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<div align="center">
<h1> 30 Days Of JavaScript: Функции</h1>
<a class="header-badge" target="_blank" href="https://www.linkedin.com/in/asabeneh/">
<img src="https://img.shields.io/badge/style--5eba00.svg?label=LinkedIn&logo=linkedin&style=social">
</a>
<a class="header-badge" target="_blank" href="https://twitter.com/Asabeneh">
<img alt="Twitter Follow" src="https://img.shields.io/twitter/follow/asabeneh?style=social">
</a>
<sub>Автор:
<a href="https://www.linkedin.com/in/asabeneh/" target="_blank">Asabeneh Yetayeh</a><br>
<small> Январь, 2020</small>
</sub>
</div>
[<< День 6](../06_Day_Loops/06_day_loops.md) | [День 8 >>](../08_Day_Objects/08_day_objects.md)
![30 Days Of JavaScript](../images/banners/day_1_7.png)
- [📔 День 7](#-День-7)
- [Функции](#функции)
- [Объявление функции](#объявление-функции)
- [Функции без параметров и возвращаемого значения](#функции-без-параметров-и-возвращаемого-значения)
- [Функции возвращающие значение](#функции-возвращающие-значение)
- [Функции с параметрами](#функции-с-параметрами)
- [Функции с двумя параметрами](#function-with-two-parameters)
- [Функции с множеством параметров](#function-with-many-parameters)
- [Функции с неограниченным числом параметров](#function-with-unlimited-number-of-parameters)
- [Неограниченное число параметров в обычных функциях](#unlimited-number-of-parameters-in-regular-function)
- [Неограниченное число параметров в стрелочных функциях](#unlimited-number-of-parameters-in-arrow-function)
- [Анонимные функции](#anonymous-function)
- [Функциональные выражения](#expression-function)
- [Самовызываемая функции](#self-invoking-functions)
- [Стрелочные функции](#arrow-function)
- [Функции с параметрами по умолчанию](#function-with-default-parameters)
- [Объявление функции против стрелочной функции](#function-declaration-versus-arrow-function)
- [💻 Упражнения](#-exercises)
- [Упражнения: Уровень 1](#exercises-level-1)
- [Упражнения: Уровень 2](#exercises-level-2)
- [Упражнения: Уровень 3](#exercises-level-3)
# 📔 День 7
## Функции
До сих пор мы видели много встроенных функций JavaScript. В этом разделе мы сосредоточимся на пользовательских функциях. Что такое функция? Прежде чем мы начнем создавать функции, давайте разберемся, что такое функция и зачем она нам нужна?
Функция, это многократно используемый блок кода или программных операторов, предназначенная для выполнения определенной задачи.
Функция объявляется ключевым словом `function`, за которым следует имя функции, за которым следуют круглые скобки (). Скобки могут принимать параметры. Если функция принимает параметры, она будет вызываться с аргументами. Функция также может принимать параметр по умолчанию. Чтобы сохранить данные в функцию, функция должна возвращать определенные типы данных. Чтобы получить значение, мы вызываем функцию.
Функции делают код:
- Чистым и простым в чтении
- Переиспользуемым
- Простым для тестирования
Функции могут быть объявлены или созданы разными путями:
- _Объявление функции_
- _Функциональное выражение_
- _Анонимные функции_
- _Стрелочные функции_
### Объявление функции
Давайте посмотрим, как объявлять функции, и как их вызывать.
```js
//Объявление функции без параметров
function functionName() {
// Здесь будет код функции
}
functionName() // Вызов функции по ее имени вместе со скобками
```
### Функции без параметров и возвращаемого значения
Функции могут быть объявлены без параметров
**Пример:**
```js
// Функция без параметров, функция которая возводит число в квадрат
function square() {
let num = 2
let sq = num * num
console.log(sq)
}
square() // 4
// Функция без параметров
function addTwoNumbers() {
let numOne = 10
let numTwo = 20
let sum = numOne + numTwo
console.log(sum)
}
addTwoNumbers() // Функция должна быть вызвана по своему имени, для того что бы она могла выполниться
```
```js
function printFullName (){
let firstName = 'Asabeneh'
let lastName = 'Yetayeh'
let space = ' '
let fullName = firstName + space + lastName
console.log(fullName)
}
printFullName() // Вызов функции
```
### Функции возвращающие значение
Функции также могут возвращать значения. Если функция ничего не возвращает, то значением функции будет `undefined`. Давайте перепишем функции выше, с возвращаемым значением. С этого момента мы возвращаем значение из функции.
