fix(npm): improve error message on directory import in npm package (#19538)

Co-authored-by: David Sherret <dsherret@users.noreply.github.com>

cli/module_loader.rs

@@ -748,13 +748,34 @@ impl NpmModuleLoader {
       .read_to_string(&file_path)
       .map_err(AnyError::from)
       .with_context(|| {
-        let mut msg = "Unable to load ".to_string();
-        msg.push_str(&file_path.to_string_lossy());
-        if let Some(referrer) = &maybe_referrer {
-          msg.push_str(" imported from ");
-          msg.push_str(referrer.as_str());
+        if file_path.is_dir() {
+          // directory imports are not allowed when importing from an
+          // ES module, so provide the user with a helpful error message
+          let dir_path = file_path;
+          let mut msg = "Directory import ".to_string();
+          msg.push_str(&dir_path.to_string_lossy());
+          if let Some(referrer) = &maybe_referrer {
+            msg.push_str(" is not supported resolving import from ");
+            msg.push_str(referrer.as_str());
+            let entrypoint_name = ["index.mjs", "index.js", "index.cjs"]
+              .iter()
+              .find(|e| dir_path.join(e).is_file());
+            if let Some(entrypoint_name) = entrypoint_name {
+              msg.push_str("\nDid you mean to import ");
+              msg.push_str(entrypoint_name);
+              msg.push_str(" within the directory?");
+            }
+          }
+          msg
+        } else {
+          let mut msg = "Unable to load ".to_string();
+          msg.push_str(&file_path.to_string_lossy());
+          if let Some(referrer) = &maybe_referrer {
+            msg.push_str(" imported from ");
+            msg.push_str(referrer.as_str());
+          }
+          msg
         }
-        msg
       })?;
 
     let code = if self.cjs_resolutions.contains(specifier) {

cli/tests/integration/npm_tests.rs

@@ -778,6 +778,22 @@ itest!(deno_run_non_existent {
   exit_code: 1,
 });
 
+itest!(directory_import_folder_index_js {
+  args: "run npm/directory_import/folder_index_js.ts",
+  output: "npm/directory_import/folder_index_js.out",
+  envs: env_vars_for_npm_tests(),
+  http_server: true,
+  exit_code: 1,
+});
+
+itest!(directory_import_folder_no_index {
+  args: "run npm/directory_import/folder_no_index.ts",
+  output: "npm/directory_import/folder_no_index.out",
+  envs: env_vars_for_npm_tests(),
+  http_server: true,
+  exit_code: 1,
+});
+
 itest!(builtin_module_module {
   args: "run --allow-read --quiet npm/builtin_module_module/main.js",
   output: "npm/builtin_module_module/main.out",

cli/tests/testdata/npm/directory_import/folder_index_js.out

@@ -0,0 +1,7 @@
+Download http://localhost:4545/npm/registry/@denotest/sub-folders
+Download http://localhost:4545/npm/registry/@denotest/sub-folders/1.0.0.tgz
+error: Directory import [WILDCARD]folder_index_js is not supported resolving import from file:///[WILDCARD]/directory_import/folder_index_js.ts
+Did you mean to import index.js within the directory?
+
+Caused by:
+    [WILDCARD]

cli/tests/testdata/npm/directory_import/folder_index_js.ts

@@ -0,0 +1,2 @@
+import test from "npm:@denotest/sub-folders/folder_index_js";
+console.log(test);

cli/tests/testdata/npm/directory_import/folder_no_index.out

@@ -0,0 +1,6 @@
+Download http://localhost:4545/npm/registry/@denotest/sub-folders
+Download http://localhost:4545/npm/registry/@denotest/sub-folders/1.0.0.tgz
+error: Directory import [WILDCARD]folder_no_index is not supported resolving import from file:///[WILDCARD]/folder_no_index.ts
+
+Caused by:
+    [WILDCARD]

cli/tests/testdata/npm/directory_import/folder_no_index.ts

@@ -0,0 +1,2 @@
+import test from "npm:@denotest/sub-folders/folder_no_index";
+console.log(test);

cli/tests/testdata/npm/registry/@denotest/sub-folders/1.0.0/folder_index_js/index.d.ts

@@ -0,0 +1 @@
+export function add(a, b): number;

cli/tests/testdata/npm/registry/@denotest/sub-folders/1.0.0/folder_index_js/index.js

@@ -0,0 +1,3 @@
+export function add(a, b) {
+    return a + b;
+}

cli/tests/testdata/npm/registry/@denotest/sub-folders/1.0.0/folder_no_index/random_name.js

@@ -0,0 +1 @@
+module.exports = 5;

cli/tests/testdata/npm/registry/@denotest/sub-folders/1.0.0/main.mjs

@@ -0,0 +1,3 @@
+export function getValue() {
+  return 5;
+}

cli/tests/testdata/npm/registry/@denotest/sub-folders/1.0.0/package.json

@@ -0,0 +1,6 @@
+{
+  "name": "@denotest/sub-folders",
+  "version": "1.0.0",
+  "type": "module",
+  "main": "main.mjs"
+}