fix(cli): Improve error message in watch mode (#15184)

cli/file_watcher.rs

@@ -1,9 +1,11 @@
 // Copyright 2018-2022 the Deno authors. All rights reserved. MIT license.
 
 use crate::colors;
+use crate::fmt_errors::format_js_error;
 use crate::fs_util::canonicalize_path;
 
 use deno_core::error::AnyError;
+use deno_core::error::JsError;
 use deno_core::futures::Future;
 use log::info;
 use notify::event::Event as NotifyEvent;
@@ -71,8 +73,15 @@ where
 {
   let result = watch_future.await;
   if let Err(err) = result {
-    let msg = format!("{}: {}", colors::red_bold("error"), err);
+    let error_string = match err.downcast_ref::<JsError>() {
-    eprintln!("{}", msg);
+      Some(e) => format_js_error(e),
+      None => format!("{:?}", err),
+    };
+    eprintln!(
+      "{}: {}",
+      colors::red_bold("error"),
+      error_string.trim_start_matches("error: ")
+    );
   }
 }
 

cli/tests/integration/watcher_tests.rs

@@ -788,6 +788,36 @@ fn run_watch_with_import_map_and_relative_paths() {
   check_alive_then_kill(child);
 }
 
+#[test]
+fn run_watch_error_messages() {
+  let t = TempDir::new();
+  let file_to_watch = t.path().join("file_to_watch.js");
+  write(
+    &file_to_watch,
+    "throw SyntaxError(`outer`, {cause: TypeError(`inner`)})",
+  )
+  .unwrap();
+
+  let mut child = util::deno_cmd()
+    .current_dir(util::testdata_path())
+    .arg("run")
+    .arg("--watch")
+    .arg(&file_to_watch)
+    .env("NO_COLOR", "1")
+    .stdout(std::process::Stdio::piped())
+    .stderr(std::process::Stdio::piped())
+    .spawn()
+    .unwrap();
+  let (_, mut stderr_lines) = child_lines(&mut child);
+
+  wait_contains("Process started", &mut stderr_lines);
+  wait_contains("error: Uncaught SyntaxError: outer", &mut stderr_lines);
+  wait_contains("Caused by: TypeError: inner", &mut stderr_lines);
+  wait_contains("Process finished", &mut stderr_lines);
+
+  check_alive_then_kill(child);
+}
+
 #[test]
 fn test_watch() {
   let t = TempDir::new();