Generate random password string with 5 letters and 3 numbers in JavaScript

I want to generate a random string that has to have 5 letters from a to z and 3 numbers.

How can I do this with JavaScript?

I've got the following script, but it doesn't meet my requirements.

var chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
var string_length = 8;
var randomstring = '';
for (var i=0; i<string_length; i++) {
    var rnum = Math.floor(Math.random() * chars.length);
    randomstring += chars.substring(rnum,rnum+1);
}

Update 2025:

It's been 13 years since I first asked this question as a complete beginner—both in tech and in the working world. I'm amazed at how many discussions and different answers it has sparked over the years.

I’d like to clarify now that the code in the original post was simply meant to generate a one-time password (OTP) for a single-use scenario. It was sent via email and only needed to live for about 2–3 minutes, so strong randomness or cryptographic methods weren’t really a concern.

Still, it's been fun to stop by now and then to see all the creative suggestions. Thanks to everyone who contributed—it's been a great ride!

Solution

Forcing a fixed number of characters is a bad idea. It doesn't improve the quality of the password. Worse, it reduces the number of possible passwords, so that hacking by bruteforcing becomes easier.

To generate a random word consisting of alphanumeric characters, use:

var randomstring = Math.random().toString(36).slice(-8);

How does it work?

Math.random()                        // Generate random number, eg: 0.123456
             .toString(36)           // Convert  to base-36 : "0.4fzyo82mvyr"
                          .slice(-8);// Cut off last 8 characters : "yo82mvyr"

Documentation for the Number.prototype.toString and string.prototype.slice methods.