```js
function printFullName (){
let firstName = 'Asabeneh'
let lastName = 'Yetayeh'
let space = ' '
let fullName = firstName + space + lastName
return fullName
}
console.log(printFullName())
```
```js
function addTwoNumbers() {
let numOne = 2
let numTwo = 3
let total = numOne + numTwo
return total
}
console.log(addTwoNumbers())
```
### Функции с параметрами
В функцию мы можем передать различные типы данных (числа, строки, логические значения, объекты, функции) в качестве параметров.
```js
// Функция с одним параметром
function functionName(parm1) {
// Здесь будет код функции
}
functionName(parm1) // Во время вызова требуется один аргумент
function areaOfCircle(r) {
let area = Math.PI * r * r
return area
}
console.log(areaOfCircle(10)) // Следует вызывать с одним аргументом
function square(number) {
return number * number
}
console.log(square(10))
```
### Function with two parameters
```js
// function with two parameters
function functionName(parm1, parm2) {
//code goes her
}
functionName(parm1, parm2) // during calling or invoking two arguments needed
// Function without parameter doesn't take input, so lets make a function with parameters
function sumTwoNumbers(numOne, numTwo) {
let sum = numOne + numTwo
console.log(sum)
}
sumTwoNumbers(10, 20) // calling functions
// If a function doesn't return it doesn't store data, so it should return
function sumTwoNumbers(numOne, numTwo) {
let sum = numOne + numTwo
return sum
}
console.log(sumTwoNumbers(10, 20))
function printFullName(firstName, lastName) {
return `${firstName} ${lastName}`
}
console.log(printFullName('Asabeneh', 'Yetayeh'))
```
### Function with many parameters
```js
// function with multiple parameters
function functionName(parm1, parm2, parm3,...){
//code goes here
}
functionName(parm1,parm2,parm3,...) // during calling or invoking three arguments needed
// this function takes array as a parameter and sum up the numbers in the array
function sumArrayValues(arr) {
let sum = 0;
for (let i = 0; i < arr.length; i++) {
sum = sum + arr[i];
}
return sum;
}
const numbers = [1, 2, 3, 4, 5];
//calling a function
console.log(sumArrayValues(numbers));
const areaOfCircle = (radius) => {
let area = Math.PI * radius * radius;
return area;
}
console.log(areaOfCircle(10))
```
### Function with unlimited number of parameters
Sometimes we do not know how many arguments the user going to pass. Therefore, we should know how to write a function which can take unlimited number of arguments. The way we do it has a significant difference between a function declaration(regular function) and arrow function. Let us see examples both in function declaration and arrow function.
#### Unlimited number of parameters in regular function
A function declaration provides a function scoped arguments array like object. Any thing we passed as argument in the function can be accessed from arguments object inside the functions. Let us see an example
```js
// Let us access the arguments object
​
function sumAllNums() {
console.log(arguments)
}
sumAllNums(1, 2, 3, 4)
// Arguments(4) [1, 2, 3, 4, callee: ƒ, Symbol(Symbol.iterator): ƒ]
```
```js
// function declaration
​
function sumAllNums() {
let sum = 0
for (let i = 0; i < arguments.length; i++) {
sum += arguments[i]
}
return sum
}
console.log(sumAllNums(1, 2, 3, 4)) // 10
console.log(sumAllNums(10, 20, 13, 40, 10)) // 93
console.log(sumAllNums(15, 20, 30, 25, 10, 33, 40)) // 173
```
#### Unlimited number of parameters in arrow function
Arrow function does not have the function scoped arguments object. To implement a function which takes unlimited number of arguments in an arrow function we use spread operator followed by any parameter name. Any thing we passed as argument in the function can be accessed as array in the arrow function. Let us see an example
```js
// Let us access the arguments object
​
const sumAllNums = (...args) => {
// console.log(arguments), arguments object not found in arrow function
// instead we use a parameter followed by spread operator (...)
console.log(args)
}
sumAllNums(1, 2, 3, 4)
// [1, 2, 3, 4]
```
```js
// function declaration
​
const sumAllNums = (...args) => {
let sum = 0
for (const element of args) {
sum += element
}
return sum
}
console.log(sumAllNums(1, 2, 3, 4)) // 10
console.log(sumAllNums(10, 20, 13, 40, 10)) // 93
console.log(sumAllNums(15, 20, 30, 25, 10, 33, 40)) // 173
```
### Anonymous Function
Anonymous function or without name
```js
const anonymousFun = function() {
console.log(
'I am an anonymous function and my value is stored in anonymousFun'
)
}
```
### Expression Function
Expression functions are anonymous functions. After we create a function without a name and we assign it to a variable. To return a value from the function we should call the variable. Look at the example below.
