The "localStorage is not defined" error arises when attempting to access the localStorage object in JavaScript within environments that do not support it, such as Node.js, Next.js, React.js, or older browsers. localStorage is a web storage API designed for storing key-value pairs in web browsers, enabling persistent data retention on the client side. To address this error, it is essential to verify that the JavaScript code runs in a compatible web browser environment. Additionally, ensuring browser compatibility and implementing fallback mechanisms like using sessionStorage or cookies can help manage situations where localStorage is unavailable. Using tools like Rollbar can further assist in tracking, analyzing, and managing errors, facilitating smoother and more confident deployment of web applications across diverse environments.