Advanced · 18 min

Literal configurations

Check configuration shapes while preserving useful literal types.

Validate with satisfies

The "satisfies" operator checks that an expression fits a contract while retaining its useful inferred type. Here the literal mode stays specific. It does not validate data arriving at runtime.

type Mode = "focus" | "review";
const settings = { mode: "focus", minutes: 20 } satisfies { mode: Mode; minutes: number };
const selected: "focus" = settings.mode;
console.log(selected);

Output

focus

Derive choices from constant data

A const assertion preserves literal values and makes literal object properties and array entries readonly in the type system. It does not freeze objects at runtime. Derive a union from the tuple so allowed choices stay aligned with the data.

const topics = ["Travel", "Music"] as const;
type Topic = typeof topics[number];
const selected: Topic = "Music";
console.log(selected);

Output

Music

Put it into practice

Predict the output and explain which relationships the types enforce.
Fix the task while preserving its type contract, then check both practice questions.

Try it yourself

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Correct a configuration

Replace the invalid mode with "focus". Keep "satisfies" and the specific type of "selected".

Your TypeScript code

Show solution

type Mode = "focus" | "review";
const settings = { mode: "focus", minutes: 20 } satisfies { mode: Mode; minutes: number };
const selected: "focus" = settings.mode;
console.log(selected);

The corrected literal meets the contract and remains specific enough for the selected variable. The check emits no runtime validation.

Practice