TypeScript Error

I'm checking empty fields in TypeScript for ionic and it tells me there are some errors and I'm unable to see it.

Error:

  

Unexpected token. A builder, method, accessor, or property was expected

Code:

import { Component } from '@angular/core';
import { NavController } from 'ionic-angular';

@Component({
  selector: 'page-about',
  templateUrl: 'about.html'
})
export class AboutPage {

    constructor(public navCtrl: NavController) {

    }

    function comprobar()  {
        let enCondiciones: boolean;
        let campoEmail: HTMLElement = document.getElementById('campoEmail');
        let campoContraseña: HTMLElement = document.getElementById('campoContraseña');

        if(campoEmail.value.length === 0 || campoContraseña.value.length === 0){
            enCondiciones = false;
        } else{
            enCondiciones = true;
        }

        if(enCondiciones === false){
            alert("Hay campos vacios!!");
        }
    }
}

Any ideas?