gulp transpile not generating the files

I have a simple Persona.ts class

class Persona {
    nombre: string;
    constructor(message: string) {
        this.nombre = message;
    }
    sayHi() {
        return "Hello, " + this.nombre;
    }
}

let persona = new Persona("world");

let button = document.createElement('button');
button.textContent = "Say Hello";
button.onclick = function() {
let persona = new Persona("world");
    alert(persona.sayHi());
}

document.body.appendChild(button);

And in the gulpfile.js I have this.

var gulp = require('gulp'); 
let ts = require('gulp-typescript');
gulp.task('transpile', () => {
    return gulp.src('ts/*.ts')
               .pipe(ts())
               .pipe(gulp.dest('js'));
});

when I run gulp transpile I get this output:

[21:33:17] Using gulpfile ~\Documents\Prueba\gulpfile.js 
[21:33:17] Starting 'transpile'... 
[21:33:18] Finished 'transpile' after 271 ms

However in the folder of the project, there are NO JS libraries with js files generated.

via Luis Valencia - MVP