In the beginning Kernighan created "Hello, world!"

Understanding programs, how the JavaScript interpreter works, and writing our first program

In the previous chapter,¹, I mentioned programming a few times. In this article we'll write our first JavaScript programs and explore what programs are in a little more depth.

Hello, world: our first JavaScript program

To use most programming languages you have to download an environment to run programs. Sometimes you also need to download a specialized text editor to write programs.

We'll actually do both of these for our purposes in a future chapter, but one of the nice things about JavaScript is if you have a web browser you can already run your own JavaScript programs without having to download any specialized tools. You can do this through the browser's console.

How to access the JavaScript console

Mac users can substitute the COMMAND key for CTRL and OPTION for ALT.

First, open a new browser tab with a blank page. You should be able to show a completely blank page by putting about:blank in the URL bar of your new tab.

This will keep items specific to any particular web page from showing up in your console.

Opening the console will vary by browser, but here are the basics:

• If you have Google Chrome, bring up the console with the keyboard shortcut CTRL+ALT+J, by pressing F12 and then selecting the console tab in the development tools that pop up, or by right-clicking anywhere on a web page, selecting Inspect element in the context menu, then selecting the console tab.
• In Firefox use the keyboard shortcut CTRL+ALT+K, press F12 and select the console tab, or press ALT to activate the main menu and then select Tools > Web Developer > Web Console.
  • Note that in Firefox the Web Console is separate from the Browser Console. The differences between the two don't matter for now.
• On Safari you may need to activate the console. Open the menu bar, choose Safari > Preferences, click the Advanced tab, then choose Show Develop menu in menu bar. Then open the console with CTRL + ALT + C.
• If you're using Microsoft Edge, follow these steps:
  1. Download and install a better browser for development work, like Chrome or Firefox.
  2. Close Edge and open the newly-installed one.
  3. Follow the above steps for the browser you've chosen.²

Writing your first JavaScript program

Now that you've got the console open, make sure your cursor is visible next to the prompt.

Console prompt in Google Chrome

Simply type into the console "Hello, world!" and press ENTER.

The console should repeat "Hello, world!" back to you.³

That's right, your first JavaScript program is simply a sentence in quotation marks!

This is actually a complete, valid JavaScript program, but it's not terribly useful. We'll write much better ones by the time we're through this series.

How the interpreter works to run your first program

The console will simply evaluate any valid JavaScript code that you type into it and return the value it calculates back to you.

A valid piece of code that forms a complete "thought" in JavaScript, analogous to a sentence in English, is called a statement.

A statement the JavaScript interpreter reads, evaluates, and returns a value from is called an expression.

Any value that's valid to the JavaScript interpreter can be used as an expression.

You should always end expressions with a semicolon (;).⁴

In this case, the interpreter takes your expression, "Hello, world!", evaluates it, and, since a simple value evaluates to itself, returns it back to you.

What is a program?

Now that you've written your first program let's talk more about what a program is.

At its most basic level, a program is a piece of written language that tells the computer to perform a computation.

How exactly a program is written depends on the language, the programmer's style, and in some cases the machine the program is written for.

For example, program can have detailed instructions on how the computer should compute a solution. This is known as imperative programming style.

Or it could simply tell the computer what to do, and let it handle the steps. This is called declarative programming style.

Some languages require you to use one style or the other.

Others, like JavaScript, allow you to use either style or a mixture of both in your programs.

We'll talk more about programming styles throughout the series.

Try it yourself: exercises for further learning

1. Type additional words and sentences into the console. Make sure to enclose them in quotation marks! Try them with single quotes ('), double quotes ("), and backticks (`).
2. Type a number into the console. Does it work the same way as with words and sentences? Try it with both integer and decimal numbers.
3. Try it with the words true and false (no quotation marks).
4. Type an empty pair of brackets ([]). Click the caret next to the output and explore the result.
5. Type a word without quotation marks. What happens?

Now that you know more about how the console works, you can code along with the examples so you get a better sense of how to type in code.

Notes

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1. In most programming languages, counting starts from zero instead of one. This note is here because in the Github repo for this series it starts with chapter 0 instead of 1. ↩

2. If you're using Internet Explorer, you may need to seek help beyond what I can give you. ↩

3. Printing "Hello world" as the first program when learning a new language is a tradition first found in Brian Kernighan's 1972 A Tutorial Introduction to the Language B. See Wikipedia for more about the history and purpose of "Hello world". ↩

4. Technically you can write most JavaScript expressions without a semicolon at the end and it will perform Automatic Semicolon Insertion; however, the rules for this are complex so you're better off always using a semicolon for now. When you have more experience you can decide whether or not to use semicolons. There are popular JavaScript style guides for both cases. I prefer to always use semicolons, but in actual projects you should go with the style guide the project you're working on uses. ↩