Reflect.construct()

The static Reflect.construct() method acts like the new operator, but as a function. It is equivalent to calling new target(...args). It gives also the added option to specify a different prototype.

Sintaxe

Reflect.construct(target, argumentsList[, newTarget])

Parametros

target
A função alvo à ser chamada.
argumentsList
Um objeto tipo array que especifica com quais argumentos target deveria ser chamada.
newTarget Optional
O construtor de quem o protótipo deveria ser usado. Veja também o new.target operador. Se newTarget não estiver presente, será target.

Return value

A new instance of target (or newTarget, if present), initialized by target as a constructor with the given arguments.

Exceptions

A TypeError, if target or newTarget are not constructors.

Description

Reflect.construct allows you to invoke a constructor with a variable number of arguments (which would also be possible by using the spread operator combined with the new operator).

var obj = new Foo(...args);
var obj = Reflect.construct(Foo, args);

 

Reflect.construct() vs Object.create()

Prior to the introduction of Reflect, objects could be constructed using an arbitrary combination of constructor and prototype by using Object.create().

function OneClass() {
    this.name = 'one';
}

function OtherClass() {
    this.name = 'other';
}

// Calling this:
var obj1 = Reflect.construct(OneClass, args, OtherClass);

// ...has the same result as this:
var obj2 = Object.create(OtherClass.prototype);
OneClass.apply(obj2, args);

console.log(obj1.name); // 'one'
console.log(obj2.name); // 'one'

console.log(obj1 instanceof OneClass); // false
console.log(obj2 instanceof OneClass); // false

console.log(obj1 instanceof OtherClass); // true
console.log(obj2 instanceof OtherClass); // true

However, while the end result is the same, there is one important difference in the process. When using Object.create() and Function.prototype.apply(), the new.target operator will point to undefined within the function used as the constructor, since the new keyword is not being used to create the object.

When invoking Reflect.construct(), on the other hand, the new.target operator will point to the newTarget parameter if supplied, or target if not.

function OneClass() {
    console.log('OneClass');
    console.log(new.target);
}
function OtherClass() {
    console.log('OtherClass');
    console.log(new.target);
}

var obj1 = Reflect.construct(OneClass, args);
// Output:
//     OneClass
//     function OneClass { ... }

var obj2 = Reflect.construct(OneClass, args, OtherClass);
// Output:
//     OneClass
//     function OtherClass { ... }

var obj3 = Object.create(OtherClass.prototype);
OneClass.apply(obj3, args);
// Output:
//     OneClass
//     undefined

 

Examples

Using Reflect.construct()

var d = Reflect.construct(Date, [1776, 6, 4]);
d instanceof Date; // true
d.getFullYear(); // 1776

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Reflect.construct' in that specification.
Padrão Initial definition.
ECMAScript (ECMA-262)
The definition of 'Reflect.construct' in that specification.
Padrão em tempo real  

Browser compatibility

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See also