Lesson Monday

We were first introduced to JavaScript functions in the Functions lesson. In that lesson we familiarized ourselves with function syntax and other important terminology and features. In this lesson, we'll review what we learned and start writing our own custom functions.

What do we know so far about functions?

  • A function is a bundle of code that performs a set of operations.
  • Functions allow us to do things in JavaScript.
  • A method is a type of function that belongs to an object, and a function is just a set of operations that isn't necessarily a method.
  • There are a few built-in functions in JavaScript, like parseInt(), but mostly we will be writing our own custom functions. This is in contrast to methods: we will only be using built-in JavaScript methods until we revisit objects in the Object-Oriented JavaScript course section.

Let's review function syntax, terminology and get started with writing our own custom functions.

Writing Functions

Formatting Code in the DevTools Console

Before we get into our first example function, let's review how to format code in the DevTools console to make new lines and indentation:

  • To create a new line, use shift + enter.
  • To tab over multiple spaces for indentation, use tab. To configure the console to use 2 spaces for indentation with tab, within the DevTools window go to Settings > Preferences > scroll to the Sources section > set "Default indentation" to 2 spaces.

Function Declarations

We're going to write a very simple function declaration to start. We'll give the function the name sayHi.

// Here we are defining the sayHi() function; this is a "function declaration".
function sayHi() {                     
  return 'Hello from Epicodus!';

When we write a function like this, it's called a function declaration. That just means we've declared (defined) a function starting with the function keyword. There are other ways of writing a function, but we won't be covering them in this lesson. Here's the breakdown:

  • The function keyword indicates that we are declaring a function. "Declaring" a function means we're defining it.
  • sayHi is the name of the function, and we always include parentheses () after the function name. The parentheses are to hold optional parameters, which we'll review later in this lesson.
  • Everything enclosed in the curly brackets { and } is called the function body; this is where we write out all the code that we want the function to execute when we call on it.

Indentation and Spacing

Note, the code snippet above shows the proper indentation and spacing for functions. It's common convention to write JavaScript functions on multiple lines to make the code more readable. While we could write our sayHi() function declaration on one line like this:

// Don't write function in one line.
function sayHi() { return 'Hello from Epicodus!'; }                                             

It's not considered best practice.

Semicolons in Function Declarations

Let's continue examining our function declaration:

// Here we are defining the sayHi function; this is a "function declaration".
function sayHi() {                     
  return 'Hello from Epicodus!';

Notice how we are using semicolons here. The first line, which uses the function keyword and includes the name of the function, ends with an opening curly bracket {. It will never have a semicolon.

The next line has a return statement. Statements and expressions have semicolons at the end of the line regardless of whether or not they are inside function declarations.

The return keyword tells JavaScript to return anything to the right of the return keyword. In this case, we're returning the string 'Hello from Epicodus!'. We could also return a variable instead:

function sayHi() {
  const greeting = 'Hello from Epicodus!';  // Each statement is on its own line.
  return greeting;                          // Each statement is on its own line.

Without a return statement, JavaScript will return undefined from the function, which is JavaScript's way of saying something does not have a value. Also notice that each new statement in the function body is written on a new line. This is also a best practice for code readability.

Finally, we close our function with a curly bracket }. We don't add a semicolon here. Why is this? Well, this is a part of the function declaration. It's not a statement. JavaScript knows that the curly bracket } represents the end of the function so it doesn't need a semicolon.

If semicolon placement feels fuzzy still, it's really not something to worry about. Understanding semicolon placement is important but there are much more important topics we need to focus on right now like learning about functions and how they work.

Calling a Function

For this next example, we're putting the code into the DevTools console. To call our sayHi function, this is what we do:

// First we input the function declaration.
> function sayHi() {                     
  return 'Hello from Epicodus!';
// Then we call the sayHi() function by including parens.
> sayHi();                               
'Hello from Epicodus!'

Every time we run the sayHi() function, it executes all JavaScript code between the opening and closing braces — { and }. In this case, our function returns the string 'Hello from Epicodus!'. This isn't terribly useful so let's write a slightly more interesting function.

A Function with a Parameter — saySomething(whatToSay)

Let's write a function in the DevTools console that has one parameter. We'll name this function saySomething:

> function saySomething(whatToSay) {  // function declaration with 1 parameter
   return whatToSay;
> saySomething("hello!");             // function call with 1 argument

Let's review the parts of this new function:

  • When we're declaring a new function, we start with the function keyword. A function declaration always starts with this keyword.

