The handler.preventExtensions()
method is a trap for the [[PreventExtensions]]
object internal method, which is used by operations such as Object.preventExtensions()
.
The handler.preventExtensions()
method is a trap for the [[PreventExtensions]]
object internal method, which is used by operations such as Object.preventExtensions()
.
new Proxy(target, { preventExtensions(target) { } });
The following parameter is passed to the preventExtensions()
method. this
is bound to the handler.
target
The target object.
The preventExtensions()
method must return a boolean value.
This trap can intercept these operations:
Or any other operation that invokes the [[PreventExtensions]]
internal method.
If the following invariants are violated, the trap throws a TypeError
when invoked.
Object.preventExtensions(proxy)
only returns true
if Object.isExtensible(proxy)
is false
.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
Specification |
---|
ECMAScript Language Specification # sec-proxy-object-internal-methods-and-internal-slots-preventextensions |
Desktop | Mobile | Server | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Chrome | Edge | Firefox | Opera | Safari | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | WebView Android | Deno | Node.js | ||
preventExtensions |
49 | 12 | 22 | 36 | 10 | 49 | 22 | 36 | 10 | 5.0 | 49 | 1.0 | 6.0.0 |
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https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/Proxy/preventExtensions