Unique Email Addresses The task
was taken from leetcode


  Every email consists of a local name and a domain name, separated by
  the @ sign.
  
  For example, in alice@leetcode.com, alice is the local name, and
  leetcode.com is the domain name.
  
  Besides lowercase letters, these emails may contain '.'s or '+'s.
  
  If you add periods ('.') between some characters in the local name
  part of an email address, mail sent there will be forwarded to the
  same address without dots in the local name.  For example,
  "alice.z@leetcode.com" and "alicez@leetcode.com" forward to the same
  email address.  (Note that this rule does not apply for domain names.)
  
  If you add a plus ('+') in the local name, everything after the first
  plus sign will be ignored. This allows certain emails to be filtered,
  for example m.y+name@email.com will be forwarded to my@email.com. 
  (Again, this rule does not apply for domain names.)
  
  It is possible to use both of these rules at the same time.
  
  Given a list of emails, we send one email to each address in the list.
  How many different addresses actually receive mails? 
  
  Example 1:
  
  Input:
  ["test.email+alex@leetcode.com","test.e.mail+bob.cathy@leetcode.com","testemail+david@lee.tcode.com"]
  
  Output: 2
  
  Explanation: "testemail@leetcode.com" and "testemail@lee.tcode.com"
  actually receive mails  
  
  Note:
  
  1 &lt;= emails[i].length &lt;= 100
  
  1 &lt;= emails.length &lt;= 100
  
  Each emails[i] contains exactly one '@' character.
  
  All local and domain names are non-empty.
  
  Local names do not start with a '+' character.


My declarative solution

/**
 * @param {string[]} emails
 * @return {number}
 */
var numUniqueEmails = emails =&gt; {
    return emails.reduce((validMails, mail) =&gt; {
      const names = mail.split('@');
      let [local, domain] = names;
      const iPlus = [...local].findIndex(x =&gt; x === '+');
      if (iPlus !== -1) { local = local.substr(0, iPlus); }
      const key = local.split('.').join('') + '@' + domain;
      if (!validMails.has(key)) { validMails.add(key); } 
      return validMails
    }, new Set).size;
};


My Imperative solution

/**
 * @param {string[]} emails
 * @return {number}
 */
var numUniqueEmails2 = emails =&gt; {
  const validMails = new Set();
  for (const mail of emails) {
    let [local, domain] = mail.split('@');
    const iPlus = [...local].findIndex(x =&gt; x === '+');
    if (iPlus !== -1) { local = local.substr(0, iPlus); }
    const key = [...local].filter(x =&gt; x !== '.').join() + '@' + domain;
    if (!validMails.has(key)) { validMails.add(key); }
  }
  return validMails.size;
};


My solution with regex

/**
 * @param {string[]} emails
 * @return {number}
 */
var numUniqueEmails3 = emails =&gt; {
  const validMails = new Set();
  for (const mail of emails) {
    let [local, domain] = mail.split('@');
    local = local.replace(/\+(.*)$/, '')
      .replace(/\./g, '');
    console.log(local);
    const key = `${local}@${domain}`;
    if (!validMails.has(key)) { validMails.add(key); }
  }
  return validMails.size;
};


Addendum

in numUniqueEmails ran the code with this snippet:

  const names = mail.split('@');
  let [local, domain] = names;


and got 90th percentile. Running it again with this snippet:

  let [local, domain] = mail.split('@');


gives me 25th percentile. I'd assume it should be the opposite. Does anyone know why this is the case and also how come the difference is so big?
I don't know what you mean by declarative in this case. All these functions are pure (except the console.log), but the implementations of the functions are all imperative (which is fine of course)
When adding to a set, you don't need to check if the key exists first. Just add it.
Splitting the string into an array of chars isn't necessary. You can use string utils instead, for example indexOf('+'). You can also do a split('+')[0] to avoid having the iPlus variable.
If you swap the reduce for a map, and wrap that in a Set, you don't need to add to set explicitly. It's probably a bit slower though.
You could add some more newlines to the code to group related concepts, and make it more readable


I came up with the solution below:

const getUniqueEmailKey = email =&gt; {
    const [local, domain] = email.split('@')
    return local
        .split('+')[0] // Take everything before +
        .split('.').join('') // Remove dots
        + '@' + domain
}

const numUniqueEmails = emails =&gt; new Set(emails.map(getUniqueEmailKey)).size


About leetcode:
Try running the code a few more times. The time you get is a bit random...