Flow doesn't like defaulting arguments to this.foo

[ghost]

I'm not sure what the spec says about this, but flow and babel are out of sync here.

class Foo {
  a = 42;
  bar(a = this.a) {
    console.log(a);
  }
}

new Foo().bar()

Flow claims that in the assignment to this.a that this is global. Babel transpiles it to the class instance.

[AugustinLF]

It should be fixed quite soon, there's an open PR #3334 to fix that!

[jeffmo]

For posterity, here's my breakdown of the spec confirming Babel's behavior:

• 12.3.4.1, Step 7.a.i gets the thisValue of a .-ed member expression
• Step 11 returns EvaluateDirectCall(func, thisValue, arguments, tailCall).
• Step 5 of EvaluateDirectCall delegates to Call ( F, V [ , argumentsList ] )
• Call runs the internal [[Call]] method of the function
• Step 4 of [[Call]] runs PrepareForOrdinaryCall (which is what sets the [[HomeObject]] for use by super by way of NewFunctionEnvironment)
• Step 6 of [[Call]] -> OrdinaryCallBindThis ( F, calleeContext, thisArgument ) which sets this in the lexical environment
• Step 7 of [[Call]] -> OrdinaryCallEvaluateBody ( F, argumentsList )
• OrdinaryCallEvaluateBody calls EvaluateBody
• EvaluateBody calls FunctionDeclarationInstantiation
• 9.2.12 - FunctionDeclarationInstantiation ( func, argumentsList ), Step 27 is where we evaluate defaults as part of a function call.
• Step 27.b creates a new lexical environment with a parent equal to the outer environment
• Step 27.f then goes on to evaluate the defaults in this new environment