satisfies, as and annotations — three ways to state a type

Level: 201 · for TypeScript users choosing how to state a type

Stub — an outline, not a lesson. There is no runnable example behind this page yet, so nothing on it has been through the check that backs every other claim in this library. The questions below are what the finished page has to answer.

One line: An annotation checks an object and forgets its details, satisfies checks it and keeps them, and as barely checks: { red: [255, 0, 0] } as Palette compiles without the required green, which satisfies Palette reports as TS2741.

Keywords: satisfies, as

What the finished page will answer

• What type does green have in an object declared : Palette and in one declared satisfies Palette, and which of them allows green.toUpperCase()?
• Why does { red: [255, 0, 0] } as Palette compile without green, and what does reading green give at run time?
• When does as refuse, as TS2352 does for 42 as string, and what does as unknown as do about it?
• Do satisfies and an annotation both report an extra property such as blue (TS2353)?
• What does as const satisfies Palette add over satisfies Palette alone?

Examples it will need

• satisfies_as_annotation_tserror.ts — TS2339 on the annotated object's green, nothing on the satisfies one, nothing on the as one that lacks green, and TS2741 when satisfies meets the same missing property
• satisfies_as_missing_property_ts.ts — the undefined that an as-asserted object holds where green should be

See also

• Annotations and inference — when to write a type at all
• Type assertions — how far an as can lie
• Primitive and literal types — the literal types satisfies keeps and an annotation widens
• Excess property checks — the extra-property check both forms apply
• Rust: Casting with as ↗ — an as in Rust converts the value; TypeScript's converts nothing

Sources to start from

• TypeScript 4.9 release notes — the satisfies operator ↗
• TypeScript Handbook — Everyday Types: type assertions ↗