So far we have seen many builtin JavaScript functions. In this section, we will focus on custom functions. What is a function? Before we start making functions, lets understand what function is and why we need function?
A function is a reusable block of code or programming statements designed to perform a certain task.
A function is declared by a function key word followed by a name, followed by parentheses (). A parentheses can take a parameter. If a function take a parameter it will be called with argument. A function can also take a default parameter. To store a data to a function, a function has to return certain data types. To get the value we call or invoke a function.
Function makes code:
- clean and easy to read
- reusable
- easy to test
A function can be declared or created in couple of ways:
- _Declaration function_
- _Expression function_
- _Anonymous function_
- _Arrow function_
### Function Declaration
Let us see how to declare a function and how to call a function.
```js
//declaring a function without a parameter
function functionName() {
// code goes here
}
functionName() // calling function by its name and with parentheses
Function can also return values, if a function does not return values the value of the function is undefined. Let us write the above functions with return. From now on, we return value to a function instead of printing it.
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
#### 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
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.
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') {
🌕 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.
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.
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_.
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.
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.
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'.
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.
