JS: Как проверить, не является ли переменная неопределенной [duplicate]

Из $ man curl:

--cert-type <type>
    (SSL) Tells curl what certificate type the provided  certificate
    is in. PEM, DER and ENG are recognized types.  If not specified,
    PEM is assumed.

    If this option is used several times, the last one will be used.

--cacert <CA certificate>
    (SSL) Tells curl to use the specified certificate file to verify
    the peer. The file may contain  multiple  CA  certificates.  The
    certificate(s)  must be in PEM format. Normally curl is built to
    use a default file for this, so this option is typically used to
    alter that default file.