ECMAScript 5 has introduced several methods to prevent modification of object. Those preventive measures ensures that no one, accidentally or otherwise change functionality of object.
There are 3 levels of preventive methods
1) Prevent Extensions
In this level, one cannot add any new properties or methods but can access existing properties or methods. Here there is an ability to delete the respective object. Object.preventExtensions() is the method used to accomplish this task. It prevents any new properties from ever being added to the object.
Example
<html>
<body>
<script>
var object1 = {
prop1: 1
};
Object.preventExtensions(object1);
delete object1.prop1 // value got deleted
try {
Object.defineProperty(object1, 'prop2', {
value: 2
});
} catch (err) {
document.write(err);
}
document.write("</br>");
document.write(object1.prop1);
</script>
</body>
</html>Output
TypeError: Cannot define property prop2, object is not extensible undefined // deleted so undefined
2) Seal
It is same as preventing extensions, in addition it doesn't allow to delete existing properties or methods. To accomplish this task Object.seal() method is used.
Example
<html>
<body>
<script>
var object1 = {
prop1: 1
};
Object.seal(object1);
object1.prop1 = 2; // value got changed
delete object1.prop1;
try {
Object.defineProperty(object1, 'prop2', {
value: 2
});
} catch (err) {
document.write(err);
}
document.write("</br>");
document.write(object1.prop1); // it gives value as 2 because of seal.
</script>
</body>
</html>Output
TypeError: Cannot define property prop2, object is not extensible 2 // because of seal the value can't be deleated but got updated
3) Freeze
In addition to seal's functionality, freeze doesn't allow even to access the existing properties. To freeze an object we use Object.freeze() method. It can also make an object immutable.
Example
<html>
<body>
<script>
var object1 = {
prop1: 1
};
Object.freeze(object1);
object1.prop1 = 2; // value got updated
delete object1.prop1; // value got deleted
try {
Object.defineProperty(object1, 'prop2', {
value: 2
});
} catch (err) {
document.write(err);
}
document.write("</br>");
document.write(object1.prop1); // it gives 1 as output despite value updated to 2
</script>
</body>
</html>Output
TypeError: Cannot define property prop2, object is not extensible 1 // because of freeze the value won't get delete and won't get update.