Random alpha-numeric string in JavaScript?

I just came across this as a really nice and elegant solution:

Math.random().toString(36).slice(2)

Notes on this implementation:

• This will produce a string anywhere between zero and 12 characters long, usually 11 characters, due to the fact that floating point stringification removes trailing zeros.
• It won't generate capital letters, only lower-case and numbers.
• Because the randomness comes from Math.random(), the output may be predictable and therefore not necessarily unique.
• Even assuming an ideal implementation, the output has at most 52 bits of entropy, which means you can expect a duplicate after around 70M strings generated.

If you only want to allow specific characters, you could also do it like this:

function randomString(length, chars) {    var result = '';    for (var i = length; i > 0; --i) result += chars[Math.floor(Math.random() * chars.length)];    return result;}var rString = randomString(32, '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ');

Here's a jsfiddle to demonstrate: http://jsfiddle.net/wSQBx/

Another way to do it could be to use a special string that tells the function what types of characters to use. You could do that like this:

function randomString(length, chars) {    var mask = '';    if (chars.indexOf('a') > -1) mask += 'abcdefghijklmnopqrstuvwxyz';    if (chars.indexOf('A') > -1) mask += 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';    if (chars.indexOf('#') > -1) mask += '0123456789';    if (chars.indexOf('!') > -1) mask += '~`!@#$%^&*()_+-={}[]:";\'<>?,./|\\';    var result = '';    for (var i = length; i > 0; --i) result += mask[Math.floor(Math.random() * mask.length)];    return result;}console.log(randomString(16, 'aA'));console.log(randomString(32, '#aA'));console.log(randomString(64, '#A!'));

Fiddle: http://jsfiddle.net/wSQBx/2/

Alternatively, to use the base36 method as described below you could do something like this:

function randomString(length) {    return Math.round((Math.pow(36, length + 1) - Math.random() * Math.pow(36, length))).toString(36).slice(1);}

Another variation of answer suggested by JAR.JAR.beans

(Math.random()*1e32).toString(36)

By changing multiplicator 1e32 you can change length of random string.