@push.rocks/smartwatch

A cross-runtime file watcher with glob pattern support for Node.js and Bun.

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Install

npm install @push.rocks/smartwatch
# or
pnpm add @push.rocks/smartwatch

Features

• Cross-Runtime — Works on Node.js 20+ and Bun; Deno is unsupported and untested
• Glob Pattern Support — Watch files using patterns like **/*.ts and src/**/*.{js,jsx}
• RxJS Observables — Subscribe to file system events using reactive streams
• Dynamic Watching — Add or remove watch patterns at runtime
• Write Stabilization — Built-in debouncing and awaitWriteFinish support for atomic writes
• TypeScript First — Full TypeScript support with comprehensive type definitions

How It Works

smartwatch selects the best file watching backend for the current platform and runtime:

Platform and runtime	Backend
Linux x64/arm64, any runtime	Bundled Rust binary built on the notify crate (inotify), driven over IPC by @push.rocks/smartrust
Node.js/Bun elsewhere	chokidar v5 (uses fs.watch() internally)

The package ships statically linked musl Rust binaries for Linux x64 and arm64. These binaries run on both glibc and musl hosts, and smartwatch prefers them on Node.js and Bun alike. Their third-party notices ship in native-notices/, generated by tsrust. Without a binary for the platform, chokidar provides robust cross-platform file watching on Node.js and Bun with features like atomic write detection, inode tracking, and write stabilization.

Glob patterns are handled through picomatch — base paths are extracted from patterns and watched natively, while events are filtered through matchers before emission. Relative patterns resolve against the working directory when start() runs, and a path without glob characters watches that file, or that directory and everything below it.

Native binaries

The package ships these files; consumers may rely on their paths:

Target	Binary	Beside it
Linux x64	dist_rust/smartwatch-rust_linux_amd64_musl	.third-party-notices.json, .third-party-notices.txt, .tsrust-build.json
Linux arm64	dist_rust/smartwatch-rust_linux_arm64_musl	.third-party-notices.json, .third-party-notices.txt, .tsrust-build.json

Both binaries are statically linked against musl, so they need no glibc. @git.zone/tsrust writes the third-party notices of each binary beside it (<binary>.third-party-notices.json in the notices format of @git.zone/tsbundle, and a .txt rendering) and its build provenance (<binary>.tsrust-build.json).

Choosing the backend

The backend option selects the watcher:

• 'auto' (default): use the Rust watcher where its binary is found, and chokidar otherwise. Before it falls back, smartwatch logs why: [smartwatch] Falling back to chokidar: … (a warning) when the binary lookup fails or a platform that ships a binary has none, and [smartwatch] Using chokidar: … on other platforms.
• 'rust': require the Rust watcher and never fall back. When its binary cannot be found, start() rejects with SmartwatchBackendUnavailableError: its cause is the lookup error (such as smartrust's RustBinaryLocatorError), or, when no binary exists, its message names the platform and the searched paths.

With either backend, a binary that is found but does not start makes start() reject with SmartwatchBackendUnavailableError, whose message names what the binary reported (bridge errors, its stderr and exit status); bridge errors are also logged as [smartwatch] Rust bridge: ….

A smoke test of an application that must use the native watcher, for example a single executable application that embeds it, asserts this way:

import { Smartwatch, SmartwatchBackendUnavailableError } from '@push.rocks/smartwatch';

const watcher = new Smartwatch(['./config/**/*'], { backend: 'rust' });
try {
  await watcher.start(); // rejects instead of falling back to chokidar
} catch (error) {
  if (error instanceof SmartwatchBackendUnavailableError) {
    console.error(error.message, error.cause);
  }
  throw error;
} finally {
  await watcher.stop();
}

Single executable applications

smartwatch looks its binary up through @push.rocks/smartrust under the name smartwatch-rust. Inside a Node.js single executable application, smartrust resolves the sidecar embedded under exactly that name and searches nothing else. An application built with @git.zone/tssea therefore embeds the binary as the sidecar smartwatch-rust; tssea embeds the .third-party-notices.json beside it with it:

