Watch mode and HMR

Deno has a built-in file watcher, so you don't need nodemon or any other external tool to reload your program as you edit. This page covers what gets watched, how to exclude paths, and hot module replacement.

Watch mode Jump to heading#

You can supply the --watch flag to deno run, deno test, and deno fmt to enable the built-in file watcher. The watcher enables automatic reloading of your application whenever changes are detected in the source files. This is particularly useful during development, as it allows you to see the effects of your changes immediately without manually restarting the application.

deno run --watch main.ts
deno test --watch
deno fmt --watch

The files that are watched will depend on the subcommand used:

• for deno run and deno test the entrypoint, and all local files that the entrypoint statically imports will be watched.
• for deno fmt all local files and directories specified as command line arguments (or the working directory if no specific files/directories is passed) are watched.

You can exclude paths or patterns from watching by providing the --watch-exclude flag. The syntax is --watch-exclude=path1,path2. For example:

deno run --watch --watch-exclude=file1.ts,file2.ts main.ts

This will exclude file1.ts and file2.ts from being watched.

To exclude a pattern, remember to surround it in quotes to prevent your shell from expanding the glob:

deno run --watch --watch-exclude='*.js' main.ts

Hot module replacement Jump to heading#

deno run also supports the --watch-hmr flag, which hot-replaces changed modules in the running process instead of restarting it. This keeps your application's state across edits. If hot replacement fails, the process falls back to a full restart.

deno run --watch-hmr main.ts

Editors with atomic save Jump to heading#

Some editors use "atomic save" (also called safe write), where the editor writes your changes to a temporary file and then renames it over the original on each save. On Linux this replaces the file with a new one on disk, which can detach the file watcher used by --watch-hmr after the first change. The symptom is that hot replacement works once and then stops detecting further edits to that module.

If you hit this, disable atomic save in your editor:

• Helix: set [editor] atomic-save = false (it is enabled by default).
• Neovim/Vim: set :set backupcopy=yes.

Plain --watch is not affected, because each change triggers a full restart that re-establishes the watchers.