handler.construct()

The handler.construct() method is a trap for the <span class="selflink">Construct</span> 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.

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 object.
argumentsList
- : The list of arguments for the constructor.
newTarget
- : The constructor that was originally called.

Return value

The construct method must return an object.

Description

Interceptions

This trap can intercept these operations:

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

Or any other operation that invokes the <span class="selflink">Construct</span> internal method.

Invariants

If the following invariants are violated, the trap throws a TypeError when invoked.

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