有没有办法在babelify中关闭“超级之前不允许这样的规则”？

Question

I'm running babelify 7.2.0 with Gulp and I'm getting an error on the following code:

class One {}

class Two extends One {
  constructor() {
    this.name = 'John';
  }
}

Here is the crux of the error:

SyntaxError: [the file path in question]: 'this' is not allowed before super()
  20 | class Two extends One {
  21 |   constructor() {
> 22 |     this.name = 'John';
     |     ^
  23 |   }
  24 | }
  25 | 

It seems to me that this should not be firing because I am not making any super calls within the constructor at all so there's no risk of conflict. I've submitted an issue already on Github but I'm wondering if there's a way I can turn it off in the mean time.

Answer 1

This isn't a bug. Subclasses must call super explicitly before attempting to access this :

class Two extends One {
    constructor(...args) {
        super(...args);
        this.name = 'John';
    }
}

This is defined in ECMAScript standard (see this answer), and Babel follows it closely.

Answer 2

No, there's no way to "turn this off" as this is a requirement defined in the ECMAScript 2015 standard.

this is not available prior to super(...) call in the constructor of an extending class.

The exact details of where in the standard this is defined can be seen in this answer .

Answer 3

This isn't a Babel thing, it's defined in ES2015. You can't turn it off.

In a derived constructor, this is undefined prior to calling super . Unlike (say) Java, calls to super are never implicit, you must write them explicitly.

This is defined primarily in §9.2.2 , [[Construct]] ( argumentsList, newTarget) , and §12.3.5.1 , the runtime semantics for super : [[Construct]] only calls OrdinaryCallBindThis if the [[ConstructorKind]] is "base" (not "derived"). Otherwise, this remains undefined until you call super , part of which calls [[Construct]] on the super constructor, which either calls its own super (if it is also a derived constructor) or OrdinaryCallBindThis (if it's a base constructor).