userRoles
.
set
(
u1
,
'User'
);
userRoles
.
set
(
u2
,
'User'
);
userRoles
.
set
(
u3
,
'Admin'
);
// poor James...we don't assign him a role
The
set()
method is also chainable, which can save some typing:
userRoles
.
set
(
u1
,
'User'
)
.
set
(
u2
,
'User'
)
.
set
(
u3
,
'Admin'
);
You can also pass an array of arrays to the constructor:
const
userRoles
=
new
Map
([
[
u1
,
'User'
],
[
u2
,
'User'
],
[
u3
,
'Admin'
],
]);
Now if we want to determine what role
u2
has, you can use the
get()
method:
userRoles
.
get
(
u2
);
// "User"
If you call
get
on a key that isn’t in the map, it will return
undefined
. Also, you can
use the
has()
method to determine if a map contains a given key:
userRoles
.
has
(
u1
);
// true
userRoles
.
get
(
u1
);
// "User"
userRoles
.
has
(
u4
);
// false
userRoles
.
get
(
u4
);
// undefined
If you call
set()
on a key that’s already in the map, its value will be replaced:
userRoles
.
get
(
u1
);
// 'User'
userRoles
.
set
(
u1
,
'Admin'
);
userRoles
.
get
(
u1
);
// 'Admin'
The
size
property will return the number of entries in the map:
userRoles
.
size
;
// 3
Use the
keys()
method to get the keys in a map,
values()
to return the values, and
entries()
to get the entries as arrays where the first element is the key and the sec‐
ond is the value. All of these methods return an iterable object, which can be iterated
over by a
for...of
loop:
for
(
let
u
of
userRoles
.
keys
())
console
.
log
(
u
.
name
);
for
(
let
r
of
userRoles
.
values
())
console
.
log
(
r
);
for
(
let
ur
of
userRoles
.
entries
())
console.log(
`
${
ur
[
0
].
name
}
:
${
ur
[
1
]
}
`
);
164 | Chapter 10: Maps and Sets
