Javascript function which produces random strings

randstr.js

/**
 * Constructs a random string with the given length and character set.
 *
 * If no character set is provided, default character set will be used.
 */
function randstr (length, charset) {
    // (Re)define the charset:
    charset = charset || 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';

    // construct and return:
    return Array.apply(null, Array(length)).map(function (x) {
        return charset[Math.floor(Math.random() * charset.length)];
    }).join("");
}