Structural typing and assignability

Shape, not name

TypeScript compares types by structure. Two separately declared types with the same members are interchangeable, and a value is assignable anywhere its shape satisfies the target — the name it was declared under is irrelevant.

The exception is excess property checking on object literals assigned directly to a typed target, which catches typos in configuration objects. Assign through a variable first and the check does not apply, which surprises people regularly.

When two structurally identical types must not mix — a user id and an order id, both strings — add a phantom property to brand them. The compiler then treats them as distinct even though the runtime value is unchanged.