function join(strs, separator) {
        // undefined inputs always return undefined
        if (typeof strs === 'undefined') {
            return undefined;
        }

        // if separator is not specified, default to empty string
        if (typeof separator === 'undefined') {
            separator = "";
        }

        return strs.join(separator);
    }
Join an array of strings
@param {Array} strs - array of string
@param {String} [separator] - the token that splits the string
@returns {String} The concatenated string