Question

I have a function which does not work.
I dont know what the syntax will be when I want to find out if the variable is not blank and then display/write the variable??

function contactUsMailer(e) {
try {
var recipient = "[e-mail]";
var timestamp = e.values[0];
var email = e.values[1];
var buffet = e.values[12];    
var message = 'Kantine Rekvisition'+'<br>'

    +'Afdeling:        '+e.values[2]+'<br>'
    +'Afd. nr.:        '+e.values[3]+'<br>'
    +'Til dato/kl.:    '+e.values[8]+'<br>'+'<br>'

    **if (buffet!='') {
      +'Antal Buffet: '+ buffet + '<br>'**

    var body = message;
 var advancedArgs = {htmlBody:body, replyTo:email};
 MailApp.sendEmail(recipient, "Kantine Rekvisition", body, advancedArgs);
} catch(e){
 MailApp.sendEmail(recipient, "Error - Contact Us Form", e.message);
}
}

Answer 1

You could simply use:

if (buffet) {
    // buffet is not undefined, null, false, 0 or empty string
}

Your existing condition buffet!='' should work as well, just keep in mind that null!='' is also truthy.

Edit

Btw, used inside a string concatenation, you should use the ternary operator:

var message = 'Kantine Rekvisition'+'<br>'
  +'Afdeling:        '+e.values[2]+'<br>'
  +'Afd. nr.:        '+e.values[3]+'<br>'
  +'Til dato/kl.:    '+e.values[8]+'<br>'+'<br>'
  +(buffet ? 'Antal Buffet: '+ buffet + '<br>' : '');