Ever visit a website and a little box pops up with a message? It might say “Welcome!” or “Please enter your email.” That’s not magic—it’s a tiny piece of JavaScript in action, and it’s one of the first ways you can start adding real behavior to a webpage. Think of it like giving your site a voice, a way to talk back to the person using it. Today, we’re going to learn how to make that happen with our very first JavaScript command: the humble alert().
To get started, you don’t need any fancy software. Just open up Chrome. Go to the menu under View > Developer > JavaScript Console. A new panel will open up. This console is amazing because it lets you type JavaScript code and run it instantly, just by hitting Enter.
Let’s try it. Type this exactly:alert("Hello");
Then press Enter. Boom! A pop-up appears with your message. That’s it. You’ve just written JavaScript that changes how the browser behaves. Click OK, and the pop-up disappears. The console is perfect for testing a single line like this. But what if you want two alerts, one after another? If you try typing a second line and hit Enter, your first line runs immediately, and you lose your place. It’s frustrating.
Here’s the trick: hold down Shift and then hit Enter. This lets you go to a new line without running the code. You can write:alert("Hello");alert("World!");
Now, press Enter (without Shift). You’ll see the “Hello” pop-up, and after you click OK, the “World!” pop-up follows. It works! But honestly, writing multi-line code this way is a bit cumbersome. If you forget to hold Shift, you lose everything you just typed.
Thankfully, Chrome has a better tool for writing little scripts. It’s a bit hidden, but super useful. In that same Developer panel, click the Sources tab. On the left, you’ll see a tiny arrow. Click it and select Snippets. This is a mini code editor right inside your browser.
Click the + New snippet button and name your file something like index.js. Now you have a clean space to write code, just like in a proper text editor. Here, you can write multiple lines without any stress. Try typing:alert("Hello!");alert("World");
When you’re done, look at the bottom right of the snippet area and click the Run button. You’ll get your two alerts in sequence. This Snippets editor is what we’ll use for learning—it’s built for writing a full script and running it all at once, not just one-line tests.
Now, how does the browser know what alert means? What if you typed say("Hello") instead? If you try that in your snippet, after the alerts run, your code will “crash.” The console will show an error: Uncaught ReferenceError: say is not defined. In plain English, the browser is saying, “I have no idea what ‘say’ is. That’s not a real instruction for me.”
alert is what we call a keyword or a function—a special word the browser recognizes as a command. It tells the browser, “Do this specific thing: show a pop-up.” To know all the keywords you can use, the best resource is the MDN Web Docs. Search for “alert” there, and you’ll find the official documentation for Window.alert().
Let’s break down the line alert("Hello"); in detail. It’s like a sentence with a very specific grammar:
alert: This is the function, the action word. It’s a command the browser understands.("Hello"): The parentheses (or round brackets) follow the function. What’s inside them is the message—the data you’re giving to the command. In this case, the text you want to show."Hello": The quotation marks (double quotes) tell the browser, “Everything between these marks is just plain text, not more code.” In programming, we call this a string—a string of characters.;: The semicolon is like a period at the end of an English sentence. It tells the browser, “This instruction is finished. Move on.”
Just like you need periods and quotation marks in English to make your meaning clear, you need these symbols in JavaScript so the computer knows exactly what you want.
A couple of crucial style tips as you start writing:
- Use “straight” quotes, not “curly” ones. Word processors like Google Docs use fancy, curly quotation marks. Code requires the simple, straight ones (
"). If you copy-paste from a document, it might break. Luckily, tools like the Snippets editor automatically give you the right quotes. - No spaces between the main parts. Write
alert("Hello");notalert ( "Hello" ) ;. While spaces sometimes work, this tighter style is the common best practice. It makes your code neat and consistent with what other programmers write. - Use double quotes for strings. In JavaScript, you can use single quotes (
'Hello'), but the common style is to use double quotes for text strings. We’ll use single quotes for specific cases later.
If you ever feel unsure about the “right” way to style your JavaScript, there’s an amazing community style guide you can bookmark: JavaScript Style Guide. Its core idea is beautiful: all code in a project should look like it was written by one person, even if a team worked on it. It’s like a grammar book for code. Don’t worry about understanding it all now, but as you grow, it’s a fantastic reference to check your style against.
Your Practical Takeaway
Open Chrome right now, go to Sources > Snippets, create a new file, and write a few alert() commands with different messages. Run it. You’ve just built a tiny behavioral script. This is the foundation. Next, we’ll look at the different types of data—like numbers and text—that we can put inside those parentheses. See you in the next lesson
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