pwg.js a strong password generator

pwg.js

function pwg(length, option) {
    var pw = "",
        arr = [],
        isOpera = false,
        chars = [
            "abcdefghijklmnopqrstuvwxyz",
            "1234567890",
            "!@#$%&*()_+}{:?-=}[;/.,]",
            "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        ],
        random = chars.join(''),
        onlyLetters = chars[0] + chars[3];

    if (window.crypto && window.crypto.getRandomValues) {
        arr = new Uint32Array(length);

        window.crypto.getRandomValues(arr);
    } else if (navigator.userAgent.indexOf('Opera') > -1) {
        isOpera = true; 
    } else { 
        throw new Error("Your browser can't generate secure random numbers");
    }

    function randomNum(len, opt) {
        var res = "", m = "";

        opt === false ? m = onlyLetters : m = random; 

        for (var i = 0; i < len; i++) {
            isOpera ? res += m[Math.floor(Math.random() * m.length)] : res += m[arr[i] % m.length];
        }

        return res;
    } 

    return randomNum(length, option);
}

This has an ever-so-slight bias in the all characters case towards the first 16 letters since Math.pow(2, 32) % random.length is 16, and towards the first 48 letters in the onlyLetters case.