From a8a82b72491a34353e3700e15f873cb0414f5bd9 Mon Sep 17 00:00:00 2001
From: Roman Kurakin <roman@kurakindev.com>
Date: Thu, 10 Sep 2026 05:53:32 +0500
Subject: [PATCH] fix(js): read the format key only when it holds formatter
 options

`Volt.Builder` reads `config :volt, :format` as the bundle format, where the
value is an atom such as `:esm`. `Volt.JS.Format` and `Volt.JS.Helpers` read
the same key as formatter options and expect a keyword list.

A project that asks the bundler for ESM therefore cannot use `Volt.Formatter`
or `mix volt.js.check`. `load_config/0` raises a `CaseClauseError`, and
`discovery_config/1` passes the atom on to `Keyword.get/3`.

Both now accept a keyword list and fall back otherwise. A configuration that
worked before still works. A value that raised before now falls back to the
JSON configuration.
---
 lib/volt/js/format.ex         | 11 ++++++++---
 lib/volt/js/helpers.ex        |  8 +++++++-
 test/volt/js/format_test.exs  |  6 ++++++
 test/volt/js/helpers_test.exs | 13 +++++++++++++
 4 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/lib/volt/js/format.ex b/lib/volt/js/format.ex
index 9743f34..0df0bb9 100644
--- a/lib/volt/js/format.ex
+++ b/lib/volt/js/format.ex
@@ -27,9 +27,14 @@ defmodule Volt.JS.Format do
   @discovery_keys ~w(root sources ignore)a
 
   def load_config do
-    case Application.get_env(:volt, :format) do
-      nil -> load_json_config()
-      opts when is_list(opts) -> Keyword.drop(opts, @discovery_keys)
+    opts = Application.get_env(:volt, :format)
+
+    # The bundler reads the same key, where the value is an atom such as :esm.
+    # Only a keyword list carries formatter options.
+    if Keyword.keyword?(opts) do
+      Keyword.drop(opts, @discovery_keys)
+    else
+      load_json_config()
     end
   end
 
diff --git a/lib/volt/js/helpers.ex b/lib/volt/js/helpers.ex
index a092c94..8c6a5d4 100644
--- a/lib/volt/js/helpers.ex
+++ b/lib/volt/js/helpers.ex
@@ -38,5 +38,11 @@ defmodule Volt.JS.Helpers do
   end
 
   defp discovery_config(nil), do: []
-  defp discovery_config(tool), do: Application.get_env(:volt, tool, [])
+
+  defp discovery_config(tool) do
+    config = Application.get_env(:volt, tool, [])
+
+    # The bundler reads :format as an atom, so it holds no discovery keys.
+    if Keyword.keyword?(config), do: config, else: []
+  end
 end
diff --git a/test/volt/js/format_test.exs b/test/volt/js/format_test.exs
index 2cb7794..bc68de7 100644
--- a/test/volt/js/format_test.exs
+++ b/test/volt/js/format_test.exs
@@ -25,4 +25,10 @@ defmodule Volt.JS.FormatTest do
 
     assert Format.load_config() == [semi: false, print_width: 100]
   end
+
+  test "load_config/0 ignores a bundle format set on the same key" do
+    Application.put_env(:volt, :format, :esm)
+
+    assert Format.load_config() == Format.load_json_config()
+  end
 end
diff --git a/test/volt/js/helpers_test.exs b/test/volt/js/helpers_test.exs
index 8eea207..9e1c764 100644
--- a/test/volt/js/helpers_test.exs
+++ b/test/volt/js/helpers_test.exs
@@ -44,6 +44,19 @@ defmodule Volt.JS.HelpersTest do
     assert Helpers.discover_files(tool: :lint) == [Path.join(tmp_dir, "lint/source.ts")]
   end
 
+  test "a bundle format on the same key holds no discovery options", %{tmp_dir: tmp_dir} do
+    Application.put_env(:volt, :format, :esm)
+
+    Application.put_env(:volt, :lint,
+      root: tmp_dir,
+      sources: ["lint/**/*.ts"],
+      ignore: []
+    )
+
+    assert Helpers.discover_format_files() == []
+    assert Helpers.discover_files(tool: :lint) == [Path.join(tmp_dir, "lint/source.ts")]
+  end
+
   defp restore_env(key, nil), do: Application.delete_env(:volt, key)
   defp restore_env(key, value), do: Application.put_env(:volt, key, value)
 end