Casting from unknown

Learn to safely cast from unknown.

The unknown type

unknown is the type-safe counterpart of any. You cannot use an unknown value directly; you must narrow it first. This is done with typeof or instanceof checks, or by asserting with as.

let hidden: unknown = "hi";
// hidden.toUpperCase();  error!
if (typeof hidden === "string") {
  console.log(hidden.toUpperCase());
}

Narrowing with typeof

The typeof operator checks primitive types. When you narrow an unknown value with a condition like typeof hidden === "string", TypeScript knows the value is string inside that block and allows string methods. This is the safest narrowing method.

Exercise Narrow unknown to string

The gizli variable is of type unknown with value "merhaba". Check if it is string with typeof; if so, call toUpperCase() and print the result with console.log.

Show example solution

const gizli: unknown = "merhaba";
if (typeof gizli === "string") {
  console.log(gizli.toUpperCase());
}

Test yourself

What is needed to directly operate on an unknown variable?
- Narrowing or asserting the type
- Copying the value to a new variable
- Redeclaring it with const
- Declaring the variable globally
Show answer

Narrowing or asserting the type — unknown values cannot be used directly; they must first be narrowed with typeof/instanceof or asserted with as.

The typeof operator

const gizli: unknown = "selam";
if (____ gizli === "string") {
  console.log(gizli.toUpperCase());
}

Show answer

const gizli: unknown = "selam";
if (typeof gizli === "string") {
  console.log(gizli.toUpperCase());
}

unknown concepts

unknown

Type requiring narrowing
typeof

Primitive type check
instanceof

Class instance check
as

Type assertion

Next lesson: Summary - Type assertion

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