Do school mathematics , compute sum of current digits and carry
let sum = ( ( str1 [ i ] . charCodeAt ( 0 ) - ' ' . charCodeAt ( 0 ) ) + ( str2 [ i ] . charCodeAt ( 0 ) - ' ' . charCodeAt ( 0 ) ) + carry ) ; str += String . fromCharCode ( sum % 10 + ' ' . charCodeAt ( 0 ) ) ;