  • Next, we have the function name saySomething. After the name of the function, we will always include parentheses to list any parameters (more on this below): saySomething(). Just like with naming variables, function names should:

    • Be lower camelCase, which means the first letter of the name should be lower-cased while the first letter of other words in the function (Something in this case) should be capitalized.
    • Be named descriptively so other developers can quickly understand the purpose of the function. In the example above, we are simply denoting that this function will say something.
  • Next, we have the function body. This is enclosed in two curly brackets, { and }. The function body includes any code that should be executed when the function is called, in this case return whatToSay;. It is convention to have the opening curly bracket on the same line as the function declaration. Meanwhile, the closing curly bracket goes on the final line of the function.

  • If a function has parameters, they go inside the parentheses next to the function name: saySomething(whatToSay). A parameter is a placeholder variable in the function declaration that doesn't initially have a value. The value of parameters are set by the arguments we pass into functions when we call them. In other words, parameters are placeholder variables for any data (called "arguments") that we want to pass into a function when we call the function. Here's more of a breakdown:

    • whatToSay is the parameter in the saySomething() function.
    • When we call the function with an argument, like saySomething("hello"), we are setting the value of the whatToSay parameter to the argument, which is the string "hello".
    • Then, anywhere the parameter whatToSay is used in the function body, like in the return statement return whatToSay;, the argument's value "hello" is used. This makes the function return "hello". If we called saySomething("Howdy"), then the function would return "Howdy".
    • Note that we can create as many parameters as we want in a function.

If you're confused about the difference between arguments and parameters, just remember that the argument is the data you pass in, and the parameter is the variable that receives the argument as a value. In the saySomething() function, "hello!" is the argument, and whatToSay is the parameter. The parameter can then be used just like any other variable. Let's look at another example to better understand functions, and the difference between parameters and arguments.

A Function with Two Parameters — add(number1, number2)

Okay, on to another slightly more complex function:

> function add(number1, number2) {  // function declaration with 2 parameters
   const sum = number1 + number2;
   return sum;
> add(3, 5);                        // function call with 2 arguments

Let's step through exactly what happens when we call add(3, 5):

  1. We call the add(3, 5) function with two arguments 3 and 5: because the add() function has two parameters, we must pass in two arguments, one for each parameter.
  2. The add function is run. The parameter number1 is set equal to 3, and the parameter number2 is set equal to 5.
  3. The expression 3 + 5 is run and we set the value of the sum variable equal to the result of the expression, which is 8.
  4. The variable sum (with a value of 8) is returned.
  5. The add function ends.

Notice our variables names: number1 and number2. We could have called them n1 and n2, and it would have taken less typing. But the name number1 very clearly expresses what the variable is for, whereas n1 would require another programmer to figure it out from context (or your future self, when you come back to your code in a few months and don't remember exactly how it works). In this example, it is pretty obvious what n1 is for. But if you practice choosing descriptive names now and resisting the temptation to abbreviate, you will be in the habit of doing it when you're writing more complex code where it matters more.

Documentation on MDN

To see the documentation on function declarations, including more examples, visit this page on MDN:

Next, let's practice what we've learned and write and call some functions!


  • An argument is what you pass into a function or method when you are calling it.

  • A parameter is a placeholder variable for the data we pass into a function via the argument. The value of a parameter is set to the argument we pass into the function when we are calling the function. In the example below, number1 and number2 are parameters while 1 and 2 are arguments.

Sample code

This function takes 2 arguments (expecting two numbers) and assigns them to the parameters number1 and number2. The function returns a number:

function add(number1, number2) {  // function declaration
  return number1 + number2; 
> add(1, 2);                      // function call
3                                 // returned value from the function

Parts of a function

  • A function declaration starts with the function keyword.
  • Next comes the name. It should be descriptive and clear. In the example above, the name is add. Always use lower camel case for function names.
  • The name is always followed with parens. These parens may or may not include parameters. In the add function above, number1 and number2 are parameters.
  • The function body is enclosed in curly brackets. The code inside the curly brackets is the code that will be executed when the function is called.
  • Generally, you will want to return a value from your function. It's very common for beginners to forget the return, which means that the function will return undefined instead of a value. In the example above, the add function will return the value of number1 + number2.


  • Use lower camelCase for function names.
  • Use descriptive variable names so that your code is easy to understand. Don't be tempted to abbreviate.

Lesson 25 of 72
Last updated July 15, 2022