return
secrets
.
get
(
this
);
}
}
})();
Here we’ve put our
WeakMap
inside an IIFE, along with a class that uses it. Outside the
IIFE, we get a class that we call
SecretHolder
whose instances can store secrets. We
can only set a secret through the
setSecret
method, and only get the secret through
the
getSecret
method:
const
a
=
new
SecretHolder
();
const
b
=
new
SecretHolder
();
a
.
setSecret
(
'secret A'
);
b
.
setSecret
(
'secret B'
);
a
.
getSecret
();
// "secret A"
b
.
getSecret
();
// "secret B"
We could have used a regular
Map
, but then the secrets we tell instances of
SecretHolder
could never be garbage-collected!
Sets
A set is a collection of data where duplicates are not allowed (consistent with sets in
mathematics). Using our previous example, we may want to be able to assign a user to
multiple roles. For example, all users might have the
"User"
role, but administrators
have both the
"User"
and
"Admin"
role. However, it makes no logical sense for a user
to have the same role multiple times. A set is the ideal data structure for this case.
First, create a
Set
instance:
const
roles
=
new
Set
();
Now if we want to add a user role, we can do so with the
add()
method:
roles
.
add
(
"User"
);
// Set [ "User" ]
To make this user an administrator, call
add()
again:
roles
.
add
(
"Admin"
);
// Set [ "User", "Admin" ]
Like
Map
,
Set
has a
size
property:
roles
.
size
;
// 2
Here’s the beauty of sets: we don’t have to check to see if something is already in the
set before we add it. If we add something that’s already in the set, nothing happens:
roles
.
add
(
"User"
);
// Set [ "User", "Admin" ]
roles
.
size
;
// 2
166 | Chapter 10: Maps and Sets
