Securing an ExpressJS web application with Helmet

Helmet allows to protect web server applications against common attacks, such as cross-site scripting (XSS), insecure requests, and clickjacking.

Helmet is a collection of 12 middleware functions that allow you to set specific HTTP headers:

1. Content Security Policy (CSP): This is an effective way to whitelist what kind of external resources are allowed in your web application, such as JavaScript, CSS, and images, for instance.
2. Certificate Transparency: This is a way of providing more transparency for certificates issued for a specific domain or specific domains https://sites.google.com/a/chromium.org/dev/Home/chromium-security/certificate-transparency.
3. DNS Prefetch Control: This tells the browser if it should perform domain name resolution (DNS) on resources that are not yet loaded, such as links.
4. Frameguard: This helps to prevent...