```js
// Function expression
const square = function(n) {
return n * n
}
console.log(square(2)) // -> 4
```
### Self Invoking Functions
Self invoking functions are anonymous functions which do not need to be called to return a value.
```js
(function(n) {
console.log(n * n)
})(2) // 4, but instead of just printing if we want to return and store the data, we do as shown below
let squaredNum = (function(n) {
return n * n
})(10)
console.log(squaredNum)
```
### Arrow Function
Arrow function is an alternative to write a function, however function declaration and arrow function have some minor differences.
Arrow function uses arrow instead of the keyword *function* to declare a function. Let us see both function declaration and arrow function.
```js
// This is how we write normal or declaration function
// Let us change this declaration function to an arrow function
function square(n) {
return n * n
}
console.log(square(2)) // 4
const square = n => {
return n * n
}
console.log(square(2)) // -> 4
// if we have only one line in the code block, it can be written as follows, explicit return
const square = n => n * n // -> 4
```
```js
const changeToUpperCase = arr => {
const newArr = []
for (const element of arr) {
newArr.push(element.toUpperCase())
}
return newArr
}
const countries = ['Finland', 'Sweden', 'Norway', 'Denmark', 'Iceland']
console.log(changeToUpperCase(countries))
// ["FINLAND", "SWEDEN", "NORWAY", "DENMARK", "ICELAND"]
```
```js
const printFullName = (firstName, lastName) => {
return `${firstName} ${lastName}`
}
console.log(printFullName('Asabeneh', 'Yetayeh'))
```
The above function has only the return statement, therefore, we can explicitly return it as follows.
```js
const printFullName = (firstName, lastName) => `${firstName} ${lastName}`
console.log(printFullName('Asabeneh', 'Yetayeh'))
```
### Function with default parameters
Sometimes we pass default values to parameters, when we invoke the function if we do not pass an argument the default value will be used. Both function declaration and arrow function can have a default value or values.
```js
// syntax
// Declaring a function
function functionName(param = value) {
//codes
}
// Calling function
functionName()
functionName(arg)
```
**Example:**
```js
function greetings(name = 'Peter') {
let message = `${name}, welcome to 30 Days Of JavaScript!`
return message
}
console.log(greetings())
console.log(greetings('Asabeneh'))
```
```js
function generateFullName(firstName = 'Asabeneh', lastName = 'Yetayeh') {
let space = ' '
let fullName = firstName + space + lastName
return fullName
}
console.log(generateFullName())
console.log(generateFullName('David', 'Smith'))
```
```js
function calculateAge(birthYear, currentYear = 2019) {
let age = currentYear - birthYear
return age
}
console.log('Age: ', calculateAge(1819))
```
```js
function weightOfObject(mass, gravity = 9.81) {
let weight = mass * gravity + ' N' // the value has to be changed to string first
return weight
}
console.log('Weight of an object in Newton: ', weightOfObject(100)) // 9.81 gravity at the surface of Earth
console.log('Weight of an object in Newton: ', weightOfObject(100, 1.62)) // gravity at surface of Moon
```
Let us see how we write the above functions with arrow functions
```js
// syntax
// Declaring a function
const functionName = (param = value) => {
//codes
}
// Calling function
functionName()
functionName(arg)
```
**Example:**
```js
const greetings = (name = 'Peter') => {
let message = name + ', welcome to 30 Days Of JavaScript!'
return message
}
console.log(greetings())
console.log(greetings('Asabeneh'))
```
```js
const generateFullName = (firstName = 'Asabeneh', lastName = 'Yetayeh') => {
let space = ' '
let fullName = firstName + space + lastName
return fullName
}
console.log(generateFullName())
console.log(generateFullName('David', 'Smith'))
```
```js
const calculateAge = (birthYear, currentYear = 2019) => currentYear - birthYear
console.log('Age: ', calculateAge(1819))
```
```js
const weightOfObject = (mass, gravity = 9.81) => mass * gravity + ' N'
console.log('Weight of an object in Newton: ', weightOfObject(100)) // 9.81 gravity at the surface of Earth
console.log('Weight of an object in Newton: ', weightOfObject(100, 1.62)) // gravity at surface of Moon
```
### Function declaration versus Arrow function
It Will be covered in other section.
🌕 You are a rising star, now you knew function . Now, you are super charged with the power of functions. You have just completed day 7 challenges and you are 7 steps a head in to your way to greatness. Now do some exercises for your brain and for your muscle.
