}
add
(
entry
) {
this
.
log
.
push
({
log
:
entry
,
timestamp
:
Date
.
now
(),
});
}
}
const
log
=
new
Logger
(
"Captain's Log"
);
Object
.
seal
(
log
);
Object
.
isSealed
(
log
);
// true
log
.
name
=
"Captain's Boring Log"
;
// OK
log
.
add
(
"Another boring day at sea...."
);
// OK
log
.
newProp
=
'test'
;
// TypeError: Can't add property newProp, object is not extensible
log
.
name
=
'test'
;
// OK
delete
log
.
name
;
// TypeError: Cannot delete property 'name' of [object Object]
Object
.
defineProperty
(
log
,
'log'
, {
enumerable
:
false
});
// TypeError: Cannot redefine property: log
Finally, the weakest protection, making an object nonextensible, only prevents new
properties from being added. Properties can be assigned to, deleted, and reconfig‐
ured. Reusing our
Logger
class, we can demonstrate
Object.preventExtensions
and
Object.isExtensible
:
const
log2
=
new
Logger
(
"First Mate's Log"
);
Object
.
preventExtensions
(
log2
);
Object
.
isExtensible
(
log2
);
// true
log2
.
name
=
"First Mate's Boring Log"
;
// OK
log2
.
add
(
"Another boring day at sea...."
);
// OK
log2
.
newProp
=
'test'
;
// TypeError: Can't add property newProp, object is not extensible
log2
.
name
=
'test'
;
// OK
delete
log2
.
name
;
// OK
Object
.
defineProperty
(
log2
,
'log'
,
{
enumerable
:
false
});
// OK
I find that I don’t use
Object.preventExtensions
very often. If I want to prevent
extensions to an object, I typically also want to prevent deletions and reconfiguration,
so I usually prefer sealing an object.
summarizes the protection options.
Protecting Objects: Freezing, Sealing, and Preventing Extension | 307
