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


new Proxy(target, {
  construct(target, argumentsList, newTarget) {


The following parameters are passed to the construct() method. this is bound to the handler.


The target object.


The list of arguments for the constructor.


The constructor that was originally called, p above.

Return value

The construct method must return an object.



This trap can intercept these operations:

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


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

  • The result must be an Object.


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


ECMAScript Language Specification
# sec-proxy-object-internal-methods-and-internal-slots-construct-argumentslist-newtarget

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See also