Hello, World

So far, we've been using a REPL to execute JavaScript one command at a time. More often, you'll write a script: a series of JavaScript expressions and statements that can be passed to the JavaScript engine all at once. The JavaScript engine will still execute them one at a time in the order you set out, but it doesn't have to wait for you to give it the next command.

Typically a script is saved as a file. Maybe you've seen Microsoft Word files that end with an extension like .doc or .docx or an image file that ends with a .jpg or .gif extension. Most JavaScript files end with a special extension: .js. The extension isn't strictly necessary, but it helps users and some programs know how to handle the file.

It's not obvious, but the editor on this page is showing you the contents of a file. When you click the ▶ button, the code you've written in that file will get passed to the JavaScript engine. Output, if there is any, will show up in the box below the editor.

Give it a try: click the ▶ button . . .

Nothing happened, right? Not quite. The JavaScript engine did run the code you see in the editor, but there's nothing in the script that tells it to output any values.

Let's add one line to the script:

let greeting = "Hello, World!";
console.log(greeting);

Try it again. Voila! You should see the output Hello, World! in the console below the editor. If we want to send a value to the console, we need to "console log" it.

You can pass more than one value in the same console.log function. Try this:

let greeting = "Hello, World!";
console.log(greeting, "Nice to meet you.");
console.log("Bye!");

Try logging a few more things to the console. So far, we've been logging words. Can you log numbers to the console ? Could you log the result of a calculation?

When you're finished experimenting, click the ✓ button.