handler.construct()
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.construct() method is a trap for the [[Construct]] object internal method, which is used by operations such as the new operator. In order for the new operation to be valid on the resulting Proxy object, the target used to initialize the proxy must itself be a valid constructor.
Try it
function Monster(disposition) {
this.disposition = disposition;
}
const handler = {
construct(target, args) {
console.log(`Creating a ${target.name}`);
// Expected output: "Creating a monster"
return new target(...args);
},
};
const ProxiedMonster = new Proxy(Monster, handler);
console.log(new ProxiedMonster("fierce").disposition);
// Expected output: "fierce"
Syntax
new Proxy(target, {
construct(target, argumentsList, newTarget) {
}
})
Parameters
The following parameters are passed to the construct() method. this is bound to the handler.
target-
The target constructor object.
argumentsList-
An
Arraycontaining the arguments passed to the constructor. newTarget-
The constructor that was originally called.
Return value
The construct() method must return an object, representing the newly created object.
Description
Interceptions
This trap can intercept these operations:
- The
newoperator:new myFunction(...args) Reflect.construct()
Or any other operation that invokes the [[Construct]] internal method.
Invariants
The proxy's [[Construct]] internal method throws a TypeError if the handler definition violates one of the following invariants:
- The
targetmust be a constructor itself. - The result must be an
Object.
Examples
Trapping the new operator
The following code traps the new operator.
const p = new Proxy(function () {}, {
construct(target, argumentsList, newTarget) {
console.log(`called: ${argumentsList}`);
return { value: argumentsList[0] * 10 };
},
});
console.log(new p(1).value); // "called: 1"
// 10
The following code violates the invariant.
const p = new Proxy(function () {}, {
construct(target, argumentsList, newTarget) {
return 1;
},
});
new p(); // TypeError is thrown
The following code improperly initializes the proxy. The target in Proxy initialization must itself be a valid constructor for the new operator.
const p = new Proxy(
{},
{
construct(target, argumentsList, newTarget) {
return {};
},
},
);
new p(); // TypeError is thrown, "p" is not a constructor
Specifications
| Specification |
|---|
| ECMAScript® 2026 Language Specification # sec-proxy-object-internal-methods-and-internal-slots-construct-argumentslist-newtarget |
Browser compatibility
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