## 💻 Exercises
### Exercises: Level 1
1. Declare a function _fullName_ and it print out your full name.
2. Declare a function _fullName_ and now it takes firstName, lastName as a parameter and it returns your full - name.
3. Declare a function _addNumbers_ and it takes two two parameters and it returns sum.
4. An area of a rectangle is calculated as follows: _area = length x width_. Write a function which calculates _areaOfRectangle_.
5. A perimeter of a rectangle is calculated as follows: _perimeter= 2x(length + width)_. Write a function which calculates _perimeterOfRectangle_.
6. A volume of a rectangular prism is calculated as follows: _volume = length x width x height_. Write a function which calculates _volumeOfRectPrism_.
7. Area of a circle is calculated as follows: _area = π x r x r_. Write a function which calculates _areaOfCircle_
8. Circumference of a circle is calculated as follows: _circumference = 2πr_. Write a function which calculates _circumOfCircle_
9. Density of a substance is calculated as follows:_density= mass/volume_. Write a function which calculates _density_.
10. Speed is calculated by dividing the total distance covered by a moving object divided by the total amount of time taken. Write a function which calculates a speed of a moving object, _speed_.
11. Weight of a substance is calculated as follows: _weight = mass x gravity_. Write a function which calculates _weight_.
12. Temperature in oC can be converted to oF using this formula: _oF = (oC x 9/5) + 32_. Write a function which convert oC to oF _convertCelsiusToFahrenheit_.
13. Body mass index(BMI) is calculated as follows: _bmi = weight in Kg / (height x height) in m2_. Write a function which calculates _bmi_. BMI is used to broadly define different weight groups in adults 20 years old or older.Check if a person is _underweight, normal, overweight_ or _obese_ based the information given below.
- The same groups apply to both men and women.
- _Underweight_: BMI is less than 18.5
- _Normal weight_: BMI is 18.5 to 24.9
- _Overweight_: BMI is 25 to 29.9
- _Obese_: BMI is 30 or more
14. Write a function called _checkSeason_, it takes a month parameter and returns the season:Autumn, Winter, Spring or Summer.
15. Math.max returns its largest argument. Write a function findMax that takes three arguments and returns their maximum with out using Math.max method.
```js
console.log(findMax(0, 10, 5))
10
console.log(findMax(0, -10, -2))
0
```
### Exercises: Level 2
1. Linear equation is calculated as follows: _ax + by + c = 0_. Write a function which calculates value of a linear equation, _solveLinEquation_.
1. Quadratic equation is calculated as follows: _ax2 + bx + c = 0_. Write a function which calculates value or values of a quadratic equation, _solveQuadEquation_.
```js
console.log(solveQuadratic()) // {0}
console.log(solveQuadratic(1, 4, 4)) // {-2}
console.log(solveQuadratic(1, -1, -2)) // {2, -1}
console.log(solveQuadratic(1, 7, 12)) // {-3, -4}
console.log(solveQuadratic(1, 0, -4)) //{2, -2}
console.log(solveQuadratic(1, -1, 0)) //{1, 0}
```
1. Declare a function name _printArray_. It takes array as a parameter and it prints out each value of the array.
1. Write a function name _showDateTime_ which shows time in this format: 08/01/2020 04:08 using the Date object.
```sh
showDateTime()
08/01/2020 04:08
```
1. Declare a function name _swapValues_. This function swaps value of x to y.
```js
swapValues(3, 4) // x => 4, y=>3
swapValues(4, 5) // x = 5, y = 4
```
1. Declare a function name _reverseArray_. It takes array as a parameter and it returns the reverse of the array (don't use method).
```js
console.log(reverseArray([1, 2, 3, 4, 5]))
//[5, 4, 3, 2, 1]
console.log(reverseArray(['A', 'B', 'C']))
//['C', 'B', 'A']
```
1. Declare a function name _capitalizeArray_. It takes array as a parameter and it returns the - capitalizedarray.
1. Declare a function name _addItem_. It takes an item parameter and it returns an array after adding the item
1. Declare a function name _removeItem_. It takes an index parameter and it returns an array after removing an item
1. Declare a function name _sumOfNumbers_. It takes a number parameter and it adds all the numbers in that range.
1. Declare a function name _sumOfOdds_. It takes a number parameter and it adds all the odd numbers in that - range.
1. Declare a function name _sumOfEven_. It takes a number parameter and it adds all the even numbers in that - range.
1. Declare a function name evensAndOdds . It takes a positive integer as parameter and it counts number of evens and odds in the number.
```sh
evensAndOdds(100);
The number of odds are 50.
