JavaScript developers must master security skills to safeguard user data, prevent breaches, and maintain user trust. Key skills include understanding secure coding practices, such as input validation, output encoding, and applying the principle of least privilege; threat modeling to identify vulnerabilities early; and using secure libraries and frameworks. Additionally, skills for fixing security vulnerabilities include familiarity with common vulnerabilities, leveraging security testing tools, and strong debugging for root cause analysis. Embracing a security-first mindset and integrating security into DevOps practices further enhances application security.