Validates an email, error on the side of letting bad ones through (best practice). Accepts emails with plus signs.

isValidEmail.js

function isValidEmail(str) {
  /**
   * These comments use the following terms from RFC2822:
   * local-part, domain, domain-literal and dot-atom.
   * Does the address contain a local-part followed an @ followed by a domain?
   * Note the use of lastIndexOf to find the last @ in the address
   * since a valid email address may have a quoted @ in the local-part.
   * Does the domain name have at least two parts, i.e. at least one dot,
   * after the @? If not, is it a domain-literal?
   * 
   * This WILL ACCEPT some invalid email addresses BUT
   * it doesn't reject valid ones, which is most important to us
   * as this should just be used to prevent gibberish input.
   *
   * Reference: http://tools.ietf.org/html/rfc2822
   */

  // First, make sure the string isn't empty/falsy
  if (!str) {
    return false;
  }

  var atSymbol = str.lastIndexOf('@');

  // no local-part
  if (atSymbol < 1) {
    return false;
  }

  // no domain
  if (atSymbol === str.length - 1) {
    return false;
  }

  // there may only be 64 octets in the local-part
  if (atSymbol > 64) {
    return false;
  }

  // there may only be 255 octets in the domain
  if (str.length - atSymbol > 255) {
    return false;
  }

  // Is the domain plausible?
  var lastDot = str.lastIndexOf('.');

  // Check if it is a dot-atom such as example.com
  if (lastDot > atSymbol + 1 && lastDot < str.length - 1) {
    return true;
  }

  //  Check if could be a domain-literal.
  if (str.charAt(atSymbol + 1) === '[' && str.charAt(str.length - 1) === ']') {
    return true;
  }

  // If you reach here we just give up and say no
  return false;
}