A class over a base runs its instance initializers after the base constructor, where C# runs them before #571

What happens

C# runs a class's instance field and property initializers BEFORE the base constructor, and JavaScript cannot touch this before super(), so a twin assigns them after it. Measured on the class and record emission branch (#413):

class Base0 { public Base0() { Log("base-ctor"); } int b = Log("base-init"); }
class Derived : Base0 { int d = Log("derived-init"); public string P { get; } = Log("derived-prop"); }

new Derived() logs derived-init derived-prop base-init base-ctor in .NET and base-init base-ctor derived-prop derived-init in the browser. A base constructor that calls a virtual member reading a derived field sees it unset in the browser and initialized in .NET.

The record twins of that branch evaluate each initializer that runs code into a local before super() and assign it after, which keeps the order of the side effects; a plain class does not yet.

Direction

The same rule for a plain class: evaluate each derived initializer before super(), in declaration order, and assign after it. Conformance cases on both sides for the order of the side effects, and for a base constructor reading a derived field through a virtual member, which no twin can give the initialized value before super() returns, and which should be fenced or documented if it cannot be reached.