Composing Types

Build bigger types out of smaller ones rather than repeating yourself.

& intersects: type Timestamped = { created: string } and then Book & Timestamped.

extends does the same for interfaces.

The point is the same as normalization in the SQL course: describe each fact once, so correcting it corrects everything.

Try it

Add a field to Entity and watch both Book and Author require it. That is the refactoring payoff, in miniature.

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checking…

Author is missing created, and the compiler says so. One change to Entity now propagates to both.

◈ Graded Types

Define a type called Book with a required title (string), a required year (number), an optional subtitle (string), and a readonly id (string). Then define a type called Shelf with a name (string) and a books property that is an array of Book. Finally create a Shelf called shelf containing exactly one book titled Kindred from 1979 with id "b1", named "Fiction". Everything must type-check in strict mode.

Checked against the compiler, not against how you wrote it. 3 visible and 3 hidden checks.

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real tsc, strict mode

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