JavaScript comments
The JavaScript from the
input
page contained this line:
// Get the user's name and say hello.
This is a comment in the JavaScript program. The page already has an HTML comment
marked by <!-- ... --> but the double forward slash marks the start of a
JavaScript comment to explain what's happening in the program itself.
There are two types of comment in JavaScript. One type starts with a double slash
and everything that follows until the end of the line will become a comment. The
other type starts with /* and everything that follows until */ will become a
comment. This second style is very useful because it can extend over several lines and
you can use it to temporarily remove a section of code from your program. Put a /* at the
start and a */ at the end and that whole section of code will be ignored.
Writing comments
Use the first type of comment for the usual purpose of explaining what the program
is doing. To be more precise, use them to explain why the program is doing what it's
doing.
There is no use at all in a comment like this:
// Use prompt to display "What is your name"
// Set the default name to be blank
userName = prompt("What is your name", "");
Anyone reading the program can see that you're using prompt to display this text
and that the default will be blank. What they need to know is why you're doing it.
This next sample would have been a more useful comment. It's shorter but it tells the reader
something that they can't see from the code itself.
// We need the user name to personalise the output
userName = prompt("What is your name", "");
Debugging with comments
If you get into the habit of using // for your regular programming comments then you
can use the other style as an aid to debugging.
You should always write programs in short bursts, testing as you go, and you will
often find that the last bit you've added has stopped the program working.
Sometimes the Error Console will tell you where you've made the mistake.
Sometimes it won't.
What you do in these circumstances is to wrap that latest section of code in a pair of
/* ... */ comments so that the browser just sees the original code. The page
should now go back to working as it did a few minutes ago. You then expose bits of
the new code by moving the comment symbols. Eventually you'll expose the block of
lines which cause the program to fail again and you can concentrate your debugging
on that section.
|
