If the current digit is the missing digit , then keep the track of its sign
if ( c [ i ] == ' ' ) { xSignIsPositive = ( w == 1 ) ; } else { c_mod_11 = ( c_mod_11 + w * ( c [ i ] . charCodeAt ( 0 ) - ' ' . charCodeAt ( 0 ) ) ) % 11 ; } w = w * - 1 ; }