2.6 Calling Functions and Variables

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What you'll learn:

  • Calling functions
  • Creating variables with let statements
  • Assigning variables and compound assignment
  • Printing function values and console.log
  • Embedding Functions as parameters
  • Browser function output

Calling Functions and Variables

Now that we’ve got sandwichMaker, how do we use this machine to make sandwiches? The technical term to "activate" the function is named "calling the function".


Inside the parentheses, we put input in and, in this case, it's bacon! However, we aren't doing anything about the output. We are simply leaving the sandwich there. It is kind of like buying a sandwich maker, making a sandwich and throwing the sandwich away. That's a waste!

Instead, we extract the output by storing it into a variable.

let lintonsLunch = sandwichMaker('🥓')

Now, we will get the output of the sandwichMaker and have a "label" on it. This way hopefully nobody will eat my lunch 👀.

The importance of this label is that we can use the output-value later in my code.

Although the sandwich is virtual, and I, unfortunately, can't eat it, we can at least watch and print it. Let’s do that.

let lintonsLunch = sandwichMaker('🥓')
sandwich string in console

The console is located at the bottom right of the preview page.

The label we used here — lintonsLunch — is called a "variable" in JavaScript and almost every other language.

You can think of it as a labeled bucket that we can put things in.

We define a variable by writing a let. Followed by the name of the variable, an equals sign, then what we want to put inside the bucket.

let variableName = someValue

In sandwichMaker, the sandwich is a variable as well since we put a let statement before it. We initialized sandwich to a string or, more specifically, a piece of bread!

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

We can replace the value of a variable anytime with an equals sign.

let sandwich = '🍞'
sandwich = newValue
sandwich = newNewValue

Compound Assignment

sandwich = sandwich + meat

This line means that we get what’s currently in the bucket, concatenate (adding string) with meat and put the result back into the bucket. It's called a compound assignment because a calculation and an assignment is occurring.

Shorthand Compound Assignment

A simplified method or more complicated, depending on how you look at it is this

sandwich += meat

This process repeats until we get a "full sandwich" (adding the veggies and then the final piece of bread) in the variable.

The variable is the third fundamental concept in JavaScript that you should know, along with objects and functions.


Now let me ask you, what’s this console.log? It must be a function because of the pair of parentheses after it, but we didn't define it.

Well, it is a function and although there is a dot here, which I’ll explain later, it is simply another function.

However, it is a bit different than our sandwichMaker, since we are not using its return value in any way, but let's try.

let lintonsLunch = sandwichMaker('🥓')
let output = console.log(lintonsLunch)
console undefined

It looks like some functions do not produce any output, but it does show something on the screen. Functions that don't produce any output can even save something to the disk and more.

Embedding Functions as Parameters

By the way, we don’t necessarily need to save the output of a function to a variable to then put it inside console.log.


We can even actually embed more functions calls like this and still get the same output.


sandwichMaker is first run, the output is put it into microwave as input, then the output is fed into console.log.

console heated sandwich

You can also call functions multiple times.


Now, we simply get four of the same outputs, and we can change the parameters if need be.

All in all, functions are very cool and flexible machines.

Browser Function Output

Now that we have a sandwich maker function, how do we show the result on the web page, rather than just in the console?

Let’s say we put it inside a div in App. Will we get the same console result?

<div className="App">

Unfortunately, this doesn't work. I get the literal text inside the <div>.

browser preview of sandwich maker

To fix this, we can add a pair of curly brackets to make the code dynamic. These curly brackets will treat what’s inside as executable code and try to run it. Here it’ll call this function and put the output on the resulting web page.

<div className="App">

Which results in

browser preview of sandwich maker


Great! You now know how to call a function and what variables are! We'll learn next the different types of variables and how to create them.