Understanding how LitElement handles data is crucial for building interactive and maintainable web components. Reactive properties are special fields on your component that LitElement observes for changes. When you declare a property as reactive, LitElement automatically tracks its value. If the property changes, the component updates its DOM to reflect the new state. This ensures your UI always matches your underlying data.

Attribute reflection refers to the synchronization between HTML attributes and JavaScript properties. In LitElement, you can configure properties to reflect their values as attributes on the host element. This is especially useful for interoperability with plain HTML and allows you to set values via markup or JavaScript interchangeably. LitElement handles type conversion for you: when an attribute is set via HTML, LitElement converts the string value to the correct JavaScript type for your property, and vice versa when reflecting a property change back to an attribute.

When you define a property in your component’s static properties getter, you can specify options such as type and reflect. The type option tells LitElement how to convert between attribute strings and property values. The reflect option, when set to true, ensures that changes to the property are mirrored as updates to the corresponding HTML attribute. This two-way synchronization is essential for creating robust, reusable components that work well with both JavaScript and HTML.