Generate random password based on user input in Javascript

Question

The assignment is to prompt for length of the password and character type from the user, then generate a random password. I think the for loop isn't working correctly. The retVal is returned empty because the for loop isn't passing it anything. I tried removing the charAt function and having the Math.floor give me just and index, that just gave me undefinedundefinedundefinedundefinedundefinedundefined. Back with the regular charAt function I'm getting nothing.

//ask for length
var length = prompt("How many characters will your password be? Enter a number between 8 and 128");

//ask for character type
var charType = prompt("Enter a character type: special, numeric, uppercase, lowercase.");

//generate password
function generatePassword() {
  //evaluate character type
  var charSet = "";
  if( charType.toLowerCase === "lowercase" ) {
    charSet = "abcdefghijklmnopqrstuvwxyz";
  } else if( charType.toLowerCase === "uppercase" ) {
    charSet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  } else if( charType.toLowerCase === "numeric" ) {
    charSet = "0123456789";
  } else if( charType.toLowerCase === "special" ) {
    charSet = " !\"#$%&'()*+,-./:;<=>?@[\]^_`{|}~";
  } 
  //return value
  var retVal = "";
  //for (var i = 0, n = charSet.length; i < length; i++) {
    for (var i = 0, n = length; i < length; i++) {
    //picks a character within charSet at index of random number
    retVal += charSet.charAt(Math.floor(Math.random() * n));
  }
  console.log(retVal);
  return retVal;
}

Is there also a way to take an answer that fits multiple criteria and generate a password? Such as one that had uppercase letters and special characters? — May Hitchings
– May Hitchings, Commented Dec 27, 2019 at 21:42
Well yes, If you have multiple strings like "abc","ABC","1@3$" you could do something like password=abc[2]+ABC[1]+123[2] . This is just an example. You can randomize it using Math.random for indexes . — Saurav
– Saurav, Commented Dec 27, 2019 at 21:52

Alexander O'Mara · Accepted Answer · 2019-12-27 18:21:47Z

1

There are a couple of subtle issues you are having.

prompt returns a string, you will need to cast it to a number to use it for your length (Number(prompt(...))).
The string toLowerCase is a method, not a property, you have to call it (charType.toLowerCase()). You also only need to do this once, if you set it to a variable you can avoid re-computing it.
You want a random character in the full charset range, not the password length (using charSet.length).

var length = Number(prompt("How many characters will your password be? Enter a number between 8 and 128"));

//ask for character type
var charType = prompt("Enter a character type: special, numeric, uppercase, lowercase.");

//generate password
function generatePassword() {
  //evaluate character type
  var charSet = "";
  var charTypeLower = charType.toLowerCase();
  if( charTypeLower === "lowercase" ) {
    charSet = "abcdefghijklmnopqrstuvwxyz";
  } else if( charTypeLower === "uppercase" ) {
    charSet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  } else if( charTypeLower === "numeric" ) {
    charSet = "0123456789";
  } else if( charTypeLower === "special" ) {
    charSet = " !\"#$%&'()*+,-./:;<=>?@[\]^_`{|}~";
  } 
  //return value
  var retVal = "";
  for (var i = 0; i < length; i++) {
    //picks a character within charSet at index of random number
    retVal += charSet.charAt(Math.floor(Math.random() * charSet.length));
  }
  return retVal;
}
alert(generatePassword());

Side Note:

I'm guessing this is just for learning purposes, but if you want to generate cryptographically secure passwords you should use a random number generator based on crypto.getRandomValues (see this question).

edited Dec 27, 2019 at 18:21

answered Dec 27, 2019 at 18:14

Alexander O'Mara

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2 Comments

PM 77-1 Over a year ago

I have a tiny DYI issue with your code. Can you please make a change to calculate charType.toLowerCase() only once?

Alexander O'Mara Over a year ago

@PM77-1 There's plenty of room for improvement but sure.

jefi · Accepted Answer · 2019-12-27 18:23:32Z

0

charType.toLowerCase is a function, what you want is charType.toLowerCase(), which is the result of the function.

edited Dec 27, 2019 at 18:23

answered Dec 27, 2019 at 18:16

jefi

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