Readonly and as const

readonly stops a property from being reassigned, and readonly T[] removes changing methods like push. Writing as const after a literal locks the whole value: every property becomes readonly and every value keeps its exact literal type, so ["north", "south"] as const has the type readonly ["north", "south"]. These checks exist only at compile time; they catch mistakes before the code runs.

How it reads

• config.apiUrl = ... would be a compile error; retries can change
• as const keeps each direction as its own literal type
• (typeof DIRECTIONS)[number] turns the array into the union of its values

Cloud tip: The as const array plus [number] pattern gives you a runtime list and a matching type from one source.