javascript

Object Lock - Down Methods
Chapter 22: The Evolution of JavaScript
One of the major downsides of ECMAScript 3 was the inability to prevent an object from being
augmented, which allowed developers to overwrite native functionality and introduced the possibility
of malicious code being run on a page. ECMAScript 3.1 seeks to remedy this situation by providing three
different levels of object lock - down: non - extensible, sealed, and frozen.
An object is non - extensible when properties cannot be added to it. You can make an object non -
extensible by calling Object.preventExtensions() and passing in the object to change. Doing so sets
the internal [[Extensible]] attribute to false , ensuring that no further properties can be added. To
detect if an object is extensible, you can use Object.isExtensible() as in the following example:
var person = {
name: “Nicholas”,
age: 29
};
alert(Object.isExtensible(person));
Object.preventExtensions(person);
alert(Object.isExtensible(person));
person.job = “Software Engineer”;
//true
//false
//ERROR!
This example makes the person object non - extensible so that no further properties may be added to the
instance. Note, however, that new properties may be added to the object ’ s prototype and inherited from
it unless the prototype is non - extensible.
The second type of lock - down is a sealed object. An object is said to be sealed when its properties cannot
be added or deleted and property attributes cannot be changed using Object.defineProperty() . You
can seal an object by using the Object.seal() method, which performs two operations. First, it sets the
[[Flexible]] attribute on each property to false , ensuring that it cannot be deleted or modified.
Second, it sets the [[Extensible]] attribute on the object to false , preventing any further properties
from being added to the object. A sealed object is also non - extensible, so Object.isExtensible() will
return false for a sealed object, as in the following example:
var person = {
name: “Nicholas”,
age: 29
};
alert(Object.isSealed(person)); //false
Object.seal(person);
alert(Object.isSealed(person)); //true
alert(Object.isExtensible(person)); //false
alert(delete person.name); //false
alert(person.name); “Nicholas”
In this example, the person object is sealed. Because it is sealed, it is also non - extensible, so
Object.isExtensible() returns false . When the delete operator is applied to person.name ,
it returns false because the property cannot be deleted. Accessing person.name afterwards yields the
same initial value.
The last lock - down mode is called frozen. An object is said to be frozen when it cannot have properties
added or deleted, property attributes cannot be changed, and all properties are read-only. Calling
Object.freeze() on an object sets its [[Extensible]] attribute to false and sets the [[Writable]]
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