exit-hook

What is exit-hook?

The exit-hook npm package allows developers to easily register callbacks to be executed when a Node.js process exits. This can be particularly useful for cleaning up resources, saving state, or performing other shutdown tasks in a Node.js application.

What are exit-hook's main functionalities?

Registering a simple exit hook

This feature allows you to register a callback function that will be called when the process exits. The code sample demonstrates how to register a simple exit hook that logs a message to the console when the process exits.

const exitHook = require('exit-hook');
exitHook(() => {
  console.log('Process is exiting');
});

Unregistering an exit hook

This feature allows for the deregistration of an exit hook. The code sample shows how to register an exit hook and then later unregister it, preventing the callback from being called when the process exits.

const exitHook = require('exit-hook');
const unsubscribe = exitHook(() => {
  console.log('Process is exiting');
});

// Later in the code, to unregister the hook
disconnect();

Other packages similar to exit-hook

signal-exit

signal-exit is a package that provides similar functionality to exit-hook, allowing you to run callbacks when a process exits. It differs in its implementation details and the breadth of exit signals it can handle, making it a good alternative depending on specific project requirements.

async-exit-hook

async-exit-hook extends the basic idea of exit-hook by adding support for asynchronous shutdown tasks. This can be particularly useful for ensuring that all asynchronous operations (e.g., database connections, file writes) are completed before the process exits.

README

exit-hook

Run some code when the process exits

The process.on('exit') event doesn't catch all the ways a process can exit.

This package is useful for cleaning up before exiting.

Install

npm install exit-hook

Usage

import exitHook from 'exit-hook';

exitHook(() => {
	console.log('Exiting');
});

// You can add multiple hooks, even across files
exitHook(() => {
	console.log('Exiting 2');
});

throw new Error('🦄');

//=> 'Exiting'
//=> 'Exiting 2'

Removing an exit hook:

import exitHook from 'exit-hook';

const unsubscribe = exitHook(() => {});

unsubscribe();

API

exitHook(onExit)

Register a function to run during process.exit.

Returns a function that removes the hook when called.

onExit

Type: function(): void

The callback function to execute when the process exits.

asyncExitHook(onExit, minimumWait)

Register a function to run during gracefulExit.

Returns a function that removes the hook when called.

Please see Async Notes for considerations when using the asynchronous API.

import {asyncExitHook} from 'exit-hook';

asyncExitHook(async () => {
	console.log('Exiting');
}, 300);

throw new Error('🦄');

//=> 'Exiting'

Removing an asynchronous exit hook:

import {asyncExitHook} from 'exit-hook';

const unsubscribe = asyncExitHook(async () => {
	console.log('Exiting');
}, {
	minimumWait: 300
});

unsubscribe();

onExit

Type: function(): void | Promise<void>

The callback function to execute when the process exits via gracefulExit, and will be wrapped in Promise.resolve.

options

minimumWait

Type: number

The amount of time in milliseconds that the onExit function is expected to take.

gracefulExit(signal?: number): void

Exit the process and make a best-effort to complete all asynchronous hooks.

If you are using asyncExitHook, consider using gracefulExit() instead of process.exit() to ensure all asynchronous tasks are given an opportunity to run.

import {gracefulExit} from 'exit-hook';

gracefulExit();

signal

Type: number
Default: 0

The exit code to use. Same as the argument to process.exit().

Asynchronous Exit Notes

tl;dr If you have 100% control over how your process terminates, then you can swap exitHook and process.exit for asyncExitHook and gracefulExit respectively. Otherwise, keep reading to understand important tradeoffs if you're using asyncExitHook.

Node.js does not offer an asynchronous shutdown API by default #1 #2, so asyncExitHook and gracefulExit will make a "best effort" attempt to shut down the process and run your asynchronous tasks.

If you have asynchronous hooks registered and your Node.js process is terminated in a synchronous manner, a SYNCHRONOUS TERMINATION NOTICE error will be logged to the console. To avoid this, ensure you're only exiting via gracefulExit or that an upstream process manager is sending a SIGINT or SIGTERM signal to Node.js.

Asynchronous hooks should make a "best effort" to perform their tasks within the minimumWait time, but also be written to assume they may not complete their tasks before termination.