# Functions

## What are Functions?

When first learning how to calculate the area of a rectangle, there’s a sequence of steps to calculate the correct answer:

• Measure the width of the rectangle.

• Measure the height of the rectangle.

• Multiply the width and height of the rectangle.

With practice, you can calculate the area of the rectangle without being instructed with these three steps every time.

We can calculate the area of one rectangle with the following code:

 1 2 3 4 const width = 10; const height = 6; const area = width * height; console.log(area); // Output: 60 
Imagine being asked to calculate the area of three different rectangles:
  1 2 3 4 5 6 7 8 9 10 11 12 13 14 // Area of the first rectangle const width1 = 10; const height1 = 6; const area1 = width1 * height1; // Area of the second rectangle const width2 = 4; const height2 = 9; const area2 = width2 * height2; // Area of the third rectangle const width3 = 10; const height3 = 10; const area3 = width3 * height3; 
In programming, we often use code to perform a specific task multiple times. Instead of rewriting the same code, we can group a block of code together and associate it with one task, then we can reuse that block of code whenever we need to perform the task again. We achieve this by creating a function. A function is a reusable block of code that groups together a sequence of statements to perform a specific task.

In these notes, we will learn how to create and use functions, and how they can be used to create clearer and more concise code.

In JS, there are many was to create a function. One way to create a unction is by using a function declaration. Just like how a variable declaration binds a function to a name, or an identifier. Take a look at the anatomy of a function declaration below:

 1 2 3 function greetWorld(){ console.log("Hello, World!"); } 
A function declaration consists of:

• The function keyword.

• The name of the function, or its identifier, followed by parentheses.

• A function body, or the block of statements required to perform a specific task, enclosed in the function's curly brackets, {}.

## Parameters and Arguments

Some functions can take inputs and use the inputs to perform a task. When declaring a function, we can specify its parameters. Parameters allow functions to accept input(s) and perform a task using the input(s). We use parameters as placeholders for information that will be passed to the function when it is called.

Let's observe how to specify parametes in our function declaration:

 1 2 3 function calculateArea(width, height) { console.log(width * height); } 
In the code above, calculateArea() computes the area of a rectangle based on two inputs, width and height. The parameters are specified between the parenthesis as width and height, and inside the function body, they act like regular variables. width and height act as placeholders for values that will be multiplied together.

When calling a function that has parameters, we specify the values in the parentheses that follow the function name (in the example above: calculateArea()). The values that are passed to the function when it is called are called arguments. Arguments can be passed to the function as values or variables.

calculateArea(10, 6);

In the function above, the number 10 is passed as the width and 6 is passed as the height. Notice that the order in which arguments are passed and assigned follows the order that the parameters are declared.

 1 2 3 4 const rectWidth = 10; const rectHeight = 6; calculateArea(rectWidth, rectHeight); 
The variables rectWidth and rectHeight are initialized with the values for the height and width of the rectangle before being used in the function call.

By using parameters, calculateArea() can be reused to compute the area of any rectangle!

Let's create an example function that takes an argument and prints out a message using the argument:

 1 2 3 4 5 function sayHello(name) { console.log("Hello, " + name + "!"); } sayHello("Nick"); 
This code would have the output Hello, Nick!. We can pass any name as the argument for the sayHello() function and it would print accordingly.

### Default Parameters

We can also set default parameters for arguments in a function. Default parameters allow parameters to have a predetermined value in case there is no argument passed into the function or if the argument is undefined when called.

 1 2 3 4 5 6 function greeting (name = 'stranger') { console.log(Hello, \${name}!); } greeting('Nick'); // Output: Hello, Nick! greeting(); // Output: Hello, stranger! 
In the code above, we set a default parameter by using a = operator to assign the string 'stranger'. This ensures that if a argument is not passed to the function, the function will display the default message instead of erroring out.

## Return

When a function is called, the computer will run through the function's code and evaluate the result of calling the function. By default that resulting value is undefined.

 1 2 3 4 function rectangleArea(width, height) { let area = width * height; } console.log(rectangleArea(5, 7)); // Prints undefined 
In the code example above, we defined our function to calculate the area of the width and height parameter. Then rectangleArea() is invoked with eh arguments 5 and 7. But when we went to print the results we got undefined. Did we write our function wrong? No! In fact, the function worked fine, and the computer did compute the area as 35, but we didn't capture it. We are able to capture it with the return keyword!

 1 2 3 4 function rectangleArea(width, height) { let area = width * height; return area; } 
To pass back information from the return call, we use a return statement. To create a return statement, we use the return keyword followed by the value that we wish to return. Like we saw above, if the value is omitted, undefined is returned instead.

When a return statement is used in a function body, the execution of the function is stopped and the code that follows it will not be executed. Look at the example below:

 1 2 3 4 5 6 function rectangleArea(width, height) { if (width < 0 || height < 0) { return 'You need positive integers to calculate area!'; } return width * height; } 
If an argument for width or height is less than 0, then rectangleArea() will return 'You need positive integers to calculate area!'. The second return statement width * height will not run.

The return keyword is powerful because it allows functions to produce an output. We can then save the output to a variable for later use.

