2.5 Functions

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What you'll learn:

  • What functions are
  • Defining functions
  • Input and output return-values of functions
  • Case-sensitivity of functions


So far, we’ve been focusing on the JSX code, that is, this piece of code that looks like HTML.

Let’s now zoom out a bit to look at the rest of the code. There’s a word "function" here, which means that what we have here is a function.

function App() {
return ( <div className="App"> <Frame width={120} height={60} borderRadius={30> <Frame size={60} borderRadius={30}/> </Frame> </div> ) }

The function is another fundamental concept in JavaScript. It describes what our code does.

A function is like a machine. It’s a machine that produces some output when we feed it some input.

image of a machine

Creating Functions

To create a function, we start by writing the word function


Followed by the name of the function and a pair of parentheses

function microwave()

Inside this pair of parentheses, we put the input of the function.

function microwave(food)

Function Inputs

Note this food here is a general representation of the input. The input depends on what we feed it when we operate the machine. The technical term for the input is "parameter".

To finish the function definition, we write a pair of curly brackets at the end. The space between those curly brackets doesn't matter.

function microwave(food) {}

What’s "inside" and between the curly brackets is called function body. It is the inner working of the function.

Function Outputs

Now if this food is input, where is the output? We can define the output by adding a return at the last line of the function body.

function microwave(food) {
  return 'heated ' + food

Because of this return, we are outputting some value out of the function often called its "return value".

This means that when we operate this microwave and put some food as into its input, we’ll get heated food in return.

Let’s create a slightly more complex function. Let’s create an imaginary machine which makes sandwiches for us.

function sandwichMaker(meat) {}

Inside the function body we can write

function sandwichMaker(meat) {
  let sandwich = '🍞'
  sandwich = sandwich + meat
  sandwich = sandwich + '🍀'
  sandwich = sandwich + '🍞'
  return sandwich

Don’t worry about the details here, but you see it’s a list of instructions about how to make a sandwich. These instructions would run from top to bottom.

Finally, when everything is done, the function returns the sandwich to whoever orders it.

In total, our functions have inputs and "recipes" on how to create the output. They then return the output.

The technical term for what we’ve been doing is called "defining a function"

Different Types of Functions

Now let me ask you, what’s the input and output of this App function?

function App() {
  return (
    <div className="App">
      <Frame width={120} height={60} borderRadius={30}>
        <Frame size={60} borderRadius={30} />

Unlike the functions we created, there is nothing inside this pair of parentheses. meaning there is no input for this function. The function's output is this group of JSX tags after the return — inside the return's pair of curly brackets.

Capital vs lowercase functions

Have you also noticed that the name of this function App starts with a capital letter? Why did I then use lowercase letters for the other functions? More so, in JavaScript, the convention is that function names start lowercase.

The App Function

In contrast to many other functions, the App function is a special React component. Therefore, if you see a function whose name starts with an uppercase letter and returns a JSX tag you should know it’s a React component.


You’ll get to know more about React components later on, for now, it’s useful to realize that App is simply a special function with unique requirements.

In the next post, you'll learn how to call functions and how to create variables.