How to avoid undefined in string concatenation?

I’m iterating over an object and I want to concatenate the name of the service. This is my code:

var servizi;

for(var i = 0; i < appointment.id_services.length; i++)
{
    servizi += appointment.id_services[i].name + " ";
}

Now the problem’s that I got this result:

undefined hair cut

In my object there’s only hair and cut, why I get undefined also?

Answer

You get undefined because you declared an uninitialized variable, and then added to it (twice).

Initialize the declared variable as an empty string first

var servizi = "";