Flow error on using this.x as a default param in a class

[hzoo]

This shouldn't be an error right? flow 0.20.0, node 5.1.0

 10:   asdf(str: string = this.b) {
                               ^ property `b`. Property cannot be accessed on
global object

/* @flow */

export default class A {
  constructor() {
    this.b = "";
  }

  b: string;

  asdf(str: string = this.b): string {
    return str;
  }
}

[dasilvacontin]

this is referencing the current scope during function definition. How would you reference the current scope otherwise? But if then, how do you reference the instance?

[samwgoldman]

It's very possible I got the scoping wrong on the default expr. I suspect you are right here, but it would be very helpful if you could link to the relevant part of the spec that indicates what this should evaluate to in a class method argument default expr.

[burgalon]

@samwgoldman
http://www.ecma-international.org/ecma-262/6.0/#sec-getthisenvironment seems to mention the binding

[Jessidhia]

A default expression is not evaluated until it is necessary, and will be re-evaluated on every call that needs it. It also does not capture the outer this -- it runs with the same this as the body of the function when evaluated.

I am not entirely sure on the spec language, but I'm trying to follow the definition of [[Call]]. By the time the default expression evaluation at FunctionDeclarationInstantiation.28 happens (called from OrdinaryCallEvaluateBody), the OrdinaryCallBindThis has already happened. The defaults should see the same this as the function body.

This, of course, still allows it to be either global (in sloppy mode) or undefined if you go and extract it and call it as a function, but that is not how you normally use a method declared in a class.

Arrow functions bind their this at declaration time so they would see the declaration scope's this. The appropriate value of this in class property declarations is still being debated, but as of TC39 2016-07 it seemed to lean towards pointing to the new instance being constructed (initialisers are evaluated as if inside a method that is called in the new instance).

[brendan-roche]

Is there any updates / workarounds on this one?