const
arr
=
[
3
,
1.5
,
9
,
2
,
5.2
];
arr
.
sum
=
function
() {
return
this
.
reduce
((
a
,
x
)
=>
a
+
x
); }
arr
.
avg
=
function
() {
return
this
.
sum
()
/
this
.
length
; }
Object
.
defineProperty
(
arr
,
'sum'
, {
enumerable
:
false
});
Object
.
defineProperty
(
arr
,
'avg'
, {
enumerable
:
false
});
We could also do this in one step per property:
const
arr
=
[
3
,
1.5
,
9
,
2
,
5.2
];
Object
.
defineProperty
(
arr
,
'sum'
, {
value
:
function
() {
return
this
.
reduce
((
a
,
x
)
=>
a
+
x
); },
enumerable
:
false
});
Object
.
defineProperty
(
arr
,
'avg'
, {
value
:
function
() {
return
this
.
sum
()
/
this
.
length
; },
enumerable
:
false
});
Lastly, there is also a
Object.defineProperties
(note the plural) that takes an object
that maps property names to property definitions. So we can rewrite the previous
example as:
const
arr
=
[
3
,
1.5
,
9
,
2
,
5.2
];
Object
.
defineProperties
(
arr
,
sum
:
{
value
:
function
() {
return
this
.
reduce
((
a
,
x
)
=>
a
+
x
); },
enumerable
:
false
}),
avg
:
{
value
:
function
() {
return
this
.
sum
()
/
this
.
length
; },
enumerable
:
false
})
);
Protecting Objects: Freezing, Sealing, and Preventing
Extension
JavaScript’s flexible nature is very powerful, but it can get you into trouble. Because
any code anywhere can normally modify an object in any way it wishes, it’s easy to
write code that is unintentionally dangerous or—even worse—intentionally mali‐
cious.
JavaScript does provide three mechanisms for preventing unintentional modifications
(and making intentional ones more difficult): freezing, sealing, and preventing exten‐
sion.
Freezing prevents any changes to an object. Once you freeze an object, you cannot:
• Set the value of properties on the object.
• Call methods that modify the value of properties on the object.
Protecting Objects: Freezing, Sealing, and Preventing Extension | 305
