handler.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:

• new myFunction(...args)
• Reflect.construct()

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 compatibility

See also

• Proxy
• handler
• new operator.
• Reflect.construct()