Careeroza

The unknown Type

unknown is the Safe any

unknown also accepts any value, but you cannot use it until you check what it is. Use it for data you do not control: JSON.parse results, API responses, caught errors.

Example

Using an unknown value directly is an error (TS18046).

const input: unknown = JSON.parse('{"price": 100}');
console.log(input.price);

Compiler output (tsc)

Line 2: error TS18046: 'input' is of type 'unknown'.

Narrow It First

Example

After a typeof or property check, TypeScript knows the exact type inside the if block. This is called narrowing.

function describe(value: unknown): string {
  if (typeof value === "string") return "text of length " + value.length;
  if (typeof value === "number") return "number " + value.toFixed(1);
  if (Array.isArray(value)) return "array of " + value.length;
  if (typeof value === "object" && value !== null && "price" in value) {
    return "object with price " + String(value.price);
  }
  return "something else";
}

console.log(describe("hello"));
console.log(describe(3.14159));
console.log(describe([1, 2, 3]));
console.log(describe(JSON.parse('{"price": 100}')));
console.log(describe(true));

Output

text of length 5
number 3.1
array of 3
object with price 100
something else