Type guard iliyofafanuliwa na mtumiaji ni function ambayo aina yake ya kurejeshwa ni type predicate (x is T). Inaporejesha true, kompaila huzuia hoja kuwa T katika msimbo unaopiga simu — ikikuwezesha kufunga vipimo maalum vya wakati wa utekelezaji (runtime checks).
interface Cat { meow(): void; }
interface Dog { bark(): void; }
// the magic is the return type `pet is Cat`, not just `boolean`
function isCat(pet: Cat | Dog): pet is Cat {
return "meow" in pet;
}
function speak(pet: Cat | Dog) {
if (isCat(pet)) {
pet.meow(); // ✅ narrowed to Cat
} else {
pet.bark(); // ✅ narrowed to Dog
}
}
Bila predicate ya pet is Cat, isCat inayorejesha boolean isingeweza kuzuia pet — kompaila bado ingeona Cat | Dog ndani ya if. Predicate ndiyo inayoifundisha.
interface User { id: number; name: string; }
function isUser(x: unknown): x is User {
return (
typeof x === "object" && x !== null &&
typeof (x as any).id === "number" &&
typeof (x as any).name === "string"
);
}
const data: unknown = await res.json();
if (isUser(data)) data.name; // ✅ safely typed as User
Kompaila huamini predicate yako — ikiwa mantiki ya mwili (body) ni mbaya, unapata uzuiaji usio salama. Kwa miundo changamano, kithibitishaji cha schema (zod) hutengeneza guards sahihi kwa niaba yako.
Type guards zinakuruhusu kugeuza vipimo vyovyote vya wakati wa utekelezaji kuwa functions za kuzuia (narrowing) zinazoweza kutumika tena — muhimu kwa kuthibitisha data ya unknown katika mipaka ya API na kwa kutofautisha wanachama wa muungano (union) wakati kipimo rahisi cha typeof/in hakitoshi peke yake.