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>