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Summary - Functions

Write a store calculation function with types.

Summary - Functions

Functions combine type annotations, optional and default parameters, rest parameters, and overloading to create powerful and flexible definitions. Choosing the right features improves both type safety and usability.

function hesapla(fiyat: number, oran: number = 10): number {
  return fiyat - (fiyat * oran) / 100;
}

These features together enable writing safe and flexible functions.

Practice Store calculation function

Define a function named indirimli with fiyat: number and oran: number = 10 parameters and return type number. The function should return fiyat - (fiyat * oran) / 100. Print the result of calling indirimli(200) with console.log.

Starter code

index.ts
// indirimli adında bir fonksiyon tanımla
// fiyat zorunlu sayı, oran varsayılan 10 ile sayı olsun
// dönüş türü number, fiyat - (fiyat * oran) / 100 döndürsün
// indirimli(200) sonucunu yazdır

Sample solution

index.ts
function indirimli(fiyat: number, oran: number = 10): number {
    return fiyat - (fiyat * oran) / 100;
}
console.log(indirimli(200));

Expected output

180

Questions and answers

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

1 Which feature lets a function accept any number of arguments?

  • Default parameters
  • Rest parameters Correct answer
  • Return type annotation
  • Optional parameters

Explanation: A rest parameter (<code>...args</code>) collects a variable number of arguments into a single array; optional and default parameters only provide skippable parameters.

2 Add the default parameter value

Question 
function indirim(fiyat: number, oran = ____): number { return fiyat - (fiyat * oran) / 100; }
Answer 
function indirim(fiyat: number, oran = 10): number { return fiyat - (fiyat * oran) / 100; }

Explanation: The default discount rate must be <code>10</code> (a number); <code>"10"</code> is a string, <code>true</code> is a boolean, and <code>oran</code> is a self-reference (undefined).

3 Function concepts summary

Term Meaning
function f(a: number): number Typed function
oran?: number Optional parameter
...sayilar: number[] Rest parameter
oran: number = 10 Default parameter

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