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JavaScript provides ways to leave messages in your code. When you or another developer review your code later, these messages (or comments) can quickly help describe what's going on in the program. You can add comments anywhere in your JavaScript code; they're ignored by a web browser and don't affect how your program works.
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It's a lot of fun to program, but
0:01
it's not always fun to return to a program
you wrote six months or a year ago.
0:03
Looking at an old program sometimes feels
like you're reading a foreign language,
0:07
it's easy to forget why you
programmed something the way you did.
0:11
Your code is not just for
the JavaScript engine to interpret,
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you should also consider other developers
who might read and work with your code.
0:18
Fortunately JavaScript provides
a way to leave messages in your code
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that can help explain what's
happening in a program.
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When you or another developer look at
your code later, these messages or
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comments can quickly help describe
what's going on in the program.
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You can add comments anywhere
in your JavaScript code.
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They're ignored by web browser, and
don't affect how your program works.
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Where should you put comments?
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Well it's a good idea to add comments
to identify different sections of code.
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For example, to introduce a long
conditional statement, or
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to explain particularly
complex blocks of code.
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Comments not only help you, but
also anyone else who works with your code.
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JavaScript provides two different types
of comments, single-line comments and
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multi-line comments.
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Single-line comments are represented
by two forward slashes,
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everything following the slashes until the
end of the line is ignored by the browser.
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If you want to add a long message to your
code, you can use a multi line comment,
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which begins with a forward slash and
asterisk.
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Anything following those two characters,
even carriage returns and multiple lines,
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are ignored by the JavaScript engine
until it encounters a final asterisk and
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forward slash.
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I'll teach you how to use
JavaScript comments by documenting
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the number guessing game you
wrote in the file booleans.js.
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First, I'll add a multi-line comment
that explains what the program does.
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The number guessing game stores
a number between 1 and 10,
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And gives the player one
attempt to guess the number.
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It's common to add a multi-line
comment like this to the beginning of
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a JavaScript file.
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This is a place where many programmers put
copyright notices and author information.
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Now, I'll add a single line
comment that says when the game
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begins the guess is false.
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Developers usually include the comment
directly above the code they're
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commenting on.
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In this case, I'm explaining what
the next line of programming does.
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Now, you don't need to comment every
line of your code, for example,
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the next two lines
are pretty straightforward.
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This line stores a number and
a variable, and
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the next line opens a prompt dialogue and
stores the user's input in a variable.
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You also might add a comment
that explains a piece of code.
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For example, the next part of this program
is the first conditional statement.
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So you might add a multi-line comment
to explain or outline what it does.
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First, test if a player's
guess matches the number.
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Then change the value of
correctGuess to true if they match.
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I'll add one last comment to explain
what happens in the final conditional
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statement.
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Test if guess is correct and
output response.
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Finally, there are a couple of other
ways you might see comments used.
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For example,
you can use single-line comments
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to annotate small specific
snippets of your code, like this.
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These are also referred
to as in line comments,
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since they're related only to
the line they're written on.
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Now, too many of these comments can
make your code harder to read, so
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you should use them sparingly.
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Finally, you can use comments to disable
code and prevent it from running.
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For example,
comment out a line of code like this.
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Now the JavaScript engine is not going to
run this line because it's commented out.
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Using comments like this can
be helpful when debugging,
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just make sure not to leave snippets of
commented out code in your final script.
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So as you've learned, adding comments to
your code is not only helpful for you, but
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also anyone else who needs to
see your code in the future, so
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get in the habit of using them.
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