short_url.js

var shortUrlKey = function(salt,l,chars){
    var out = '',
        n = chars.length,
        c,
        i = 1;
    while (i <= l) {
        if (l > 0) {
            c = ((salt / i) % n) >> 0;
            out = chars[c] + out;
        }
        i++;
    }
    return out;
};
var i = 10;
var l = i+100;
var str = [];
var chars = 'abcdefghijklmnopqrstu0123456789'.split('');
var keyLength = 4;
while(i < l) {
    str.push(shortUrlKey (i,keyLength,chars));
    i++;
}
console.log(str);