handler.preventExtensions()
The handler.preventExtensions()
method is a trap for Object.preventExtensions()
.
Try it
Syntax
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
The handler.preventExtensions()
method is a trap for Object.preventExtensions()
.
Interceptions
This trap can intercept these operations:
Invariants
If the following invariants are violated, the proxy will throw a TypeError
:
Object.preventExtensions(proxy)
only returnstrue
ifObject.isExtensible(proxy)
isfalse
.
Examples
Trapping of preventExtensions
The following code traps Object.preventExtensions()
.
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.
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 |
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