I actually found the solution and got the locale via jsreport.req.options.localization.language

Hello,

I have a global function to format date.

function formatDate(timestampStr) {
  const timestamp = parseInt(timestampStr)
  const date = new Date(timestamp);
  
  const options = {
    day: 'numeric',
    month: 'long',
    year: 'numeric'
  };

  return date.toLocaleDateString('fr-FR', options);
}

As you can see the locale in this example is "fr-FR", instead I would like to have the locale of the template. I can't find a way to get it from the javascript.
Any ideas?