Can return value of `.jsify()` be non-nullable if receiver is non-nullable?

[dgreensp]

I've noticed that you always need a ! assertion after .jsify() to get a JSAny:

  document.body!.addEventListener(
      "click",
      (JSAny e) {
        print("click");
        console.log(e);
      }.toJS,
      {"once": true}.jsify()!);

Since extension methods on overlapping types (one more specific than another) work great in Dart, it seems like you could add this to dart:js_interop so that the result of .jsify() is non-nullable if the receiver is non-nullable:

extension ObjectUtilExtension on Object {
  JSAny jsify() => NullableObjectUtilExtension(this).jsify()!;
}

Am I missing a case where the result of .jsify() would be null for a non-null receiver?

[srujzs]

I think you're right that jsify() can't return null if the input is non-nullable. There is a discussion about what to do with undefined values here: #54025 that may affect this in the future, but for now, you can indeed declare an extension method like that.

[dgreensp]

@srujzs Thanks. I hope this is fixed/improved in dart:js_interop itself, because adding it to my codebase has downsides (it has to be imported, and it subtly changes the behavior of a well-known method, which could be confusing, all to save one character); I decided against it for now. Whereas if it comes "for free," it is a nice improvement.