TypeScript "as const" explained

as const in TypeScript is known as const assertion. By appending it to a literal value (e.g. object literals, string literals or array literals), we tell the TypeScript compiler to assume the most narrow type definition for that value.

This can be best shown with an example:

const fruits = ["apple", "banana"]
// const fruits: string[]
const vegetables = ["lettuce", "broccoli", "beans"] as const
// const vegetables: readonly ["lettuce", "broccoli", "beans"]

In the first array, TypeScript assumes that fruits is an array of strings. However, in the 2^nd array with the use of a const assertion, TypeScript assumes vegetables is an array with 3 elements of type lettuce, broccoli and beans, in that exact order.

Most importantly, however, it will render them immutable.

vegetables[2] = "corn"
// Cannot assign to '2' because it is a read-only property.
vegetables.push("corn")
// Property 'push' does not exist on type 'readonly ["lettuce", "broccoli", "beans"]'.

Yet, be aware that there exist a few exceptions to this rule of immutability.