handler.construct()

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.

The construct() method must return an object, representing the newly created object.

This trap can intercept these operations:

• The new operator: new myFunction(...args)
• Reflect.construct()

Or any other operation that invokes the [[Construct]] internal method.

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.

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

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