How to use the node-opcua-hostname.get_fully_qualified_domain_name function in node-opcua-hostname

}

util.inherits(OPCUAClient, OPCUAClientBase);


OPCUAClient.prototype._nextSessionName = function () {
    if (!this.___sessionName_counter) {
        this.___sessionName_counter = 0;
    }
    this.___sessionName_counter += 1;
    return this.clientName + this.___sessionName_counter;
};

const makeApplicationUrn = require("node-opcua-common").makeApplicationUrn;
const hostname = require("node-opcua-hostname").get_fully_qualified_domain_name();

OPCUAClient.prototype._getApplicationUri = function () {

    // get applicationURI from certificate
    const exploreCertificate = require("node-opcua-crypto").exploreCertificate;

    const certificate = this.getCertificate();
    let applicationUri;
    if (certificate) {
        const e = exploreCertificate(certificate);
        if (!e.tbsCertificate.extensions  || !e.tbsCertificate.extensions.subjectAltName) {
            console.log(chalk.red(" Warning: client certificate is invalid : subjectAltName is missing"));
            applicationUri = makeApplicationUrn(hostname, this.applicationName);
        } else {
            applicationUri = e.tbsCertificate.extensions.subjectAltName.uniformResourceIdentifier[0];
        }

constructor(options) {

        const default_certificate_file = constructFilename("certificates/server_selfsigned_cert_2048.pem");
        options.certificateFile = options.certificateFile || default_certificate_file;
        assert(fs.existsSync(options.certificateFile));

        const default_private_key_file = constructFilename("certificates/PKI/own/private/private_key.pem");
        options.privateKeyFile = options.privateKeyFile || default_private_key_file;
        assert(fs.existsSync(options.certificateFile));

        const defaultApplicationUri = makeApplicationUrn(get_fully_qualified_domain_name(), "NodeOPCUA-DiscoveryServer");

        super(options);

        const serverInfo = options.serverInfo || {};

        serverInfo.applicationType = ApplicationType.DiscoveryServer;
        serverInfo.applicationUri = serverInfo.applicationUri || defaultApplicationUri;
        serverInfo.productUri = serverInfo.productUri || "SampleDiscoveryServer";
        serverInfo.applicationName = serverInfo.applicationName || {text: "SampleDiscoveryServer", locale: null};
        serverInfo.gatewayServerUri = serverInfo.gatewayServerUri || "";
        serverInfo.discoveryProfileUri = serverInfo.discoveryProfileUri || "";
        serverInfo.discoveryUrls = serverInfo.discoveryUrls || [];

        this.serverInfo = serverInfo;

        const port = options.port || 4840;