Code:

// Check that the achievement hasn't been claimed before.
ChilliConnect.Logger.info(achievementsUserData);
ChilliConnect.Logger.info(idToClaim.toString());
ChilliConnect.Logger.info(achievementsUserData[idToClaim.toString()]);

for(var i in achievementsUserData)
{
    ChilliConnect.Logger.info(i);
}

if (achievementsUserData[idToClaim.toString()].completiondate != "0")
    return false;

Output:
1   {"2210":{"progress":0,"completiondate":"0"},"2211":{"progress":1,"completiondate":"Mon Oct 21 2019"},"2212":{"progress":1,"completiondate":"0"},"2216":{"progress":1,"completiondate":"0"},"2220":{"progress":0,"completiondate":"0"}}
2   "2211"
3   null
4   "$2210"
5   "$2211"
6   "$2212"
7   "$2216"
8   "$2220"
9   "An Error has occured."
10  "Error:TypeError: Cannot read property 'completiondate' of undefined"