It involves making an HTTPS request of the HEAD type and comparing the expiration date of the certificate obtained from the response object and the successful status of the SSL transaction (handshake) as reported by the underlying socket.

checkCertificate.js

'use strict';

const https = require('https');
const validator = require('validator');


const getDaysBetween = (validFrom, validTo) => {
    return Math.round(Math.abs(+validFrom - +validTo) / 8.64e7);
};

const getDaysRemaining = (validFrom, validTo) => {
    const daysRemaining = getDaysBetween(validFrom, validTo);
    if (new Date(validTo).getTime() < new Date().getTime()) {
        return -daysRemaining;
    }
    return daysRemaining;
};

const getSSLCertificateInfo = host => {
    if(!validator.isFQDN(host)) {
        return Promise.reject(new Error('Invalid host.'));
    }
    const options = {
        agent: false,
        method: 'HEAD',
        port: 443,
        rejectUnauthorized: false,
        hostname: host
    };

    return new Promise((resolve, reject) => {
        try {
            const req = https.request(options, res => {
                const crt = res.connection.getPeerCertificate(),
                    vFrom = crt.valid_from, vTo = crt.valid_to;
                var validTo = new Date(vTo);
                resolve({
                    daysRemaining: getDaysRemaining(new Date(), validTo),
                    valid: res.socket.authorized || false,
                    validFrom: new Date(vFrom).toISOString(),
                    validTo: validTo.toISOString()
                });
            });
            req.on('error', reject);
            req.end();
        } catch (e) {
            reject(e);
        }
    });
};