# Functions 

## 1. Named Function

#### Function definition 

```javascript 
function printHello() {
  console.log('function says hello');
}

console.log(typeof printHello); // function
```

#### Function invocation - calling a function 

```javascript 
printHello(); // function says hello
```

#### Naming conventions  

Functions names are allowed to be made of $, _, a-z, A-z and 0-9. 

```javascript 
function $print() {}
function _print() {}
function p8rint() {}
```

#### Function parameters  

```javascript 
function greeting(name) { // name here is the parameter
  console.log(`Hello, ${name}`); 
}

// 'Alex' here is the argument passed into the function call
greeting('Alex'); // "Hello, Alex" 
```

#### Function return 

```javascript 
function addNumbers(a, b) {
  return a + b; 
}

const result = addNumbers(10, 5); 
console.log(result); // 15
```

## 2. Arrow Function or Fat Arrow Function 

> Please Note: `this` context in arrow functions inherits from it's parent's scope.

#### Single line and implicit return 

```javascript 
const add = (a, b) => a + b; 
console.log(add(4, 5)); // 9  
```


#### Multiple line and explicit return 

```javascript 
const subtract = (a, b) => {
  const result = a - b; 
  return result; 
}
console.log(subtract(1, 1)); // 0 
```

#### Swift closures as shown below is similar to arrow functions in JavaScript 

```swift 
let add: (Int, Int) -> Int = { $0 + $1 } // shorthand argument names and implicit return

print(add(4, 5)) // 9
```

## Challenges 

#### Challenge 1 

You know how old your dog is in human years, but what about dog years? Calculate it!

Write a function named calculateDogAge that:

takes 1 argument: your puppy's age.
calculates your dog's age based on the conversion rate of 1 human year to 7 dog years.
outputs the result to the screen like so: "Your doggie is NN years old in dog years!"
Call the function three times with different sets of values.
Bonus: Add an additional argument to the function that takes the conversion rate of human to dog years.

<details> 
  <summary>Solution</summary> 
  
```javascript 
function calculateDogAge(puppysAge, conversionRate) {
  if ((puppysAge < 1) || (conversionRate < 1)) {
    console.log('Invalid arguments.'); 
    return; 
  }
  const age = puppysAge * conversionRate; 
  console.log(`Your doggie is ${age} years old in dog years!`);
}

calculateDogAge(1, 7);  // Your doggie is 7 years old in dog years!
```
  
</details> 

***

#### Challenge 2

Write a JavaScript function that reverses a number.

_Example x = 32243_  
_Expected Output : 34223_   

<details> 
  <summary>Solution</summary> 
  
```javascript 
function reverseNumber(number) {
  const str = number.toString();
  let strReverse = '';
  for (const char of str) {
    strReverse = char + strReverse; 
  } 
  return parseInt(strReverse);
}

console.log(reverseNumber(32243)); // 34223
```
  
</details> 

***

#### Challenge 3 

Write a function to find the largest value in an array 

_Input: [-3, 5, 1, 89, 3, 0, -78, 1]_  
_Output: 89_  

<details> 
  <summary>Solution</summary> 
  
```javascript 
function largestValue(inputArray) {
  if (inputArray.length === 0) {
    return -1;
  }
  let currentLargest = inputArray[0]
  for (const element of inputArray) {
    if (element > currentLargest) {
      currentLargest = element;
    }
  }
  return currentLargest; 
}

console.log(largestValue([-3, 5, 1, 89, 3, 0, -78, 1])); // 89
console.log(largestValue([])); // -1 
```
  
</details> 

***

#### Challenge 4 

Write a function called `squareNumber` that will take one argument (a number), square that number, and return the result. It should also log a string like "The result of squaring the number 3 is 9."

Write a function called `halfNumber` that will take one argument (a number), divide it by 2, and return the result. It should also log a string like "Half of 5 is 2.5.".

Write a function called `percentOf` that will take two numbers, figure out what percent the first number represents of the second number, and return the result. It should also log a string like "2 is 50% of 4."

