handler.preventExtensions()

The handler.preventExtensions() method is a trap for the [[PreventExtensions]] object internal method, which is used by operations such as Object.preventExtensions().

Return value: The preventExtensions() method must return a Boolean indicating whether or not the operation was successful. Other values are coerced to booleans.

Return value: Many operations, including Object.preventExtensions(), throw a TypeError if the [[PreventExtensions]] internal method returns false.

Parameters: The following parameter is passed to the preventExtensions() method. this is bound to the handler.

Syntax example: new Proxy(target, {