Make last digit as highest prime less than given digit .
if ( prevprime [ N [ size - 1 ] . charCodeAt ( 0 ) - ' ' . charCodeAt ( 0 ) ] + ' ' . charCodeAt ( 0 ) != ' ' . charCodeAt ( 0 ) ) ans [ size - 1 ] = String . fromCharCode ( prevprime [ N [ size - 1 ] . charCodeAt ( 0 ) - ' ' . charCodeAt ( 0 ) ] + ' ' . charCodeAt ( 0 ) ) ;