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boolean type

Learn boolean for logical values.

What is the boolean type?

The boolean type takes only two values: true or false. It is used to express a two-state condition such as on/off or yes/no.

let aktif: boolean = true;
let silindi: boolean = false;
console.log(aktif);  // true

The values are always written lowercase.

Boolean operations

boolean values can be combined with && (and), || (or), ! (not). Comparison results are also boolean.

let acik: boolean = true;
let kil: boolean = false;
console.log(acik && kil);  // false
console.log(5 > 3);  // true

Practice Your first boolean variable

Declare a boolean variable named aktif and assign true. Then print its value with console.log.

Starter code

index.ts
// aktif adında boolean değişken tanımla, true ata

// değeri yazdır

Sample solution

index.ts
let aktif: boolean = true;
console.log(aktif);

Expected output

true

Questions and answers

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

1 Which of the following is a valid boolean variable declaration?

  • let aktif: boolean = "true";
  • let aktif: boolean = 1;
  • let aktif: boolean = true; Correct answer
  • let aktif: boolean = True;

Explanation: Only true or false (lowercase) can be assigned to a boolean variable; "true" is text, 1 is a number, and True is invalid.

2 Write the correct value

Question 
let aktif: boolean = ____;
Answer 
let aktif: boolean = true;

Explanation: boolean values are written lowercase true or false; capital True is not valid.

3 boolean concepts

Term Meaning
boolean two-state type
true true value
false false value
: boolean type tag

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