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unknown type

Learn safe type checking with unknown.

The unknown type

The unknown type, like any, accepts any value but requires you to check its type before using it. This provides flexibility while keeping type safety. You cannot access an unknown value without a type check.

let x: unknown = "merhaba";
if (typeof x === "string") {
  console.log(x.toUpperCase());  // "MERHABA"
}

unknown is used as a safe alternative to any.

Type guard

To use an unknown value, you narrow its type with a type guard. The most common type guard is the typeof operator. After narrowing, you can safely perform type-specific operations.

let deger: unknown = 42;
if (typeof deger === "number") {
  console.log(deger * 2);  // 84
}

A type guard converts unknown to a specific type and enables safe access.

Practice Type guard with unknown

Declare a variable named girdi of type unknown with value "merhaba". Use typeof to check whether it is a string; if so, convert it to uppercase with toUpperCase and print it with console.log.

Starter code

index.ts
// girdi adında bir unknown değişkeni tanımla
// değeri "merhaba" olsun
// typeof ile string kontrolü yap
// string ise büyük harfe çevirip yazdır

Sample solution

index.ts
let girdi: unknown = "merhaba";
if (typeof girdi === "string") {
    console.log(girdi.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 the key difference between <code>unknown</code> and <code>any</code>?

  • unknown is faster than any
  • unknown allows no value assignments
  • unknown requires type checking before use Correct answer
  • There is no difference between unknown and any

Explanation: <code>unknown</code> accepts any value but requires a type check (type guard) before use; <code>any</code> has no such check and is therefore unsafe.

2 Add the type guard

Question 
let deger: unknown = "merhaba";
if (typeof deger === ____) { console.log(deger.toUpperCase()); }
Answer 
let deger: unknown = "merhaba";
if (typeof deger === "string") { console.log(deger.toUpperCase()); }

Explanation: Since the variable's value is a string, the <code>typeof</code> check must be against <code>"string"</code> to use the <code>toUpperCase</code> method.

3 unknown concepts

Term Meaning
unknown Type check required before use
typeof x === "string" Type guard
unknown vs any unknown is safer
Tür koruyucu Type narrowing operation

All lessons in Beginner

1 TypeScript intro and setup

  1. What is TypeScript?
  2. Why TypeScript?
  3. tsc compiler
  4. tsconfig basics
  5. Summary - TypeScript basics

2 Primitive types

  1. string type
  2. number type
  3. boolean type
  4. let and const
  5. Summary - Primitive types

3 Arrays

  1. Array type
  2. Array methods
  3. readonly arrays
  4. Multidimensional arrays
  5. Summary - Arrays

4 Tuples

  1. What is a tuple?
  2. Tuple usage
  3. readonly tuple
  4. Summary - Tuple

5 Enums

  1. Numeric enum
  2. String enum
  3. const enum
  4. Enum members
  5. Summary - Enum

6 any, unknown, never

  1. any type
  2. unknown type
  3. never type
  4. void type
  5. Summary - Special types

7 Type annotation

  1. Variable annotation
  2. Parameter annotation
  3. Return type annotation
  4. Summary - Type annotation

8 Type inference

  1. Basic inference
  2. Inference with const
  3. Best common type
  4. Summary - Type inference

9 Objects

  1. Object type
  2. Optional properties
  3. Readonly properties
  4. Summary - Objects

10 Functions

  1. Function declaration
  2. Optional and default params
  3. Rest params
  4. Function overloads
  5. Summary - Functions

11 Arrow functions

  1. Arrow syntax
  2. this with arrow
  3. Arrow type
  4. Summary - Arrow functions

12 Interfaces

  1. What is an interface?
  2. Property types
  3. Interface vs type alias
  4. Summary - Interfaces

13 Type alias

  1. What is a type alias?
  2. Composition with alias
  3. Alias vs interface
  4. Summary - Type alias

14 Union and intersection

  1. Union types
  2. Intersection types
  3. Literal types
  4. Summary - Union & intersection

15 null and undefined

  1. null vs undefined
  2. Optional chaining
  3. Nullish coalescing
  4. Summary - null & undefined

16 Strict mode

  1. strict: true
  2. strictNullChecks
  3. strictFunctionTypes
  4. Summary - Strict mode

17 Project: Shopping cart

  1. Cart design
  2. Add product
  3. Total calculation
  4. Apply discount
  5. Integration

18 Project: Todo CLI

  1. Task model
  2. Add and list
  3. Completion
  4. Filtering
  5. Integration

19 Project: Library

  1. Book model
  2. Add and search
  3. Borrow
  4. Return
  5. Integration

20 Beginner final

  1. Challenge 1: Types
  2. Challenge 2: Functions
  3. Challenge 3: Unions
  4. Challenge 4: Mini app