Getting around the mailto character limit

I was faced with a problem recently where a web page needed to create an email with the user's email client. This is normally trivial, simply redirect the user to mailto:<emails>. The issues was that the maximum length of a URL across different platforms is approximately 2000 characters, and the amount of emails required far exceeded that in some cases. So a solution would need to make multiple mailto requests.

Here is the function I came up with:

function sendEmails(emails) {
  var timeout = 2000;
  var mailtoPrefix = 'mailto:?bcc=';
  var maxUrlCharacters = 1900;
  var separator = ';';
  var currentIndex = 0;
  var nextIndex = 0;

  if (emails.length < maxUrlCharacters) {
    window.location = mailtoPrefix + emails;
    return;
  }

  do {
    currentIndex = nextIndex;
    nextIndex = emails.indexOf(separator, currentIndex + 1);
  } while (nextIndex != -1 && nextIndex < maxUrlCharacters)

  if (currentIndex == -1) {
    window.location = mailtoPrefix + emails;
  } else {
    window.location = mailtoPrefix + emails.slice(0, currentIndex);
    setTimeout(function () { 
      sendEmails(emails.slice(currentIndex + 1)); 
    }, timeout);
  }
}

// usage
var emails = 'a@a.com;b@b.com;c@c.com';
sendEmails(emails); 

You may have noticed in the code above I set a rather large timeout for opening email windows (2000ms). Lower amounts seem to not create some of the emails, this is probably just a matter of Outlook (what I was testing against) not accepting multiple at a time.

An alternative solution for the whole problem, if mail client integration isn't required you can always of course fall back on the usual in page form that sends an email.