Add PackageResolver.relative_import_path/3

Author: dannote

lib/npm/package_resolver.ex

@@ -203,6 +203,44 @@ defmodule NPM.PackageResolver do
     end
   end
 
+  # ---------------------------------------------------------------------------
+  # Relative import paths
+  # ---------------------------------------------------------------------------
+
+  @doc """
+  Compute a relative import path from `importer` to `target` within `project_root`.
+
+  Both paths must be absolute. Returns a POSIX-style relative path with
+  a guaranteed `./` or `../` prefix, suitable for use as an import specifier.
+
+  ## Examples
+
+      iex> NPM.PackageResolver.relative_import_path(
+      ...>   "/app/src/pages/home.js",
+      ...>   "/app/src/utils/format.js",
+      ...>   "/app"
+      ...> )
+      "../utils/format.js"
+
+      iex> NPM.PackageResolver.relative_import_path(
+      ...>   "/app/src/index.js",
+      ...>   "/app/src/app.js",
+      ...>   "/app"
+      ...> )
+      "./app.js"
+  """
+  @spec relative_import_path(String.t(), String.t(), String.t()) :: String.t()
+  def relative_import_path(importer, target, project_root) do
+    importer_dir = importer |> Path.relative_to(project_root) |> Path.dirname()
+    target_label = Path.relative_to(target, project_root)
+    relative = Path.relative_to(target_label, importer_dir)
+    ensure_relative_prefix(relative)
+  end
+
+  defp ensure_relative_prefix("./" <> _ = path), do: path
+  defp ensure_relative_prefix("../" <> _ = path), do: path
+  defp ensure_relative_prefix(path), do: "./" <> path
+
   # ---------------------------------------------------------------------------
   # Private
   # ---------------------------------------------------------------------------

test/npm/package_resolver_test.exs

@@ -463,6 +463,52 @@ defmodule NPM.PackageResolverTest do
   # Helpers
   # ---------------------------------------------------------------------------
 
+  # ---------------------------------------------------------------------------
+  # relative_import_path/3
+  # ---------------------------------------------------------------------------
+
+  describe "relative_import_path/3" do
+    test "sibling file gets ./ prefix" do
+      assert PackageResolver.relative_import_path(
+               "/app/src/index.js",
+               "/app/src/app.js",
+               "/app"
+             ) == "./app.js"
+    end
+
+    test "file in subdirectory" do
+      assert PackageResolver.relative_import_path(
+               "/app/src/index.js",
+               "/app/src/utils/format.js",
+               "/app"
+             ) == "./utils/format.js"
+    end
+
+    test "file in parent directory" do
+      assert PackageResolver.relative_import_path(
+               "/app/src/pages/home.js",
+               "/app/src/utils/format.js",
+               "/app"
+             ) == "../utils/format.js"
+    end
+
+    test "deeply nested upward traversal" do
+      assert PackageResolver.relative_import_path(
+               "/app/src/a/b/c/deep.js",
+               "/app/src/lib/helper.js",
+               "/app"
+             ) == "../../../lib/helper.js"
+    end
+
+    test "same directory different extensions" do
+      assert PackageResolver.relative_import_path(
+               "/app/components/button.tsx",
+               "/app/components/button.module.css",
+               "/app"
+             ) == "./button.module.css"
+    end
+  end
+
   defp write_pkg_json(dir, data) do
     File.write!(Path.join(dir, "package.json"), :json.encode(data))
   end