Compatible shapes, excess properties and shallow readonly

First, build a mental model

A structural contract lists what a value must provide. It usually does not promise that the value has no other fields, and readonly restricts a typed access path rather than freezing an object.

1. A value with required compatible properties can satisfy an interface.
2. Fresh object literals receive extra excess-property checks that catch likely typos.
3. Intersections require all contracts at once; conflicting properties can become never.
4. Readonly is shallow unless nested contracts are also readonly. It does not freeze runtime objects.

interface Player { id: string; score: number }
const stored = { id: 'p1', score: 10, debug: true };
const player: Player = stored;
const profile: Readonly<{ stats: { score: number } }> = { stats: { score: 10 } };
profile.stats.score = 20;
// profile.stats = { score: 30 }; // blocked through this access path
type Impossible = { id: string } & { id: number };

1. Structural match

   stored has the Player fields, even though it also has debug.

2. Readonly

   Replacing stats is restricted; modifying its mutable nested score is not.

3. Intersection

   A shared id would need to be both string and number, producing never.