"@git.zone/tssea": {
  "trees": {
    "smartwatch": {
      "from": {
        "linux-x64": "./node_modules/@push.rocks/smartwatch/dist_rust/smartwatch-rust_linux_amd64_musl",
        "linux-arm64": "./node_modules/@push.rocks/smartwatch/dist_rust/smartwatch-rust_linux_arm64_musl"
      },
      "path": "smartwatch-rust",
      "targets": ["linux-x64", "linux-arm64"]
    }
  },
  "sidecars": { "smartwatch-rust": { "tree": "smartwatch", "path": "smartwatch-rust" } }
}

A single executable application that does not embed the sidecar logs the smartrust lookup error (RustBinaryLocatorError (ERR_RUST_BINARY_SEA_SIDECAR_UNAVAILABLE)) and uses chokidar; with backend: 'rust', start() rejects with SmartwatchBackendUnavailableError carrying that error as cause.

Usage

Basic Setup

import { Smartwatch } from '@push.rocks/smartwatch';

// Create a watcher with glob patterns
const watcher = new Smartwatch([
  './src/**/*.ts',
  './public/assets/**/*'
]);

// Start watching
await watcher.start();

start() resolves once the initial scan of the watched paths is complete. Files and directories that already exist at that point are the baseline and are not reported: subscribers receive add, addDir, change, unlink and unlinkDir only for changes made after start() resolved. This holds on every platform and runtime, and again for every start() that follows a stop().

A start() that cannot complete rejects and leaves the watcher idle, so it can be started again:

Error	When
SmartwatchStoppedError	stop() was called before start() completed
SmartwatchStartTimeoutError	the initial scan did not complete within startTimeoutMs (default 30 seconds)
SmartwatchScanError	the initial scan failed, for example with EMFILE (too many open files); cause holds the underlying error
SmartwatchBackendUnavailableError	with backend: 'rust', the Rust watcher binary cannot be found (cause holds the lookup error; without one, the message names the platform and the searched paths); with either backend, the binary found does not start (the message names the bridge errors, its stderr and exit status)

All four extend SmartwatchError and are exported from the package:

import { Smartwatch, SmartwatchStoppedError } from '@push.rocks/smartwatch';

try {
  await watcher.start();
} catch (error) {
  if (!(error instanceof SmartwatchStoppedError)) throw error;
  // stop() was called while starting: nothing to do
}

Subscribing to File Events

Use RxJS observables to react to file system changes. getObservableFor() resolves at once, before or after start():

// Get an observable for file changes
const changeObservable = await watcher.getObservableFor('change');
changeObservable.subscribe(([path, stats]) => {
  console.log(`File changed: ${path}`);
  console.log(`New size: ${stats?.size} bytes`);
});

// Watch for new files
const addObservable = await watcher.getObservableFor('add');
addObservable.subscribe(([path]) => {
  console.log(`File added: ${path}`);
});

// Watch for deleted files
const unlinkObservable = await watcher.getObservableFor('unlink');
unlinkObservable.subscribe(([path]) => {
  console.log(`File deleted: ${path}`);
});

Supported Events

Event	Description
add	File has been added
addDir	Directory has been added
change	File has been modified
unlink	File has been removed
unlinkDir	Directory has been removed
error	Error occurred
ready	Initial scan complete (emitted just before start() resolves; subscribe before start() to observe it)

Dynamic Watch Management

Add or remove patterns while the watcher is running:

const watcher = new Smartwatch(['./src/**/*.ts']);
await watcher.start();

// Add more patterns to watch
watcher.add(['./tests/**/*.spec.ts', './config/*.json']);

// Remove a pattern
watcher.remove('./src/**/*.test.ts');

Stopping the Watcher

await watcher.stop();

stop() also cancels a start() that is still running: that start() rejects with SmartwatchStoppedError. stop() resolves once the watcher is released, and a later start() begins afresh.