The number of evens are 51.
```
1. Write a function which takes any number of arguments and return the sum of the arguments
```js
sum(1, 2, 3) // -> 6
sum(1, 2, 3, 4) // -> 10
```
1. Writ a function which generates a _randomUserIp_.
1. Write a function which generates a _randomMacAddress_
1. Declare a function name _randomHexaNumberGenerator_. When this function is called it generates a random hexadecimal number. The function return the hexadecimal number.
```sh
console.log(randomHexaNumberGenerator());
'#ee33df'
```
1. Declare a function name _userIdGenerator_. When this function is called it generates seven character id. The function return the id.
```sh
console.log(userIdGenerator());
41XTDbE
```
### Exercises: Level 3
1. Modify the _userIdGenerator_ function. Declare a function name _userIdGeneratedByUser_. It doesn’t take any parameter but it takes two inputs using prompt(). One of the input is the number of characters and the second input is the number of ids which are supposed to be generated.
```sh
userIdGeneratedByUser()
'kcsy2
SMFYb
bWmeq
ZXOYh
2Rgxf
'
userIdGeneratedByUser()
'1GCSgPLMaBAVQZ26
YD7eFwNQKNs7qXaT
ycArC5yrRupyG00S
UbGxOFI7UXSWAyKN
dIV0SSUTgAdKwStr
'
```
1. Write a function name _rgbColorGenerator_ and it generates rgb colors.
```sh
rgbColorGenerator()
rgb(125,244,255)
```
1. Write a function **_arrayOfHexaColors_** which return any number of hexadecimal colors in an array.
1. Write a function **_arrayOfRgbColors_** which return any number of RGB colors in an array.
1. Write a function **_convertHexaToRgb_** which converts hexa color to rgb and it returns an rgb color.
1. Write a function **_convertRgbToHexa_** which converts rgb to hexa color and it returns an hexa color.
1. Write a function **_generateColors_** which can generate any number of hexa or rgb colors.
```js
console.log(generateColors('hexa', 3)) // ['#a3e12f', '#03ed55', '#eb3d2b']
console.log(generateColors('hexa', 1)) // '#b334ef'
console.log(generateColors('rgb', 3)) // ['rgb(5, 55, 175)', 'rgb(50, 105, 100)', 'rgb(15, 26, 80)']
console.log(generateColors('rgb', 1)) // 'rgb(33,79, 176)'
```
1. Call your function _shuffleArray_, it takes an array as a parameter and it returns a shuffled array
1. Call your function _factorial_, it takes a whole number as a parameter and it return a factorial of the number
1. Call your function _isEmpty_, it takes a parameter and it checks if it is empty or not
1. Call your function _sum_, it takes any number of arguments and it returns the sum.
1. Write a function called _sumOfArrayItems_, it takes an array parameter and return the sum of all the items. Check if all the array items are number types. If not give return reasonable feedback.
1. Write a function called _average_, it takes an array parameter and returns the average of the items. Check if all the array items are number types. If not give return reasonable feedback.
1. Write a function called _modifyArray_ takes array as parameter and modifies the fifth item of the array and return the array. If the array length is less than five it return 'item not found'.
```js
console.log(modifyArray(['Avocado', 'Tomato', 'Potato','Mango', 'Lemon','Carrot']);
```
```sh
['Avocado', 'Tomato', 'Potato','Mango', 'LEMON', 'Carrot']
```
```js
console.log(modifyArray(['Google', 'Facebook','Apple', 'Amazon','Microsoft', 'IBM']);
```
```sh
['Google', 'Facebook','Apple', 'Amazon','MICROSOFT', 'IBM']
```
```js
console.log(modifyArray(['Google', 'Facebook','Apple', 'Amazon']);
```
```sh
'Not Found'
```
1. Write a function called _isPrime_, which checks if a number is prime number.
1. Write a functions which checks if all items are unique in the array.
1. Write a function which checks if all the items of the array are the same data type.
1. JavaScript variable name does not support special characters or symbols except \$ or \_. Write a function **isValidVariable** which check if a variable is valid or invalid variable.
1. Write a function which returns array of seven random numbers in a range of 0-9. All the numbers must be unique.
```js
sevenRandomNumbers()
[(1, 4, 5, 7, 9, 8, 0)]
```
1. Write a function called reverseCountries, it takes countries array and first it copy the array and returns the reverse of the original array
🎉 CONGRATULATIONS ! 🎉
[<< Day 6](../06_Day_Loops/06_day_loops.md) | [Day 8 >>](../08_Day_Objects/08_day_objects.md)
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