TypeScript · Narrowing and unions

Type predicate

A user-defined guard that teaches TypeScript how a function narrows its argument.

Syntax

function isUser(v: unknown): v is User

Example

type User = { id: string };
function isUser(value: unknown): value is User {
  return typeof value === "object" && value !== null && "id" in value;
}
console.log(isUser({ id: "u1" }));

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Type predicate

Example

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