TC39 Delegates
2022-03-01
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22:22
<mgaudet>

Hey; I've got a bug which essentially boils down to this little test case

class A {
    #x = "hello";
    constructor(o = this.#x) {
        this.value = o;
    }
};

var a = new A;
// Does new A throw? if not, does a.value have the value of `hello`?
22:22
<mgaudet>
I'm trying to figure out what the spec says and got a little confused
22:24
<mgaudet>
https://tc39.es/ecma262/#sec-evaluatenew says that ArgumentListEvaluation happens before the constructor is invoked; but AFAICT it's the constructor that actually allocates the object...
22:24
<mgaudet>
does ArgumentListEvaluation not actually... evaluate the arguments? ie, is the processing of the default argument deferred until later?
22:31
<bakkot>
ArgumentListEvaluation evaluates the arguments, not the parameters
22:32
<bakkot>
it's the thing which would trigger the console.log in f(console.log(0));, not the thing which would trigger the console.log in function f(x = console.log(0)) {}; f()
22:32
<mgaudet>
yeah; I think I've got the latter traced down further into https://tc39.es/ecma262/#sec-functiondeclarationinstantiation
22:33
<mgaudet>
(which seems to imply that the answer is that private fields ought to be available in that parameter expression)
22:33
<mgaudet>
(I always get bit when coming back to arguments/formals/parameters)
22:36
<bakkot>
yeah
22:37
<bakkot>
specifically, for base classes, class fields are created/installed by [[construct]] step 6.b: https://tc39.es/ecma262/multipage/ordinary-and-exotic-objects-behaviours.html#sec-ecmascript-function-objects-construct-argumentslist-newtarget
22:39
<bakkot>
and parameter defaults (and other expressions) are evaluated later, in [[construct]] step 8, via OrdinaryCallEvaluateBody -> EvaluateBody -> EvaluateFunctionBody -> FunctionDeclarationInstantiation -> IteratorBindingInitialization
22:40
<bakkot>
so private fields (and all other fields) are available (for base classes) by the time the parameter list is evaluated
22:40
<mgaudet>
Yep :) Was writing that out... less nicely than you in the bug :P