handler.preventExtensions()
The handler.preventExtensions()
method is a trap for the [[PreventExtensions]]
object internal method, which is used by operations such as Object.preventExtensions()
.
Try it
Syntax
js
new Proxy(target, {
preventExtensions(target) {
}
});
Parameters
The following parameter is passed to the preventExtensions()
method. this
is bound to the handler.
target
-
The target object.
Return value
The preventExtensions()
method must return a boolean value.
Description
Interceptions
This trap can intercept these operations:
Or any other operation that invokes the [[PreventExtensions]]
internal method.
Invariants
If the following invariants are violated, the trap throws a TypeError
when invoked.
Object.preventExtensions(proxy)
only returnstrue
ifObject.isExtensible(proxy)
isfalse
.
Examples
Trapping of preventExtensions
The following code traps Object.preventExtensions()
.
js
const p = new Proxy(
{},
{
preventExtensions(target) {
console.log("called");
Object.preventExtensions(target);
return true;
},
},
);
console.log(Object.preventExtensions(p));
// "called"
// false
The following code violates the invariant.
js
const p = new Proxy(
{},
{
preventExtensions(target) {
return true;
},
},
);
Object.preventExtensions(p); // TypeError is thrown
Specifications
Specification |
---|
ECMAScript Language Specification # sec-proxy-object-internal-methods-and-internal-slots-preventextensions |
Browser compatibility
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