the handler field.
What is this validation method? I’m new to ionic.

presentPrompt() {
  let alert = this.alertCtrl.create({
    title: 'Login',
    inputs: [
      {
        name: 'username',
        placeholder: 'Username'
      },
      {
        name: 'password',
        placeholder: 'Password',
        type: 'password'
      }
    ],
    buttons: [
      {
        text: 'Cancel',
        role: 'cancel',
        handler: data => {
          console.log('Cancel clicked');
        }
      },
      {
        text: 'Login',
        handler: data => {
          if (User.isValid(data.username, data.password)) {
            // logged in!
          } else {
            // invalid login
            return false;
          }
        }
      }
    ]
  });
  alert.present();
}
2

Where does the code come from?

This isnt explainable without context ( no out of the box angular nor ionic)

3

This is the example of AlertController in ionic documentation (https://ionicframework.com/docs/api/components/alert/AlertController/).

4

I think that line in the code snippet should be ignored. Pseudo code I would say.

If u want complete examples from the ionic team check out the preview app