More tweaks and comments

Author: acozzette

js/BUILD.bazel

@@ -4,8 +4,8 @@ load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
 load("@bazel_skylib//rules:common_settings.bzl", "bool_flag")
 
 # Underscore prefix signals this is internal — do not rely on it.
-# It exists temporarily to allow opt-in to the new default (False) while
-# the ecosystem migrates, and will be removed once the default is flipped.
+# This controls the default value of the hoist_runfiles_to_exec_cfg flag on
+# js_run_binary, and is for CI testing only.
 bool_flag(
     name = "_hoist_runfiles_to_exec_cfg",
     build_setting_default = False,

js/private/js_run_binary.bzl

@@ -158,8 +158,8 @@ def js_run_binary(
             execution platform, which may not be the same as the target platform.
 
             When set to `None` (the default), the value of the internal `//js:_hoist_runfiles_to_exec_cfg` build flag
-            is used. Pass `--//js:_hoist_runfiles_to_exec_cfg=true` on the command line or in `.bazelrc` to enable
-            globally. This flag is temporary and will be removed once the default is changed to `True`.
+            is used. That flag is for internal testing only and should not be used. It is temporary and will be removed once
+            the default is changed to `True`.
 
         copy_srcs_to_bin: When True, all srcs files are copied to the output tree that are not already there.
 
@@ -394,11 +394,12 @@ See https://github.com/aspect-build/rules_js/tree/main/docs#using-binaries-publi
             testonly = kwargs.get("testonly", False),
         )
         runfiles_label = ":{}".format(js_runfiles_name)
-        if hoist_runfiles_to_exec_cfg == True:
+        if hoist_runfiles_to_exec_cfg:
             tools.append(runfiles_label)
         elif hoist_runfiles_to_exec_cfg == False:
             extra_srcs.append(runfiles_label)
         else:
+            # hoist_runfiles_to_exec_cfg is None, so we will defer to the flag.
             tools = tools + select({
                 "@aspect_rules_js//js:_hoist_runfiles_to_exec_cfg_true": [runfiles_label],
                 "//conditions:default": [],