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Proxy.proxy.construct

The handler.construct() method is a trap for 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 have a [[Construct]] internal method (i.e. new target must be valid).

Syntax

const p = new Proxy(target, {
  construct: function(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, p above.

Return value

The construct method must return an object.

Description

The handler.construct() method is a trap for the new operator.

Interceptions

This trap can intercept these operations:

Invariants

If the following invariants are violated, the proxy will throw a TypeError:

  • The result must be an Object.

Examples

Trapping the new operator

The following code traps the new operator.

const p = new Proxy(function() {}, {
  construct: function(target, argumentsList, newTarget) {
    console.log('called: ' + argumentsList.join(', '));
    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: function(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: function(target, argumentsList, newTarget) {
    return {};
  }
});

new p(); // TypeError is thrown, "p" is not a constructor

Specifications

Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
construct 49 12 18 No 36 10
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
construct 49 49 18 36 10 5.0
Server
Node.js
construct 6.0.0

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/Proxy/construct