If a property of an object is sealed, what can you not do?

When a property of an object is sealed, it signifies that the object cannot be modified in several ways. The primary implications of sealing an object include:

1. Adding New Properties: Sealing an object prevents you from adding new properties to it. Once an object is sealed, you can no longer introduce any new keys or fields.

2. Changing Existing Property Values: Sealed properties can have their values changed unless they are also marked as non-writable. However, any changes made to properties that aren't non-writable are permissible, meaning this point alone might seem misleading based on specific property settings.

3. Deleting Properties: Sealing an object specifically prohibits the deletion of any existing properties. This means you cannot remove or unset any of the properties present within the object.

By understanding these constraints, it becomes clear that sealing an object imposes restrictions on altering its structure or content. This is why the assertion that you cannot add new properties, change existing property values (in the context of non-writable properties), or delete properties is pivotal. The collective impossibility of all these actions characterizes the effect of sealing an object. Therefore, the correct conclusion is that all these modifications are disallowed with a sealed object.