Reflect.construct() - JavaScript 编辑

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.

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Syntax

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

Parameters

target

The target function to call.

argumentsList

An array-like object specifying the arguments with which target should be called.

newTarget Optional

The constructor whose prototype should be used. See also the new.target operator. If newTarget is not present, its value defaults to target.

Return value

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

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. (This would also be possible by using the spread syntax combined with the new operator.)

let obj = new Foo(...args)
let 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:
let obj1 = Reflect.construct(OneClass, args, OtherClass)

// ...has the same result as this:
let 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

//Another example to demonstrate below:

function func1(a, b, c, d) {
  console.log(arguments[3]);
}

function func2(d, e, f, g) {
    consol.log(arguments[3]);
}

let obj1 = Reflect.construct(func1, ['I', 'Love', 'my', 'India'])
obj1

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)
}

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

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

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

Examples

Using Reflect.construct()

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

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• Reflect
• new
• new.target