Variables or Constants: Which to Use?
In general, you should prefer constants over variables. You’ll find that more often
than not you want a handy name for some piece of data, but you don’t need its value
to change. The advantage of using constants is that it makes it harder to accidentally
change the value of something that shouldn’t be changed. For example, if you’re
working on a part of your program that performs some kind of action on a user, you
might have a variable called
user
. If you’re dealing with only one user, it would prob‐
ably indicate an error in your code if the value of
user
changed. If you were working
with two users, you might call them
user1
and
user2
, instead of simply reusing a sin‐
gle variable
user
.
So your rule of thumb should be to use a constant; if you find you have a legitimate
need to change the value of the constant, you can always change it to a variable.
There is one situation in which you will always want to use variables instead of con‐
stants: variables used in loop control (which we’ll learn about in
). Other
situations where you might want to use variables are when the value of something is
naturally changing over time (such as
targetTempC
or
currentTemp
in this chapter).
If you get in the habit of preferring constants, however, you might be surprised by
how seldom you really need a variable.
In the examples in this book, I have tried to use constants instead of variables when‐
ever possible.
Identifier Names
Variable and constant names (as well as function names, which we’ll cover in
) are called identifiers, and they have naming rules:
• Identifiers must start with a letter, dollar sign ($), or underscore (_).
• Identifiers consist of letters, numbers, the dollar sign ($), and underscore (_).
• Unicode characters are allowed (for example, π or ö).
• Identifiers cannot be a reserved word (see
).
Note that the dollar sign is not a special character the way it is in some other lan‐
guages: it’s simply another character you can use in identifier names (many libraries,
such as jQuery, have taken advantage of this, and used the dollar sign by itself as an
identifier).
Reserved words are words that JavaScript might confuse with part of the language.
For example, you can’t have a variable called
let
.
There’s no single convention for JavaScript identifiers, but the two most common are:
Variables or Constants: Which to Use? | 35
