Functions in JavaScript

Learning Goals

In this lesson we’ll cover:

  • How to declare functions with and without parameters
  • How to call functions with and without parameters
  • Write our own functions


  • Function A predefined and reusable group of behavior
  • Call/Invoke Running a function
  • Parameters The variables declared in a function definition
  • Arguments The variables passed to a function when it’s called/invoked


Functions are a way to group statements together to perform a specific task. Functions are reusable blocks of code. To create a function, you must give it a name and then write the statements required for the function to achieve its task inside the function’s curly braces. Let’s work through the pieces and parts of a function.

Anatomy of a Function:

function makeStatements() {
  var firstStatement = 'Pizza is AMAZING!';
  console.log('Burritos taste delicious!');
  • You declare a function using the keyword function.
  • Since functions perform an action, it is generally encouraged to use an action verb in the function name. i.e. getSize, saveNote, generateRandomNumber.
  • You can name the function anything you want, but it must include a set of parentheses after the name, which can be empty or accept parameters. (we’ll go over parameters shortly, but for now let’s keep it empty)
  • You must follow the parentheses with a set of curly braces, which act as bookends to hold the set of statements you want the function to run when it is called.
  • In order to call a function, type the function name with it’s parentheses and any associated arguments (Take a look below)

Invoke a Function:

// Invoke a function

When this code is read, makeStatements() is “invoked”, all three statements within the function’s body (those curly braces) get run, one line at a time. What do you think will happen if we swap the 2nd and 3rd statements?

Your Turn

With the person across from you, create your own functions in the console:

  • Write a function that logs to the console a message of “YO!”, and then logs a sum of five different integers.
  • Write a function that assigns three different math equations to three different variables, then logs the sum of the values of all three variables.
  • Write a function that declares a firstName variable and a lastName variable, then logs a message that incorporates the full name, and then logs a random number to the console.

Passing Information to a Function:

Sometimes you need to give a function some information in order for it to do its job. You can give that function the information it needs by providing parameters in the function declaration. These are place holders that you identify with appropriately named labels inside the parentheses of your named function. The words you use for your parameters act like variables INSIDE the function, which means they serve as a means to pass values.

Parameters vs. Arguments

Its a subtle difference. Basically, when you declare a function, and you stipulate the function will accept some bits of information, those are parameters. Then, when you pass the values of the parameters, those are called arguments. Like this:

// parameters named on declaration of function
function createDreamCar(make, model) {
  return "My dream car is a " + make + " " + model;

// arguments "Audi" and "R8" passed into a called function
createDreamCar("Audi", "R8");

Getting A Value from Functions:

Some functions return information to the code that called them. Wait - what? When a function performs a calculation, like 2 + 2, it will return the “answer” of 4, right?I still need the keyword return in order to return that value back from a function. Let’s work with some return statements in functions with parameters in the console:

function addTwoNumbers(num1, num2) {
  num1 + num2;

function addTwoNumbers(num1, num2) {
  return num1 + num2;

function buildAHouse(material, cost) {
  console.log("I'm building a house!");
  return "My house is made of " + material + " and cost me $" + cost;

function buildAHouse(material, cost) {
  return "My house is made of " + material + " and cost me $" + cost;
  console.log("I'm building a house!");

Notice in the last example that when the return statement is called in a function, it ends function execution (the following lines do not run), and it specifies a value to be returned to the function caller. Here is the basic structure:

// This is the formula:
return [expression];
// The expression to return. If the return is omitted, undefined is returned instead. The [] are meant to show that the expression is dynamic.


If the expression is omitted/there is no return statement, undefined is returned instead.

The following return statements are all examples that would break function execution:

return true;
return false;
return x;
return x + y / 3;

You Do

  • Create a function called saySomething that requires no parameters. This function should return a message of your choice.
  • Create a function called showFavorites that requires three parameters - a food, a movie, and an animal. Your function should return a string that uses either concatenation or interpolation to return the three favorite things in a string.</
  • Create a function called calculateAge that takes a single parameter - a year. Your function should return the difference in years from the birth year from our current year, 2019.

If you finish early, try writing tackling these functions!

Additional Resources & Practice

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