Always sanitize users' content that comes from forms.
Always use serialize instead of JSON.stringify.
Use dangerouslySetInnerHTML only when absolutely necessary.
Do unit tests for your components, and try to cover all the possible XSS attacks (we are going to see unit tests in Chapter 12, Testing and Debugging).
Always encrypt passwords with sha1 and md5, and do not forget to add a salt value (for example, if the password is abc123, then your salt can be encrypted like this: sha1(md5('$4ltT3xt_abc123')).
If you use cookies to store sensitive information (personal information and passwords mainly), you can save the cookie with Base64 to obfuscate the data.
Add some protection to your API (security tokens) unless you need to be public. There is a recipe about security tokens in Chapter 8, Creating an API with Node.js Using MongoDB and MySQL.