How to decode certificate extension value?

[jdai-godaddy]

No description provided.

[El-Virus]

Assuming you're talking about X.509 certificates:
It depends on the extension, as different extensions have different value types. Take a look at: RFC 5280 § 4.2.
It is worth noting that the following extensions, have already been decoded and if present can be accessed as: certObj.attributes.extensions: keyUsage, basicConstraints, extKeyUsage, nsCertType, subjectAltName, issuerAltName, subjectKeyIdentifier. Non-decoded extensions are also stored in the same array.