An optional property and a property with an undefined value appear similar but are fundamentally different. If an object lacks a property, accessing it yields undefined. Similarly, if a property exists but its value is explicitly undefined, accessing it also yields undefined. However, the in operator reveals the underlying distinction: value in object is false when the property is absent and true when it is present with an undefined value. In TypeScript, an optional property denoted by ? signifies that the property may not exist on the object. This distinction is crucial for APIs where omitting a property means "no change," but explicitly setting it to undefined signifies a different intent. The exactOptionalPropertyTypes compiler option enforces this difference, disallowing explicit assignment of undefined to optional properties unless | undefined is explicitly added to the type. This precise typing is valuable for configuration objects and update mechanisms where property presence carries semantic meaning. For instance, an update function might ignore omitted fields but process fields set to undefined differently. Understanding this difference is vital for robust API design and accurate object manipulation.