TypeScript readonly Arrays Offer Surface-Level Safety, Not Deep Immutability

TypeScript's readonly modifier prevents reassignment and blocks mutation methods like push or splice on arrays and tuples, but only at the top level — nested objects within a readonly array remain freely mutable. The two readonly syntaxes, ReadonlyArray<T> and readonly T[], compile to identical runtime code and differ only in ergonomic preference. A common pitfall is type widening during function calls, which silently strips readonly from literals unless developers use as const assertions or explicit type annotations. Achieving true deep immutability requires recursive utility types like DeepReadonly or as const assertions, both of which carry ergonomic and type-inference trade-offs. Experts recommend applying strict readonly patterns selectively based on domain risk, such as in financial logic, rather than treating it as a universal safety guarantee.