5 files changed, 90 insertions, 11 deletions

diff --git a/cli/media_type.rs b/cli/media_type.rs
index 7be494a3a..8280c1180 100644
--- a/cli/media_type.rs
+++ b/cli/media_type.rs
@@ -94,19 +94,16 @@ impl MediaType {
         },
       },
       Some(os_str) => match os_str.to_str() {
-        Some("ts") => match path.file_stem() {
-          Some(os_str) => match os_str.to_str() {
-            Some(file_name) => {
+        Some("ts") => {
+          if let Some(os_str) = path.file_stem() {
+            if let Some(file_name) = os_str.to_str() {
               if file_name.ends_with(".d") {
-                MediaType::Dts
-              } else {
-                MediaType::TypeScript
+                return MediaType::Dts;
               }
             }
-            None => MediaType::TypeScript,
-          },
-          None => MediaType::TypeScript,
-        },
+          }
+          MediaType::TypeScript
+        }
         Some("tsx") => MediaType::TSX,
         Some("js") => MediaType::JavaScript,
         Some("jsx") => MediaType::JSX,
diff --git a/cli/module_graph.rs b/cli/module_graph.rs
index 493bc2dc4..bf3972904 100644
--- a/cli/module_graph.rs
+++ b/cli/module_graph.rs
@@ -2008,6 +2008,27 @@ pub mod tests {
   }
 
   #[tokio::test]
+  async fn fix_graph_check_mjs_root() {
+    let specifier = ModuleSpecifier::resolve_url_or_path("file:///tests/a.mjs")
+      .expect("could not resolve module");
+    let (graph, handler) = setup(specifier).await;
+    let result_info = graph
+      .check(CheckOptions {
+        debug: false,
+        emit: true,
+        lib: TypeLib::DenoWindow,
+        maybe_config_path: None,
+        reload: false,
+      })
+      .expect("should have checked");
+    assert!(result_info.maybe_ignored_options.is_none());
+    assert!(result_info.diagnostics.is_empty());
+    let h = handler.borrow();
+    assert_eq!(h.cache_calls.len(), 1);
+    assert_eq!(h.tsbuildinfo_calls.len(), 1);
+  }
+
+  #[tokio::test]
   async fn fix_graph_check_types_root() {
     let specifier = ModuleSpecifier::resolve_url_or_path("file:///typesref.js")
       .expect("could not resolve module");
diff --git a/cli/tests/module_graph/file_tests-a.mjs b/cli/tests/module_graph/file_tests-a.mjs
new file mode 100644
index 000000000..72b3a67bc
--- /dev/null
+++ b/cli/tests/module_graph/file_tests-a.mjs
@@ -0,0 +1,3 @@
+import * as b from "./b.ts";
+
+console.log(b);
diff --git a/cli/tests/module_graph/file_tests-b.ts b/cli/tests/module_graph/file_tests-b.ts
new file mode 100644
index 000000000..59d168993
--- /dev/null
+++ b/cli/tests/module_graph/file_tests-b.ts
@@ -0,0 +1 @@
+export const b = "b";
diff --git a/cli/tsc.rs b/cli/tsc.rs
index 7f90dd7b2..bcaeaaeb6 100644
--- a/cli/tsc.rs
+++ b/cli/tsc.rs
@@ -22,6 +22,7 @@ use deno_core::Snapshot;
 use serde::Deserialize;
 use std::cell::RefCell;
 use std::collections::HashMap;
+use std::path::PathBuf;
 use std::rc::Rc;
 
 /// Provide static assets that are not preloaded in the compiler snapshot.
@@ -63,6 +64,41 @@ fn get_maybe_hash(
   }
 }
 
+/// tsc only supports `.ts`, `.tsx`, `.d.ts`, `.js`, or `.jsx` as root modules
+/// and so we have to detect the apparent media type based on extensions it
+/// supports.
+fn get_tsc_media_type(specifier: &ModuleSpecifier) -> MediaType {
+  let url = specifier.as_url();
+  let path = if url.scheme() == "file" {
+    if let Ok(path) = url.to_file_path() {
+      path
+    } else {
+      PathBuf::from(url.path())
+    }
+  } else {
+    PathBuf::from(url.path())
+  };
+  match path.extension() {
+    None => MediaType::Unknown,
+    Some(os_str) => match os_str.to_str() {
+      Some("ts") => {
+        if let Some(os_str) = path.file_stem() {
+          if let Some(file_name) = os_str.to_str() {
+            if file_name.ends_with(".d") {
+              return MediaType::Dts;
+            }
+          }
+        }
+        MediaType::TypeScript
+      }
+      Some("tsx") => MediaType::TSX,
+      Some("js") => MediaType::JavaScript,
+      Some("jsx") => MediaType::JSX,
+      _ => MediaType::Unknown,
+    },
+  }
+}
+
 #[derive(Debug, Clone, Default, Eq, PartialEq)]
 pub struct EmittedFile {
   pub data: String,
@@ -337,7 +373,7 @@ pub fn exec(
     .root_names
     .iter()
     .map(|(s, mt)| {
-      let ext_media_type = MediaType::from(&s.as_str().to_owned());
+      let ext_media_type = get_tsc_media_type(s);
       if mt != &ext_media_type {
         let new_specifier = format!("{}{}", s, mt.as_ts_extension());
         root_map.insert(new_specifier.clone(), s.clone());
@@ -490,6 +526,27 @@ mod tests {
     );
   }
 
+  #[test]
+  fn test_get_tsc_media_type() {
+    let fixtures = vec![
+      ("file:///a.ts", MediaType::TypeScript),
+      ("file:///a.tsx", MediaType::TSX),
+      ("file:///a.d.ts", MediaType::Dts),
+      ("file:///a.js", MediaType::JavaScript),
+      ("file:///a.jsx", MediaType::JSX),
+      ("file:///a.cjs", MediaType::Unknown),
+      ("file:///a.mjs", MediaType::Unknown),
+      ("file:///a.json", MediaType::Unknown),
+      ("file:///a.wasm", MediaType::Unknown),
+      ("file:///a.js.map", MediaType::Unknown),
+      ("file:///.tsbuildinfo", MediaType::Unknown),
+    ];
+    for (specifier, media_type) in fixtures {
+      let specifier = ModuleSpecifier::resolve_url_or_path(specifier).unwrap();
+      assert_eq!(get_tsc_media_type(&specifier), media_type);
+    }
+  }
+
   #[tokio::test]
   async fn test_emit() {
     let mut state = setup(None, None, None).await;