// note that we can use destructuring to make
// this iteration even more natural:
for
(
let
[
u
,
r
]
of
userRoles
.
entries
())
console.log(
`
${
u
.
name
}
:
${
r
}
`
);
// the entries() method is the default iterator for
// a Map, so you can shorten the previous example to:
for
(
let
[
u
,
r
]
of
userRoles
)
console.log(
`
${
u
.
name
}
:
${
r
}
`
);
If you need an array (instead of an iterable object), you can use the spread operator:
[
...
userRoles
.
values
()];
// [ "User", "User", "Admin" ]
To delete a single entry from a map, use the
delete()
method:
userRoles
.
delete
(
u2
);
userRoles
.
size
;
// 2
Lastly, if you want to remove all entries from a map, you can do so with the
clear()
method:
userRoles
.
clear
();
userRoles
.
size
;
// 0
Weak Maps
A
WeakMap
is identical to
Map
except:
• Its keys must be objects.
• Keys in a
WeakMap
can be garbage-collected.
• A
WeakMap
cannot be iterated or cleared.
Normally, JavaScript will keep an object in memory as long as there is a reference to it
somewhere. For example, if you have an object that is a key in a
Map
, JavaScript will
keep that object in memory as long as the
Map
is in existence. Not so with a
WeakMap
.
Because of this, a
WeakMap
can’t be iterated (there’s too much danger of the iteration
exposing an object that’s in the process of being garbage-collected).
Thanks to these properties of
WeakMap
, it can be used to store private keys in object
instances:
const
SecretHolder
=
(
function
() {
const
secrets
=
new
WeakMap
();
return
class
{
setSecret
(
secret
) {
secrets
.
set
(
this
,
secret
);
}
getSecret
() {
Weak Maps | 165
