[zamadatix]

You shouldn't have to port anything. If you mean in the opposite case of two separate languages between client and server side then yeah, of course - by definition you're rewriting everything and there is no way to reuse code. I'm not clear how you're reaching anywhere near 50x complexity though. You're writing something like this on the client side (please excuse the lazy checks):

  function basicValidatePassword(password) {
    const minLength = 8;
    const maxLength = 64;
    const isValidChars = /^[A-Za-z0-9]+$/;
    const hasUpperCase = /[A-Z]/;
    const hasLowerCase = /[a-z]/;
    const hasSpecialChar = /[\W_]/;

    const results = {minLength: password.length >= minLength,
      maxLength: password.length <= maxLength,
      hasUpper: hasUpperCase.test(password),
      hasLower: hasLowerCase.test(password),
      hasSpecial: hasSpecialChar.test(password),
      hasValidChars: isValidChars.test(password)}
    results.isValid = Object.values(results).every(item => item);

    return results;
  }

Then instead of writing another one on the server that only checks the password isn't blank, is less than the maximum, and has valid characters you're just reusing the full 6 check code. That's only twice as many checks, not even twice as many lines, and it's already written. You really should check all 6 again on the server anyways, but that's beside the point. Better still, if you do the reuse as a build step via shared function library file or similar you don't need to copy/paste and it stays in sync automatically.

Of particular note there is no UI code here because the meter's UI code is not related to the check function beyond it reads the return value.

[Dylan16807]

If that's all you want then sure, but that's not what I would call a good password quality meter. It makes no attempt to look for patterns or words or super-common passwords.

[zamadatix]

As noted excuse the basic check functions and use whatever you actually want for check criteria and the amount of work on the server side is still a factor <1 compared to writing that and then a different check on the server. If your password check logic is 50x the size of that though you might be overdoing it, but that's just an opinion. Again I'd argue you should really be validating server side as well anyways, fewer chances to mess something up and accept a weak password.

[Dylan16807]

Your check functions are fine, for both client and server.

I'm not saying not to reuse things, because I specifically think it should be two separate functions on the client, one of which is copied to the server. But if you insist on having only one client function, I think the server function should be cut down.

And the premise is doing client-only advice on strength so I'm not going to challenge that premise.

As far as 50x, your code doesn't need those consts saying the exact same thing as the results object, so that simplifies to 8 lines, and I think 400 lines for a good password estimator isn't unreasonable. zxcvbn's scoring function is around that size.