as const

Yesterday, in one sentence

Interview question

Type inferenceType Inference sets a different type depending on whether a value can be reassigned.

let hero = "xavier"; // inferred as string
const villain = "magneto"; // inferred as "magneto"

hero is a let and so can be reassigned later, which is why TypeScript infers string. villain is a const and so cannot be reassigned, which is why TypeScript infers the literal typeLiteral Type "magneto".

When you assign a const to an object or an array, the values inside can still be reassigned, so TypeScript does not infer a literal type for them.

const jean = { side: "hero" }; // { side: string }
const powers = ["telepathy", "telekinesis"]; // string[]

as const tells TypeScript to infer the narrowest type it can and make everything readonly. It applies all the way down, so nested object properties and array values are inferred as their literal types and made readonly as well.

const jean = { side: "hero" } as const; // { readonly side: "hero" }

jean.side = "villain"; // ❌ Cannot assign to 'side', it is read-only

It does the same to an array, which also becomes a tuple typeTuple with a fixed length.

const powers = ["telepathy", "telekinesis"] as const;
// readonly ["telepathy", "telekinesis"]

powers[0] = "flight"; // ❌ Cannot assign to '0', it is read-only

Check yourself

const roster = ["Scott", "Jean"] as const