How to determine if a value of type unknown is a specific named type? How to make TypeScript understand that the check has been performed (type guard)?

To determine that a value of type unknown matches a specific named type in TypeScript, you need to write a type guard function. Such a function checks the structure or properties of an object and informs the compiler that after the check, the value has the desired type.

Example: suppose there is an interface

interface User {
  name: string;
  age: number;
}

And there is a value value: unknown. To check that value is a User, we write a function:

type User = { name: string; age: number };

function isUser(value: unknown): value is User {
  return typeof value === 'object' && value !== null &&
         'name' in value && typeof (value as any).name === 'string' &&
         'age' in value && typeof (value as any).age === 'number';
}

Here, value is User is a special syntax that tells TypeScript that if the function returns true, then value can be considered of type User.

Usage:

const data: unknown = getData();

if (isUser(data)) {
  // Here, TypeScript knows that data is a User
  console.log(data.name);
} else {
  // data is not a User
}

Thus, a type guard allows safe working with the unknown type by refining it to a specific named type.