dart2js using `self` when it should (likely) use `globalThis`

[kevmoo]

These docs: https://api.dart.dev/dart-js_interop/globalContext.html

Say globalContext can be treated like JS's globalThis.

But when compiling, self is used, which blows up in Node.

Replacing self with globalThis in the output fixes issues.

Seeing this both in the use of globalContext from js_interop and with the code generated with createJSInteropWrapper

diff --git a/out.js b/out.js
index 0c4c10f..fe9f8d3 100644
--- a/out.js
+++ b/out.js
@@ -2636,7 +2636,7 @@
     },
     main() {
       A.print("Hello World!");
-      type$.JSObject._as(self).exportValue = new A.main_closure(new A.Export()).call$0();
+      type$.JSObject._as(globalThis).exportValue = new A.main_closure(new A.Export()).call$0();
     },
     main_closure: function main_closure(t0) {
       this.thing = t0;
@@ -3146,7 +3146,7 @@
     call$0() {
       var result,
         t1 = type$.JSObject,
-        t2 = t1._as(t1._as(self).Object),
+        t2 = t1._as(t1._as(globalThis).Object),
         _jsExporter = t1._as(t2.create.apply(t2, [null]));
       t2 = this.thing.get$printMessage();
       if (typeof t2 == "function")

[kevmoo]

CC @srujzs and @rakudrama

[dart-github-bot]

Summary: dart2js uses self instead of globalThis in generated JS code, causing errors in Node.js environments. Replacing self with globalThis resolves the issue.

[srujzs]

There are subtle differences depending on the compiler, but in general, globalContext can be treated like JavaScript's globalThis.

Unfortunately, you came across the subtle differences. :D Historically, package:js used self and dart.global in dart2js and ddc. When some users migrated to dart:js_interop, they saw failures in older browsers that didn't support globalThis. So, to make the migration easier, the global context that's used in every compiler is slightly different.

[sigmundch]

For context, we made the intentional change to use self here: https://dart-review.googlesource.com/c/sdk/+/319301

[rakudrama]

@kevmoo If you can load some code before the dart2js code is loaded, adding globalThis.self = globalThis; should fix the problem.

@srujzs Do we still support any browser versions missing globalThis?

In the intentional change, the message says "It allows users to workaround issues with lowerings without having to worry about browser compatibility differences."
Could you fill me in on how users might 'workaround' issues?
Or is this all in the past?

dart2js could be made to use a captured version of globalThis or self, whichever is defined.
It would be a net code-size win, since the access to the captured version could be smaller than self or globalThis.

[kevmoo]

Looking at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/globalThis#browser_compatibility

[rakudrama]

Hmm. "Important customer" still claims to support Safari 12. I'll ping them.

[srujzs]

In the intentional change, the message says "It allows users to workaround issues with lowerings without having to worry about browser compatibility differences."

Could you fill me in on how users might 'workaround' issues?

Or is this all in the past?

I don't quite remember what the issues I was referring to were, but it is desirable to work around limitations of interop declarations using this global context (for example, declarations require a fixed name). This was also to make this object consistent with what gets used when users use interop declations.

[rakudrama]

2d21ca7 makes dart2js use self if it is defined, else use globalThis.

@kevmoo Does that solve your scenario?