scope. Let’s consider a simple example: keeping track of information about a user.
Your program keeps track of a user’s name and age, and there are some functions that
operate on that information. One way to do this is with global variables:
let
name
=
"Irena"
;
// global
let
age
=
25
;
// global
function
greet
() {
console.log(
`Hello,
${
name
}
!`
);
}
function
getBirthYear
() {
return
new
Date
().
getFullYear
()
-
age
;
}
The problem with this approach is that our functions are highly dependent on the
context (or scope) that they’re called from. Any function—anywhere in your entire
program—could change the value of
name
(accidentally or intentionally). And “name”
and “age” are generic names that might reasonably be used elsewhere, for other rea‐
sons. Because
greet
and
getBirthYear
rely on global variables, they are basically
relying on the rest of the program using
name
and
age
correctly.
A better approach would be to put user information in a single object:
let
user
=
{
name
=
"Irena"
,
age
=
25
,
};
function
greet
() {
console.log(
`Hello,
${
user
.
name
}
!`
);
}
function
getBirthYear
() {
return
new
Date
().
getFullYear
()
-
user
.
age
;
}
In this simple example, we’ve only reduced the number of identifiers in the global
scope by one (we got rid of
name
and
age
, and added
user
), but imagine if we had 10
pieces of information about the user…or 100.
We could do better still, though: our functions
greet
and
getBirthYear
are still
dependent on the global
user
, which can be modified by anything. Let’s improve
those functions so they’re not dependent on global scope:
function
greet
(
user
) {
console.log(
`Hello,
${
user
.
name
}
!`
);
}
function
getBirthYear
(
user
) {
return
new
Date
().
getFullYear
()
-
user
.
age
;
}
120 | Chapter 7: Scope
