Programming Fundamentals › Type Systems

any vs unknown

TypeScript's escape hatches, and why unknown is the safe one.

Also known as: any, unknown, any vs unknown TypeScript

TypeScript has two ways to say “I don’t know the type of this value”, and they behave very differently.

any: turns the checks off

let x: any = "hello";
x.toFixed(2);          // compiles! crashes at runtime
x.foo.bar.baz();       // compiles, anything goes

any removes the type checker for that value, and spreads: anything you get from an any is any too. It’s an escape hatch, used for convenience, and it silently throws away the safety you adopted TypeScript for.

unknown: the safe version

let y: unknown = JSON.parse(text);

y.name;                               // error: can't use it yet
if (typeof y === "object" && y !== null && "name" in y) {
  console.log(y.name);                // fine after narrowing
}

unknown means “could be anything, so prove what it is before using it”. You must check the type first (type narrowing), or validate it (runtime validation).

Guidance

• Use unknown for data from outside (JSON, catch (e), event payloads), and narrow or validate it.
• Avoid any. Turn on strict mode and lint rules (no-explicit-any) to catch it.
• If you must use any, keep it local and small, with a comment on why.
• Casts (as Type) are also unchecked promises. A validation library is safer.
• Many packages give you any by accident, such as a missing type definition. Install types or write a small declaration.