Write a function called `areaOfCircle` that will take one argument (the radius), calculate the area based on that, and return the result. It should also log a string like "The area for a circle with radius 2 is 12.566370614359172."

  * Bonus: Round the result so there are only two digits after the decimal.

Write a function that will take one argument (a number) and perform the following operations, using the functions you wrote earlier1:
  1. Take half of the number and store the result.
  2. Square the result of #1 and store that result.
  3. Calculate the area of a circle with the result of #2 as the radius.
  4. Calculate what percentage that area is of the squared result (#3).
  
<details> 
  <summary>Solution</summary> 
  
```javascript 
/*
Write a function called squareNumber that will take one argument (a number), square that number, and return the result. It should also log a string like "The result of squaring the number 3 is 9."
*/
function squareNumber(number) {
  if (typeof number !== 'number') {
    console.log(`${number} is an invalid number`); 
    return
  }
  const square = number * number; 
  console.log(`The result of squaring the number ${number} is ${square}`); 
  return square; 
}

//squareNumber(25); 


/*
Write a function called halfNumber that will take one argument (a number), divide it by 2, and return the result. It should also log a string like "Half of 5 is 2.5.".
*/
function halfNumber(number) {
  if (typeof number !== 'number') {
    return; 
  }
  const result = number / 2; 
  console.log(`Half of ${number} is ${result}`);
  return result; 
}

//halfNumber(10); 


/*
Write a function called percentOf that will take two numbers, figure out what percent the first number represents of the second number, and return the result. It should also log a string like "2 is 50% of 4."
*/

function percentOf(number1, number2) {
  if (typeof number1 !== 'number' || typeof number2 !== 'number') {
    console.log('percentOf - Not a number'); 
    console.log(typeof number1);
    console.log(typeof number2);  
    return;
  }
  if (number2 === 0) { 
    console.log('Cannot divide by 0');
    return;  
  }
  const percent = (number1 * 100) / number2; 
  console.log(`${number1} is ${percent}% of ${number2}`); 
  return percent; 
}

//percentOf(1, 2); 


/*
Write a function called areaOfCircle that will take one argument (the radius), calculate the area based on that, and return the result. It should also log a string like "The area for a circle with radius 2 is 12.566370614359172."

* Bonus: Round the result so there are only two digits after the decimal.
*/

function areaOfCircle(radius) {
  if (typeof radius !== 'number') {
    console.log('In areaOfCircle not a number'); 
    return
  }
  // area of a circle is pi * r ^ 2
  // toFixed(2) - converts a number to string with 2 decimal place formatting 
  const formattedStringResult = (Math.PI * (radius * radius)).toFixed(2); 
  const area = parseFloat(formattedStringResult); 

  if (typeof area === 'string') {
    console.log('Converting area to number'); 
    area = parseFloat(area); 
  }

  console.log(`The area for a circle with radius ${radius} is ${area}.`); 
  return area; 
}

//areaOfCircle(2); 


/*
Write a function that will take one argument (a number) and perform the following operations, using the functions you wrote earlier1:
1. Take half of the number and store the result.
2. Square the result of #1 and store that result.
3. Calculate the area of a circle with the result of #2 as the radius.
4. Calculate what percentage that area is of the squared result (#3).
*/

function calculateOperation(number) {
  if (typeof number !== 'number') {
    console.log('Invalid number.'); 
    return; 
  }
  // 1. 
  const halfResult = halfNumber(number);

  // 2. 
  const squaredResult = squareNumber(halfResult); 

  // 3. 
  const areaResult = areaOfCircle(squaredResult); 

  // 4.
  percentOf(areaResult, squaredResult);
}

calculateOperation(10); 

/*
Half of 10 is 5
The result of squaring the number 5 is 25
The area for a circle with radius 25 is 1963.5.
1963.5 is 7854% of 25
*/


```
</details> 

***

## Resources 

1. [MDN - Functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Functions)