Evan X. Merz

musician / technologist / human being

Introduction to JavaScript

In this post, I'm going to introduce the JavaScript programming language and show you a few ways it's commonly used on the web.

WARNING: Some prior programming required!

I'm not introducing programming in general. This isn't a good first tutorial for someone who has never written a line of code in their life. I'm assuming that you know what functions are, and that methods and subroutines are the same thing. I'm assuming that you've written at least a little of some kind of code in the past.

TLDR: JavaScript is the engine of a webpage

HTML is used to define the structure of a web page. CSS defines the look and feel of a page. JavaScript defines how a page acts and responds to user input.

If a web page is a car, then HTML is the frame, CSS is the body, and JavaScript is the engine.

A haiku about JavaScript

Dearest Javascript,

You are my favorite tool

that costs me nothing.

Why not Java?

JavaScript has nothing to do with Java. The name JavaScript came about as an attempt to steal some of the hype around the Java programming language in the 1990s. In the 30 years since then, their positions have entirely flipped, with JavaScript now being the programming language that sees tons of innovation and market support, while Java is withering. Why has this happened? Why has JavaScript been so successful?

In my opinion, Java has been uniquely unsuccessful as a language because it has refused to grow and innovate. Java set some standards and some conventions for the language, and even thirty years later, the committees who guide specification for that language continue to be far too conservative.

If you're a Java programmer and you don't agree with me, then I encourage you to try Microsoft C#. That's the fun language that Java could have been.

And why is that?

C# and JavaScript have become incredibly open-ended. You can write code in lots of different ways, using features borrowed from other cool languages and frameworks. Both are sort of like Frankenstein's monster inasmuch as they are collections of mismatched parts that come together to form a surprisingly intimidating beast.

And this scares some people.

But, much like Shelley's actual monster, they are actually quite nice, and I hope to convey that to you in this introduction.

But what is JavaScript?

It's a programming language that runs in the web browser and on nearly every device that exists. On a webpage, you include JavaScript within the script element, or by including code in an external file. There's more to it than that, but let's learn by doing.

A JavaScript function

One of the key responsibilities of JavaScript is to respond to user input. Beside a few basic interactions, like clicking a link or a dropdown, every action that occurs on a webpage is handled by custom event handlers that are usually written in JavaScript. In this case, we're going to write a script that handles a click event.

When the user clicks on something, the browser creates what is called a click event. It then runs a bit of code called a function that is tied to that event. Here's the click handler function that we're going to write.

function showWarningOnClick() {
  alert("Warning! The elders of the internet have been notified!");

That might look like nonsense to you, so let's break it down. This graphic shows how each of the pieces come together.

A javascript function

The first line is the function declaration. The word "function" declares that the next word is the name of a function. So "showWarningOnClick" is the name of the function. The parentheses, "()", would enclose the method arguments, if we needed any. The open bracket, "{", indicates that the next line will be the first line of the function body.

The second line is the actual code that is executed by this function. In this case, it calls another function. It calls "alert", with the argument "Warning! The elders of the internet have been notified!". Notice that the argument is enclosed with parentheses. Then the line ends with a semicolon. Most lines of JavaScript code must end in a semicolon.

That's the code that we will run when a button is clicked. We will see in the next section how we can trigger that function using a button.

A JavaScript event

The following line of code creates a button and adds a handler for the onClick event.

<button onClick="showWarningOnClick();">Don't click me</button>

Notice that the button isn't created using a div or span element, as you might expect. When creating buttons, it's good to use the button element if you can. This element is friendly to screen readers, and other accessibility technology.

The only attribute given is "onClick", and it's set to call the function that we defined earlier. The onClick attribute tells the browser what to do when the button is clicked.

It's important to note that onClick events can be attached to any elements. They aren't something unique to buttons. You can attach methods to the onClick attributes of divs or spans, or any other HTML elements.

Responding to a click event in JavaScript

So let's put it all together. The code should look something like this. And as a reminder, please type this in, don't copy paste it. The point is to learn, and you will learn and remember more if you type it out.

<!DOCTYPE html>
    <title>JavaScript Hello World</title>
      function showWarningOnClick() {
        alert("Warning! The elders of the internet have been notified!");
      <button onClick="showWarningOnClick();">Don't click me</button>

When you run it, by double clicking it and opening it in a web browser, you should see a single button in the upper left that says "DOn't click me". Click it, and you should see a little popup with the message "Warning! The elders of the internet have been notified!".


JavaScipt is a fun, open-ended programming language that is particularly useful in web browsers. In this post we learned about JavaScript functions and event handlers.


Here are some other introductions to JavaScript from around the web. I suggest that you spend time to check them out, even if you found this really easy. It's good to go over the fundamentals often, and it's particularly good to get multiple perspectives.