Extending built-in Iterator class prevents tree-shaking

[nathanielyoon]

I'd expect --tree-shaking to eliminate this class if unused, but it doesn't.

class IteratorSubclass extends Iterator<number> {
  next() {
    return { value: 0 };
  }
}

This doesn't get rid of the unused class

Removing the extends works

Extending something else works

Could this have something to do with #3561?

[hyrious]

This was because there is a reference to the identifier 'Iterator', although not likely happen, but it may raise a ReferenceError if there's no such class.

> delete globalThis.Iterator
true
> class A extends Iterator {}
Uncaught ReferenceError: Iterator is not defined

So esbuild preserves the possibility of raise an error at runtime.

[nathanielyoon]

I think the third link provides a counterexample to that. For comparison:

> delete globalThis.Array
true
> class A extends Array {}
Uncaught ReferenceError: Array is not defined
    at <anonymous>:1:38

And wouldn't that prevent tree-shaking of any global reference?

• https://esbuild.github.io/try/#YgAwLjI1LjExAC0tdHJlZS1zaGFraW5nAGUAZW50cnkudHMAQXJyYXk7
• https://esbuild.github.io/try/#YgAwLjI1LjExAC0tdHJlZS1zaGFraW5nAGUAZW50cnkudHMAUmVnRXhwOw

Seems like Iterator is special: https://esbuild.github.io/try/#YgAwLjI1LjExAC0tdHJlZS1zaGFraW5nAGUAZW50cnkudHMASXRlcmF0b3I7

[chirsz-ever]

After adding "Iterator" to knownGlobals, --tree-shaking would work:

diff --git a/internal/config/globals.go b/internal/config/globals.go
index 1bbc9786..7501d45c 100644
--- a/internal/config/globals.go
+++ b/internal/config/globals.go
@@ -38,6 +38,7 @@ var knownGlobals = [][]string{
 	{"Object"},
 	{"RegExp"},
 	{"String"},
+	{"Iterator"},
 
 	// Object: Static methods
 	// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object#Static_methods

The absence of Iterator seems to be because it only appeared after ES6 ES2025. However, the list includes global objects that are only available in newer versions, such as Reflect, Symbol, Proxy, and Promise, so I think Iterator should also be added.

Related issue: #2620

[evanw]

It looks like the Iterator constructor is new in ES2025. The built-in list of globals could use an update as there have been many new additions. I think adding it to the list (as well as some other new ones such as Float16Array) makes sense. I'll do that.

It's true that you could hypothetically delete JavaScript built-ins and cause references to built-in globals to have side-effects, but that's not why this list of globals exists. It exists to guard against unknown global properties potentially having built-in getters with side-effects. Some examples from globals.go: the global scrollY property has the side-effect of triggering layout, and the global localStorage property can potentially throw an error if the browser doesn't want you to use it (e.g. when private browsing mode is active) which is also a side-effect.