Parsing an email for the text before the "@" symbol

Another possibility:

function emailUsername(emailAddress) {
  return emailAddress.split('@')[0]
}

This splits the string in half at the @ symbol, creating an array with the two parts and then retrieves the first item in the array, which will be the part before the @.

function emailUsername(emailAddress) {
   return emailAddress.match(/^(.+)@/)[1];
}

Like this:

return emailAddress.substring(0, emailAddress.indexOf("@"));