not-random.js

var ALPHABET = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_';

random_base64 = function random_base64(length) {
    var str = "";
    for (var i=0; i < length; ++i) {
        var rand = Math.floor(Math.random() * ALPHABET.length);
        str += ALPHABET.substring(rand, rand+1);
    }
    return str;
}

var stringLength = 5; // because I don't have all day
var i;
var nums = [];
var safety = 100000;

console.log('Generating random strings with', Math.pow(ALPHABET.length, stringLength).toLocaleString(), 'combinations');

for (i = 0; i < safety; i++) {
    var num = random_base64(stringLength);

    if (nums.indexOf(num) > -1) {
        console.log(num, 'was generated twice after', i.toLocaleString(), 'tries');
        break;
    }

    nums.push(num);
}

Run this in your console (or Chrome Dev Tools snippet). You'll get something interesting like this:

Generating random strings with 1,073,741,824 combinations
sH1Ru was generated twice after 43,132 tries