handler.preventExtensions()
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2016.
The handler.preventExtensions() method is a trap for the [[PreventExtensions]] object internal method, which is used by operations such as Object.preventExtensions().
Try it
const monster = {
canEvolve: true,
};
const handler = {
preventExtensions(target) {
target.canEvolve = false;
Object.preventExtensions(target);
return true;
},
};
const proxy = new Proxy(monster, handler);
console.log(monster.canEvolve);
// Expected output: true
Object.preventExtensions(proxy);
console.log(monster.canEvolve);
// Expected output: false
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 indicating whether or not the operation was successful. Other values are coerced to booleans.
Many operations, including Object.preventExtensions(), throw a TypeError if the [[PreventExtensions]] internal method returns false.
Description
Interceptions
This trap can intercept these operations:
Or any other operation that invokes the [[PreventExtensions]] internal method.
Invariants
The proxy's [[PreventExtensions]] internal method throws a TypeError if the handler definition violates one of the following invariants:
- The result is only
trueifReflect.isExtensible()on the target object returnsfalseafter callinghandler.preventExtensions().
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® 2026 Language Specification # sec-proxy-object-internal-methods-and-internal-slots-preventextensions |
Browser compatibility
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