Allows preload to be determined by provided entry_point

README.md

@@ -141,6 +141,25 @@ pin "md5", preload: false # file lives in vendor/javascript/md5.js
 ...
 ```
 
+You can also specify which entry points to preload a particular dependency in by providing `preload:` a string or array of strings.
+
+Example:
+
+```ruby
+# config/importmap.rb
+pin "@github/hotkey", to: "@github--hotkey.js", preload: 'application'
+pin "md5", preload: ['application', 'alternate']
+
+# app/views/layouts/application.html.erb
+<%= javascript_importmap_tags 'alternate' %>
+
+# will include the following link before the importmap is setup:
+<link rel="modulepreload" href="/assets/javascript/md5.js">
+...
+```
+
+
+
 ## Composing import maps
 
 By default, Rails loads import map definition from the application's `config/importmap.rb` to the `Importmap::Map` object available at `Rails.application.importmap`.

app/helpers/importmap/importmap_tags_helper.rb

@@ -3,7 +3,7 @@ module Importmap::ImportmapTagsHelper
   def javascript_importmap_tags(entry_point = "application", importmap: Rails.application.importmap)
     safe_join [
       javascript_inline_importmap_tag(importmap.to_json(resolver: self)),
-      javascript_importmap_module_preload_tags(importmap),
+      javascript_importmap_module_preload_tags(importmap, entry_point:),
       javascript_import_module_tag(entry_point)
     ], "\n"
   end
@@ -24,8 +24,8 @@ def javascript_import_module_tag(*module_names)
   # Link tags for preloading all modules marked as preload: true in the `importmap`
   # (defaults to Rails.application.importmap), such that they'll be fetched
   # in advance by browsers supporting this link type (https://caniuse.com/?search=modulepreload).
-  def javascript_importmap_module_preload_tags(importmap = Rails.application.importmap)
-    javascript_module_preload_tag(*importmap.preloaded_module_paths(resolver: self))
+  def javascript_importmap_module_preload_tags(importmap = Rails.application.importmap, entry_point: "application")
+    javascript_module_preload_tag(*importmap.preloaded_module_paths(resolver: self, entry_point:))
   end
 
   # Link tag(s) for preloading the JavaScript module residing in `*paths`. Will return one link tag per path element.

lib/importmap/map.rb

@@ -40,9 +40,9 @@ def pin_all_from(dir, under: nil, to: nil, preload: true)
   # resolver that has been configured for the `asset_host` you want these resolved paths to use. In case you need to
   # resolve for different asset hosts, you can pass in a custom `cache_key` to vary the cache used by this method for
   # the different cases.
-  def preloaded_module_paths(resolver:, cache_key: :preloaded_module_paths)
+  def preloaded_module_paths(resolver:, entry_point: "application", cache_key: :preloaded_module_paths)
     cache_as(cache_key) do
-      resolve_asset_paths(expanded_preloading_packages_and_directories, resolver: resolver).values
+      resolve_asset_paths(expanded_preloading_packages_and_directories(entry_point:), resolver:).values
     end
   end
 
@@ -118,8 +118,8 @@ def resolve_asset_paths(paths, resolver:)
       end.compact
     end
 
-    def expanded_preloading_packages_and_directories
-      expanded_packages_and_directories.select { |name, mapping| mapping.preload }
+    def expanded_preloading_packages_and_directories(entry_point:)
+      expanded_packages_and_directories.select { |name, mapping| mapping.preload.in?([true, false]) ? mapping.preload : (Array(mapping.preload) & Array(entry_point)).any? }
     end
 
     def expanded_packages_and_directories

test/importmap_test.rb

@@ -8,6 +8,9 @@ def setup
         pin "editor", to: "rich_text.js", preload: false
         pin "not_there", to: "nowhere.js", preload: false
         pin "md5", to: "https://cdn.skypack.dev/md5", preload: true
+        pin "leaflet", to: "https://cdn.skypack.dev/leaflet", preload: 'application'
+        pin "chartkick", to: "https://cdn.skypack.dev/chartkick", preload: ['application', 'alternate']
+        pin "tinyMCE", to: "https://cdn.skypack.dev/tinymce", preload: 'alternate'
 
         pin_all_from "app/javascript/controllers", under: "controllers", preload: true
         pin_all_from "app/javascript/spina/controllers", under: "controllers/spina", preload: true
@@ -78,10 +81,32 @@ def setup
     end
   end
 
-  test "preloaded modules are included in preload tags" do
+  test "preloaded modules are included in preload tags when no entry_point specified" do
     preloading_module_paths = @importmap.preloaded_module_paths(resolver: ApplicationController.helpers).to_s
     assert_match /md5/, preloading_module_paths
     assert_match /goodbye_controller/, preloading_module_paths
+    assert_match /leaflet/, preloading_module_paths
+    assert_no_match /application/, preloading_module_paths
+    assert_no_match /tinymce/, preloading_module_paths
+  end
+
+  test "preloaded modules are included in preload tags based on single entry_point provided" do
+    preloading_module_paths = @importmap.preloaded_module_paths(resolver: ApplicationController.helpers, entry_point: "alternate").to_s
+    assert_no_match /leaflet/, preloading_module_paths
+    assert_match /tinymce/, preloading_module_paths
+    assert_match /chartkick/, preloading_module_paths
+    assert_match /md5/, preloading_module_paths
+    assert_match /goodbye_controller/, preloading_module_paths
+    assert_no_match /application/, preloading_module_paths
+  end
+
+  test "preloaded modules are included in preload tags based on multiple entry_points provided" do
+    preloading_module_paths = @importmap.preloaded_module_paths(resolver: ApplicationController.helpers, entry_point: ["application", "alternate"]).to_s
+    assert_match /leaflet/, preloading_module_paths
+    assert_match /tinymce/, preloading_module_paths
+    assert_match /chartkick/, preloading_module_paths
+    assert_match /md5/, preloading_module_paths
+    assert_match /goodbye_controller/, preloading_module_paths
     assert_no_match /application/, preloading_module_paths
   end