CHAPTER 7
Scope
Scope determines when and where variables, constants, and arguments are considered
to be defined. We’ve already had some exposure to scope: we know that the argu‐
ments of a function exist only in the body of the function. Consider the following:
function
f
(
x
) {
return
x
+
3
;
}
f
(
5
);
// 8
x
;
// ReferenceError: x is not defined
We know that
x
lived very briefly (otherwise, how would it have successfully calcula‐
ted
x + 3
?), but we can see that outside of the function body, it’s as if
x
doesn’t exist.
Thus, we say the scope of
x
is the function
f
.
When we say that the scope of a variable is a given function, we must remember that
the formal arguments in the function body don’t exist until the function is called
(thereby becoming actual arguments). A function may be called multiple times: each
time the function is called, its arguments come into existence, and then go out of
scope when the function returns.
We have also taken it for granted that variables and constants do not exist before we
create them. That is, they aren’t in scope until we declare them with
let
or
const
(
var
is a special case we’ll cover later in this chapter).
In some languages, there’s an explicit distinction between declara‐
tion and definition. Typically, declaring a variable means that you
are announcing its existence by giving it an identifier. Definition,
on the other hand, usually means declaring it and giving it a value.
In JavaScript, the two terms are interchangeable, as all variables are
given a value when they’re declared (if not explicitly, they implicitly
get the value undefined).
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