What is the difference between Object.freeze(), Object.seal(), and Object.preventExtensions()?

Direct answer

preventExtensions blocks new own properties, seal additionally makes existing properties non-configurable, and freeze additionally makes existing data properties non-writable; all three are shallow operations.

Object.preventExtensions(obj) — no new own properties can be added, but existing configurable/writable properties can still be changed or removed according to their descriptors.
Object.seal(obj) — prevents extensions and makes existing own properties non-configurable, so they cannot normally be deleted or reconfigured; writable data properties can still change value.
Object.freeze(obj) — seals the object and makes existing data properties non-writable, providing the strongest of these three shallow restrictions.

“Shallow” matters: Object.freeze({settings:{theme:"dark"}}) does not automatically freeze the nested settings object. A deep-freeze utility must traverse the graph and handle cycles deliberately.

Freezing also does not mean all observable state reachable from the object can never change; accessor functions or referenced objects can still encapsulate mutable state.