new.target - JavaScript 编辑

The new.target pseudo-property lets you detect whether a function or constructor was called using the new operator. In constructors and functions invoked using the new operator, new.target returns a reference to the constructor or function. In normal function calls, new.target is undefined.

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Syntax

new.target

Description

The new.target syntax consists of the keyword new, a dot, and the identifier target. Normally, the left-hand side of the dot is the object on which property access is performed, but here, new is not an object.

The new.target pseudo-property is available in all functions.

In class constructors, it refers to the constructed class.

In ordinary functions, it refers to the function itself, assuming it was invoked via the new operator; otherwise new.target is undefined.

In arrow functions, new.target is inherited from the surrounding scope.

Examples

new.target in function calls

In normal function calls (as opposed to constructor function calls), new.target is undefined. This lets you detect whether a function was called with new as a constructor.

function Foo() {
  if (!new.target) { throw 'Foo() must be called with new' }
  console.log('Foo instantiated with new')
}

new Foo()  // logs "Foo instantiated with new"
Foo()      // throws "Foo() must be called with new"

new.target in constructors

In class constructors, new.target refers to the constructor that was directly invoked by new. This is also the case if the constructor is in a parent class and was delegated from a child constructor.

class A {
  constructor() {
    console.log(new.target.name)
  }
}

class B extends A { constructor() { super() } }

let a = new A()  // logs "A"
let b = new B()  // logs "B"

class C { constructor() { console.log(new.target)  } }
class D extends C { constructor() { super()  } }

let c = new C()  // logs class C{constructor(){console.log(new.target);}}
let d = new D()  // logs class D extends C{constructor(){super();}}

Thus from the above example of class C and D, it seems that new.target points to the class definition of class which is initialized. For example, when d was initialized using new D(), the class definition of D was printed; and similarly, in case of c, the class C was printed.

Specifications

Browser compatibility

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See also

• Functions
• Classes
• new
• this