Generate secure and readable password

generatePassword.js

// just translated from the php version (I have to optimise it)

function generatePassword (length) {
    if(!length) length = 8;
    var sets = ['abcdefghjkmnpqrstuvwxyz', 'ABCDEFGHJKMNPQRSTUVWXYZ', '23456789', '~!@#$^*()_.-=+'];
    var all = '';
    var password = '';
    sets.forEach(function(set){
        set = shuffle(set);
        password += set[0];
        all += set;
    });
    for(var i=0; i<length-sets.length; i++) {
        all = shuffle(all);
        password += all[0];
    }
    password = shuffle(password);
    return password; //'Uc74T?y$'
}

function shuffle (str) {
    var a = str.split(""), n = a.length;
    for(var i = n - 1; i > 0; i--) {
        var j = Math.floor(Math.random() * (i + 1));
        var tmp = a[i];
        a[i] = a[j];
        a[j] = tmp;
    }
    return a.join("");
}

just translated from the php version (I have to optimise it)