Lets Get The complete guide to JavaScript functions

Functions are one of the most useful features of JavaScript. Learn about function inputs, outputs, variable scopes and more in this article!

Getting started!

First of all, I encourage you to follow along in this article. It will help you learn better, and also help you to remember what you have done. Let’s start by making a new HTML file with a <script> tag in it:

Once that’s done, open it up in your web browser and you’re ready to go! (don’t forget to save and reload the page every time you make a change)

What is a function?

A function is simply a bit of JavaScript code which you can run again and again. You can even give it with different inputs and outputs!

The syntax

For the moment, let’s look at the simplest type of function:

First, we’re declaring the function. We do this by saying the word function and then the name of your function. This is similar to how we declare variable (var variableName is similar to function functionName). After the name, there is an empty set of brackets / parentheses / whatever you call these things: ().

What’s the point of them if they’re empty though? Well, this is where we put inputs for the function. At the moment, our function doesn’t have any inputs so we leave it empty. I’ll get onto inputs a bit further on in this article (get excited ????).

After that, we have a set of curly brackets (the ending one is on the fourth line). Inside these curly brackets goes all the code in the function.

Running (aka ‘calling’) a function

Now you know how it works, let’s run it! Put it in your <script> tag, save, and reload the page…

What!?? Nothing happened! Well, here’s the thing — we’ve only declared the function, not actually run it. We’ve just said “hey magical computer, here’s this bit of code and it’s called myFirstFunction”. However, we haven’t told the computer to run the code inside the function! Here’s how we do it:

As you can see, we run it by referring to its name. We then have an empty set of brackets. This is where the inputs for the function would go, but we don’t have any inputs just yet so again we leave them empty. Time to run your function! See if you can work out where to put it in the code:

In this bit of code, it’s at the end. However, it can be before the function too! This is very different to variables, where you have to declare the variable before using it. This wouldn’t work:

However, this would:

It is common practice to put all your function definitions at the bottom of your script, but it doesn’t really matter that much.

Ooookay, here’s the moment that you’ve probably been waiting for — time to try it out! Make sure that your script looks like the code above (although remember that it doesn’t matter if you run the function before or after declaring it). Save, reload and Ta-da! Our code inside the function has now been run!

At first, this may seem like overkill — why not just run the code? Well, imagine if you wanted to make this particular bit of code run at many different points inside your script — inside loops, if statements, and more. Well, then only having to say myFirstFunction(); would come in handy! Especially if your function was 30 lines long ????

Functions with inputs

In the function myFirstFunction, we set a variable (x) to 5 and then alerted that (10). What if we were to make a function called alertDouble which alerted double any number?

In this case, we’d have one input — let’s call it num. Here’s how the function declaration would look with num as our input:

Now inside the curly brackets, we can use num as a variable! (only inside the curly brackets)

See if you can work out how the finished function would look…

Did you get it right? If so, great! If not, don’t worry — hopefully, you’ll start to get it by the end of this article ????

Running functions with inputs

Time to run our function! Let’s try doubling a few different numbers:

  • 2
  • 70
  • 1024

First of all, you can leave this part of your existing code if you want:

However, we’re done with it and don’t want to run it anymore. So, delete or comment out this line:

Instead, we want to call our new function alertDouble! First, we need to add the function declaration to the end of the code:

Time to run our function! If it didn’t have any inputs, we would simply say alertDouble(). However, we need to put the input for num inside the brackets! So, let’s try doubling 2 — see if you can guess the code!

The 2 inside the brackets then becomes num inside the function when it is run! Try it out…

Woo! It alerted 4! ???? Let’s try with the other values…

Yay! Now it alerts 4, 140 and then 2048. Time to move on…

Functions with multiple inputs

In the function alertDouble, we multiplied num by 2. What if we wanted to make a function called alertMultiplied where we alerted one input multiplied by another input? Here we’d need two inputs: let’s say num1 and num2.

First of all, we need to declare the function! When functions have multiple inputs, they are separated by commas — see if you can work it out!

Then, it’s just a simple matter of alerting num1 multiplied by num2!

Running functions with multiple inputs

First of all, comment out (or delete) all the doubleMe function calls like so:

Now, let’s add the alertMultiplied function declaration to the end of the code!

Finally, it’s time to run the function. Let’s try multiplying 5 by 3 and 8 by 21. See if you can work out the code (hint: the inputs are separated by commas just like in the declaration ????)

Save, reload and see what happens! It should alert 15 followed by 168.

Functions with an output (functions that ‘return’ something)

As well as having inputs, functions can also have outputs! However, a function can only have one output (unlike inputs).

Let’s make a function called triple. However, unlike alertDouble, it won’t alert the result. It will output it!

To output the result, we simply say inside the function return and then whatever we want to output. Here’s an example:

Note that it doesn’t require brackets! However, you can put them in because they’re optional. I prefer to put them in because it makes it easier to understand, but it’s up to you. It would look like this with brackets:

Obviously, the value returned doesn’t have to be a string.

Let’s try making our triple function! It will need one input, num. And it will need to output triple that number. See if you can work out how the function would look!

Running functions with an output (getting the output)

That’s all fine and well, but how do we actually get the output of this function? We do that by running it!

The actual function call will become equal to the output, just like a variable is equal to a value. We can then use the function just like a variable. For example, we could say the following:

Here, we are alerting the output of triple(10). Try it out! Don’t forget to add the declaration and comment out the calls of alertMultiplied:

Woo! The code will alert 30! We can also use the triple function just like any other number:

This is simply like saying (18 + 3) * 2, which is equal to 42 (the meaning of life of course ????). Try it out!

Now after alerting 30, our code will also alert 42! ????

Variable scopes within functions

We’re nearly done now! ???? However, there’s one more important thing that you need to know about functions — Variables declared inside functions do not work outside of functions. Whaaat? Here’s an example — try it out!

It should alert 3, right? Nope! Because myVar was declared inside the function, it does not exist outside the function. The code will not work and give you the following error:

Uncaught ReferenceError: myVar is not defined

However, this does work!

The code will alert 3! Because myVar has already been declared outside the function, the code knows that it exists. It will be changed inside the function, and then the code can still alert it because it was defined outside the function.

This is known as the global scope. It means that for the rest of the code, myVar will be defined. In the first example where myVar was declared inside the function, it declared in the function scope. It means that the variable will only be defined inside the curly brackets of the function.

This might take a while to wrap your head around, but you’ll get it eventually. The main thing that you need to remember is that if you want to use a variable outside a function, declare it outside the function first. You don’t even have to say var myVar = 1; — you can literally just say var myVar; and that is enough. I may do an article focusing more on variable scopes in future.


Whew! ???? That was a lot to cover, so good job making it all the way through! As always, keep practicing and you will eventually get it. If you need any help, feel free to shoot me an email or hit the comments.

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