Complete Example

import { Smartwatch } from '@push.rocks/smartwatch';

async function watchProject() {
  const watcher = new Smartwatch([
    './src/**/*.ts',
    './package.json'
  ]);

  await watcher.start();
  console.log('Watching for changes...');

  const changes = await watcher.getObservableFor('change');
  changes.subscribe(([path, stats]) => {
    console.log(`Modified: ${path} (${stats?.size ?? 'unknown'} bytes)`);
  });

  const additions = await watcher.getObservableFor('add');
  additions.subscribe(([path]) => {
    console.log(`New file: ${path}`);
  });

  const deletions = await watcher.getObservableFor('unlink');
  deletions.subscribe(([path]) => {
    console.log(`Deleted: ${path}`);
  });

  // Handle graceful shutdown
  process.on('SIGINT', async () => {
    await watcher.stop();
    process.exit(0);
  });
}

watchProject();

API Reference

Smartwatch

Constructor

new Smartwatch(patterns: string[], options?: ISmartwatchOptions)

Creates a new Smartwatch instance with the given glob patterns.

Parameters:

• patterns — Array of glob patterns to watch (e.g., ['./src/**/*.ts', './config/*.json']); relative patterns resolve against the working directory, and a path without glob characters watches that file or directory tree
• options — Optional settings:

Option	Type	Default	Description
startTimeoutMs	number	30000	How long start() may take to complete its initial scan before it rejects with SmartwatchStartTimeoutError, in milliseconds. A positive integer up to 2147483647; any other value makes the constructor throw a RangeError.
backend	'auto' | 'rust'	'auto'	'auto' uses the Rust watcher where available and falls back to chokidar, logging why, when no binary is found or the lookup fails; 'rust' requires the Rust watcher, and start() rejects with SmartwatchBackendUnavailableError instead of falling back. With either backend, a binary that is found but does not start makes start() reject with SmartwatchBackendUnavailableError (see Choosing the backend). Any other value makes the constructor throw a RangeError.

Methods

Method	Returns	Description
start()	Promise<void>	Starts watching; resolves once the initial scan is complete, rejects with a SmartwatchError if it cannot
stop()	Promise<void>	Stops the file watcher and cleans up resources; cancels a running start()
add(patterns: string[])	void	Adds additional patterns to watch
remove(pattern: string)	void	Removes a pattern from the watch list
getObservableFor(event: TFsEvent)	Promise<Observable<[string, Stats | undefined]>>	Returns an RxJS observable for the specified event; resolves at once

Properties

Property	Type	Description
status	'idle' | 'starting' | 'watching'	Current watcher status

Types

type TFsEvent = 'add' | 'addDir' | 'change' | 'error' | 'unlink' | 'unlinkDir' | 'ready' | 'raw';
type TSmartwatchStatus = 'idle' | 'starting' | 'watching';

type TSmartwatchBackend = 'auto' | 'rust';

interface ISmartwatchOptions {
  startTimeoutMs?: number; // default 30000
  backend?: TSmartwatchBackend; // default 'auto'
}

class SmartwatchError extends Error {}
class SmartwatchStoppedError extends SmartwatchError {}
class SmartwatchStartTimeoutError extends SmartwatchError { readonly timeoutMs: number }
class SmartwatchScanError extends SmartwatchError {} // cause: the scan error
class SmartwatchBackendUnavailableError extends SmartwatchError {} // cause: the lookup error, if any

Requirements

Runtime	Version
Node.js	20+
Bun	Uses Node.js compatibility layer

License and Legal Information

This repository contains open-source code licensed under the MIT License. A copy of the license can be found in the LICENSE file.

Please note: The MIT License does not grant permission to use the trade names, trademarks, service marks, or product names of the project, except as required for reasonable and customary use in describing the origin of the work and reproducing the content of the NOTICE file.

Trademarks

This project is owned and maintained by Task Venture Capital GmbH. The names and logos associated with Task Venture Capital GmbH and any related products or services are trademarks of Task Venture Capital GmbH or third parties, and are not included within the scope of the MIT license granted herein.

Use of these trademarks must comply with Task Venture Capital GmbH's Trademark Guidelines or the guidelines of the respective third-party owners, and any usage must be approved in writing. Third-party trademarks used herein are the property of their respective owners and used only in a descriptive manner, e.g. for an implementation of an API or similar.

Company Information

Task Venture Capital GmbH Registered at District Court Bremen HRB 35230 HB, Germany

For any legal inquiries or further information, please contact us via email at hello@task.vc.

By using this repository, you acknowledge that you have read this section, agree to comply with its terms, and understand that the licensing of the code does not imply endorsement by Task Venture Capital GmbH of any derivative works.