## Helper Functions

We can also use the return value of a function inside another function. These functions being called within another function are often referred to as helper functions. Since each function is carrying out a specific task, it makes our code easier to read and debug if necessary.

If we wanted to define a function that converts the temperature from Celsius to Fahrenheit, we could write two functions like:

 1 2 3 4 5 6 7 8 9 function multiplyByNineFifths(number) { return number * (9/5); }; function getFahrenheit(celsius) { return multiplyByNineFifths(celsius) + 32; }; getFahrenheit(15); // Returns 59 
In the example above:

• getFahrenheit() is called and 15 is passed as an argument.

• The code block inside of getFahrenheit() calls multiplyByNineFifths() and passes 15 as an argument.

• multiplyByNineFifths() takes the argument of 15 for the number parameter.

• The code block inside of multiplyByNineFifths() function multiplies 15 by (9/5), which evaluates to 27.

• 27 is returned back to the function call in getFahrenheit().

• getFahrenheit() continues to execute. It adds 32 to 27, which evaluates to 59.

• Finally, 59 is returned back to the function call getFahrenheit(15).

We can use functions to section off small bits of logic or tasks, then use them when we need to. Writing helper functions can help take large and difficult tasks and break them into smaller and more manageable tasks.

## Function Expressions

Another way to define a function is to use a function expression. To define a function inside an expression, we can use the function keyword. In a function expression, the function name is usually omitted. A function with no name is called an anonymous function. A function expression is often store in a variable in order to refer to it.

Consider the following function expression:

 1 2 3 4 const calculateArea = function (width, height) { const area = width * height; return area; } 
To declare a function expression:
1. Declare a variable to make the variable’s name be the name, or identifier, of your function. Since the release of ES6, it is common practice to use const as the keyword to declare the variable.

1. Assign as that variable’s value an anonymous function created by using the function keyword followed by a set of parentheses with possible parameters. Then a set of curly braces that contain the function body.

To invoke a function expression, write the name of the variable in which the function is stored followed by parentheses enclosing any arguments being passed into the function.

variableName(argument1, argument2)


Unlike function declarations, function expressions are not hoisted so they cannot be called before they are defined.

## Arrow Functions

ES6 introduced arrow function syntax, a shorter way to write functions by using the special "fat arrow" () => notation.

Arrow functions remove the need to type out the keyword function every time you need to create a function. Instead, you first include the parameters inside the () and then add an arrow => that points to the function body surrounded in {} like the following:

 1 2 3 4 const rectangleArea = (width, height) => { let area = width * height; return area; } 

## Concise Body Arrow Functions

JavaScript also provides several ways to refactor arrow function syntax. The most condense form of the function is known as concise body. We'll explore a few of these techniques below:

• Functions that take only a single parameter do not need that parameter to be enclosed in parentheses. However, if a function takes zero or multiple parameters, parentheses are required.
 1 2 3 4 5 6 7 8 // Zero Parameters const functionName = () => {}; // One Parameter const functionName = paramOne => {}; // Two or More Parameters const functionName = (paramOne, paramTwo) => {}; 
• A function body composed of a single-line block does not need curly braces. Without the curly braces, whatever the line evaluates will be automatically returned. The contents of the block should immediately follow the arrow => and the return keyword can be removed. This is referred to as implicit return.
 1 2 3 4 5 6 7 8 // Single-line Block const sumNumbers = number => number + number; // Multi-line Block const sumNumbers = number => { const sum = number + number; return sum; // requires the return statement } 

For example, if we have the following function:

 1 2 3 const squareNum = (num) => { return num * num; } 
We can refactor the function to:
const squareNum = num => num * num;

Notice the following changes:

• The parentheses around num have been removed, since it has a single parameter.

• The curly braces {} have been removed since the function consists of a single-line block.

• The return keyword has been removed since the function consists of a single-line block.

## Rock, Paper, Scissors

// Command Line Rock, Paper, Scissors game
// User input is set in the playGame() function in the const userChoice

const getUserChoice = userInput => {
userInput = userInput.toLowerCase();
if (userInput === 'rock' || userInput === 'paper' || userInput === "scissors" ) {
return userInput;
} else {
console.log('Error!');
}
}

const getComputerChoice = () => {
randomNumber = Math.floor(Math.random() * 3);
switch (randomNumber) {
case 0:
return "rock";
case 1:
return "paper";
case 2:
return "scissors";
}
}

const determineWinner = (userInput, computerChoice) => {
if (userInput === computerChoice) {
return "The game is a tie!";
}
if (userInput === 'rock') {
if (computerChoice === 'paper') {
return 'The computer won!';
} else {
return 'You won!';
}
}
if (userInput === 'paper') {
if (computerChoice === 'scissors') {
return 'The computer won!';
} else {
return 'You won!';
}
}
if (userInput === 'scissors') {
if (computerChoice === 'rock') {
return 'The computer won!';
} else {
return 'You won!';
}
}
}

const playGame = () => {
const userChoice = getUserChoice('scissors');
const computerChoice = getComputerChoice();
console.log('You threw: ' + userChoice);
console.log('The computer threw: ' + computerChoice);
console.log(determineWinner(userChoice, computerChoice));
}

playGame();