JS03 - Functions

Main Question

How do functions help programmers organize code, reuse logic, and decide when to return a value versus cause an effect?

Learning Objectives

You should be able to:

• explain why functions are useful in larger programs

• define and call a function correctly

• use parameters so one function can work with different inputs

• use return to send a value back to other code

• distinguish between a function with a side effect and a function with a return value

• write simple JavaScript functions using declaration syntax and arrow syntax, and solve beginner function exercises similar to the chapter tasks in Eloquent JavaScript

Directions

Part A: Short Response

Answer these in complete sentences.

1. Why are functions useful in a program?

2. What is the difference between defining a function and calling a function?

3. What does a parameter do?

4. What does return do?

5. What is the difference between a side effect and a return value?

Part B: Read the Code

For each example, write:

• the function name

• the parameter(s), if any

• whether it mainly has a side effect or returns a value

• what happens when it runs

function showTrackTitle(title) {
	console.log(title);
}

function double(number) {
  return number * 2;
}

const greet = name => "Hello, " + name;

Part C: Write the Functions

Write code that solves each task.

A. Reusable greeting

Create a function called sayHello that prints:

"Hello from the player!"

B. Track title

Create a function called showTrackTitle with one parameter, title. When called, it should print the title.

C. Volume display

Create a function called showVolume with one parameter, level. When called, it should print:

"Volume: " followed by the level.

D. Double

Create a function called double that takes one number and returns twice that number.

E. Build a label

Create a function called buildTrackLabel with parameters title and artist.

It should return a string in this format:

"Track 1 - Artist A"

F. Return or side effect?

Write:

• one function that mainly causes a side effect

• one function that mainly returns a value

Add a comment above each one explaining which kind it is.

Part D: Call and Use The Functions

Use your functions to complete this mini-player task.

Write code that:

• calls showTrackTitle("Blinding Lights")

• calls showVolume(70)

• stores the result of double(5) in a variable called result

• prints result

• stores the result of buildTrackLabel("Levitating", "Dua Lipa") in a variable called label

• prints label

Part E: Extend Your Understanding

Choose 2 of the 3 challenges below.

Challenge 1: Minimum

Write a function min(a, b) that returns the smaller value. This mirrors one of the chapter exercises.

Challenge 2: Count Characters

Write a function countChar(text, char) that counts how many times a character appears in a string. Then write countBs(text) using countChar. This is also based on a chapter exercise.

Challenge 3: Even or Odd with Recursion

Write a recursive function isEven(n):

• 0 is even

• 1 is odd

• otherwise, use n - 2

Test it with:

• isEven(50)

• isEven(75)

Then explain what happens with isEven(-1). This comes directly from the chapter’s recursion exercise.

Submission Checklist

Turn in:

• your short responses for Parts A and B

• completed code for Parts C and D

• two completed challenge problems