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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.

Practice 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.

Starter code

index.ts
// gizli degiskeninin string olup olmadigini typeof ile kontrol et
// eger stringse toUpperCase metodunu cagir ve yazdir

const gizli: unknown = "merhaba";

Sample solution

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

Expected output

MERHABA

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What is needed to directly operate on an unknown variable?

  • Narrowing or asserting the type Correct answer
  • Copying the value to a new variable
  • Redeclaring it with const
  • Declaring the variable globally

Explanation: unknown values cannot be used directly; they must first be narrowed with typeof/instanceof or asserted with as.

2 The typeof operator

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

Explanation: The typeof operator checks primitive types; instanceof is for class instances.

3 unknown concepts

Term Meaning
unknown Type requiring narrowing
typeof Primitive type check
instanceof Class instance check
as Type assertion

All lessons in Intermediate

1 Classes

  1. Class basics
  2. Constructor and fields
  3. Methods
  4. Static members
  5. Summary - Classes

2 Access modifiers

  1. public
  2. private
  3. protected
  4. Parameter properties
  5. Summary - Access modifiers

3 Readonly properties

  1. readonly fields
  2. Readonly parameter
  3. Readonly vs const
  4. Summary - Readonly

4 Generics

  1. What is a generic?
  2. Generic functions
  3. Generic classes
  4. Multiple type parameters
  5. Summary - Generics

5 Generic constraints

  1. extends constraint
  2. keyof constraint
  3. Default type parameter
  4. Summary - Generic constraints

6 Utility types

  1. Partial
  2. Required and Pick
  3. Omit and Record
  4. Summary - Utility types

7 keyof and typeof

  1. keyof operator
  2. typeof operator
  3. Indexing with keyof
  4. Summary - keyof and typeof

8 Mapped types

  1. What is a mapped type?
  2. Readonly mapped type
  3. Optional mapped type
  4. Summary - Mapped types

9 Conditional types

  1. What is a conditional type?
  2. Distributive conditional
  3. Logic with conditional
  4. Summary - Conditional types

10 infer

  1. infer keyword
  2. Extracting from Promise
  3. Array element type
  4. Summary - infer

11 Decorators

  1. What is a decorator?
  2. Method decorator
  3. Property decorator
  4. Decorator factories
  5. Summary - Decorators

12 Modules

  1. export and import
  2. Default export
  3. Re-export
  4. Type-only import
  5. Summary - Modules

13 Namespaces

  1. What is a namespace?
  2. Nested namespace
  3. Namespace vs module
  4. Summary - Namespaces

14 Declaration files

  1. What is .d.ts?
  2. Ambient module
  3. Triple-slash directive
  4. Summary - Declaration files

15 Advanced classes

  1. abstract class
  2. implements
  3. Get and set
  4. Summary - Advanced classes

16 Type assertion

  1. as keyword
  2. angle bracket syntax
  3. Type assertion vs casting
  4. Casting from unknown
  5. Summary - Type assertion

17 Project: Library package

  1. Package design
  2. Exporting types
  3. Generic helpers
  4. Declaration file
  5. Integration

18 Project: Quiz app

  1. Question model
  2. Question pool
  3. Answer check
  4. Score calculation
  5. Integration

19 Project: Weather app

  1. Weather model
  2. API response type
  3. Data transform
  4. Display
  5. Integration

20 Intermediate final

  1. Challenge 1: Classes
  2. Challenge 2: Generics
  3. Challenge 3: Type systems
  4. Challenge 4: Mini app