• Invoke setters on the object (that modify the value of properties on the object).
• Add new properties.
• Add new methods.
• Change the configuration of existing properties or methods.
In essence, freezing an object makes it immutable. It’s most useful for data-only
objects, as freezing an object with methods will render useless any methods that mod‐
ify the state of the object.
To freeze an object, use
Object.freeze
(you can tell if an object is frozen by calling
Object.isFrozen
). For example, imagine you have an object that you use to store
immutable information about your program (such as company, version, build ID, and
a method to get copyright information):
const
appInfo
=
{
company
:
'White Knight Software, Inc.'
,
version
:
'1.3.5'
,
buildId
:
'0a995448-ead4-4a8b-b050-9c9083279ea2'
,
// this function only accesses properties, so it won't be
// affected by freezing
copyright
() {
return
`©
${
new
Date
().
getFullYear
()
}
,
${
this
.
company
}
`
;
},
};
Object
.
freeze
(
appInfo
);
Object
.
isFrozen
(
appInfo
);
// true
appInfo
.
newProp
=
'test'
;
// TypeError: Can't add property newProp, object is not extensible
delete
appInfo
.
company
;
// TypeError: Cannot delete property 'company' of [object Object]
appInfo
.
company
=
'test'
;
// TypeError: Cannot assign to read-only property 'company' of [object Object]
Object
.
defineProperty
(
appInfo
,
'company'
, {
enumerable
:
false
});
// TypeError: Cannot redefine property: company
Sealing an object prevents the addition of new properties, or the reconfiguration or
removal of existing properties. Sealing can be used when you have an instance of a
class, as methods that operate on the object’s properties will still work (as long as
they’re not attempting to reconfigure a property). You can seal an object with
Object.seal
, and tell if an object is sealed by calling
Object.isSealed
:
class
Logger
{
constructor
(
name
) {
this
.
name
=
name
;
this
.
log
=
[];
306 | Chapter 21: Object Property Configuration and Proxies
