ECMAScript 5引入了几种防止修改对象的方法。这些预防措施确保没有任何人意外或以其他方式更改对象的功能。
在此级别，不能添加任何新的属性或方法，但可以访问现有的属性或方法。这里可以删除相应的对象。Object.preventExtensions()是用于完成此任务的方法。这样可以防止将任何新属性添加到对象中。
<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>TypeError: Cannot define property prop2, object is not extensible undefined // deleted so undefined
它与防止扩展相同，此外，它不允许删除现有的属性或方法。为了完成此任务，使用了Object.seal()方法。
<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>TypeError: Cannot define property prop2, object is not extensible 2 // because of seal the value can't be deleated but got updated
除了Seal的功能之外，冻结甚至不允许访问现有属性。要冻结对象，我们使用 Object.freeze()方法。它还可以使对象不可变。
<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>TypeError: Cannot define property prop2, object is not extensible 1 // because of freeze the value won't get delete and won't get update.