Understanding TypeScript's `satisfies` vs `as`

[Lomanu4]

In TypeScript, the satisfies operator is a safer, more precise alternative to as when you want to ensure a value conforms to a type without losing type inference.

Key Differences

Feature	as	satisfies
Type checking	Not enforced	Enforced
Keeps extra properties	No
Narrows const types	No
Safer for configuration objects	No

1. Keeping Extra Properties

Using as (unsafe):


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

const user = {
name: 'Alice',
age: 30,
email: 'alice@example.com',
} as Person; // No error, but `email` is lost from the type
Using satisfies (safe):


const user = {
name: 'Alice',
age: 30,
email: 'alice@example.com',
} satisfies Person;
// TS checks that name and age exist and retains `email` in the inferred type
2. Narrowing const Types

Using as:


const colors = ['red', 'green', 'blue'] as string[];
Using satisfies:


const colors = ['red', 'green', 'blue'] satisfies readonly ['red', 'green', 'blue'];
3. Ensuring Correct Object Shape

Using as (can lie):


type Config = { port: number };

const config = {
port: '3000',
} as Config; // No error, even though it's wrong!
Using satisfies (type checked):


const config = {
port: '3000',
} satisfies Config; // Type error: 'string' is not assignable to 'number'
4. With Utility Types


type Route = {
path: string;
name: string;
};

const routes = {
home: { path: '/', name: 'Home' },
about: { path: '/about', name: 'About' },
} satisfies Record<string, Route>;
Conclusion


Use satisfies when you want strong type validation without losing inferred details. It's ideal for configuration objects, constant arrays,

Пожалуйста Авторизируйтесь или Зарегистрируйтесь для просмотра скрытого текста.