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 Array containing 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 new operator: 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 target must 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

Browser compatibility

See also

• Proxy
• Proxy() constructor
• new
• Reflect.construct()