ES6 js“with”关键字

Question

In the deep internet I encountered structure like class A extends B with C { ... } , I haven't link right now, it was like macro for class while extending abstract class.

Is this in standard? (I googled, only finding old with that is removed right now, chrome inline console throws error)

Answer 1

No this is not a part of the ECMAScript 6 Classes standard. See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes

The with keyword is used in some programming languages to signify multiple inheritance.

Scala for example has traits which classes can extend from with the with keyword

trait Drawable {
    def draw() { }
}

trait Cowboy extends Drawable {
    override def draw() { println("Bang!") }
}

trait Artist extends Drawable {
    override def draw() { println("A pretty painting") }
}


class CowboyArtist extends Cowboy with Artist

Answer 2

It's not standard. From Annex A.4 - Functions and Classes , the syntax is

ClassDeclaration[Yield, Default] :
     BindingIdentifier[?Yield] ClassTail[?Yield]
    [+Default]  ClassTail[?Yield]

ClassExpression[Yield] :
     BindingIdentifier[?Yield]opt ClassTail[?Yield]

ClassTail[Yield] :
    ClassHeritage[?Yield]opt  ClassBody[?Yield]opt 

ClassHeritage[Yield] :
     LeftHandSideExpression[?Yield]

So the ClassTail is the part which includes both the optional extends something and the class body. And that something must be a LeftHandSideExpression , without any with in there.