The Javascript handler preventExtensions() method traps the Object.preventExtensions method. New properties can not be added to the object when extensions are prevented.
Syntax:
preventExtensions: function(target)
Parameters:
target: It represents the target object.
Return:
Boolean.
Example:
<!DOCTYPE html>
<html>
<body>
<script>
const handler = new Proxy({}, {
preventExtensions: function(a) {
Object.preventExtensions(a);
return !Object.isExtensible(a);
}});
document.writeln('Value: ');
document.writeln(Object.isExtensible(handler));
</script>
</body>
</html>