@poppinss/hooks

A simple yet effective implementation for executing hooks around an event.

This package is a zero-dependency implementation for running lifecycle hooks around an event. Following are some of the notable features.

• Register and run lifecycle hooks.
• Hooks can return cleanup functions that are executed to perform the cleanup.
• Super lightweight

Setup

Install the package from the npm packages registry.

1npm i @poppinss/hooks
2
3# yarn lovers
4yarn add @poppinss/hooks

And import the Hooks class as follows.

1import Hooks from '@poppinss/hooks'
2
3const hooks = new Hooks()
4
5hooks.add('saving', function () {
6  console.log('called')
7})
8
9// Execute hooks using runner
10await hooks.runner('saving').run()

Defining hooks

The hooks are defined using the hooks.add method. The method accepts the event name and a callback function to execute.

1const hooks = new Hooks()
2
3hooks.add('saving', function () {
4  console.log('called')
5})

You can also define hook as an object with the name and the handle method property. This is usually helpful when you want to specify a custom name for the hook, or re-use the same handle method multiple times.

1const hooks = new Hooks()
2
3function handleSave() {}
4
5hooks.add('saving', { name: 'beforeSave', handle: handleSave })
6hooks.add('creating', { name: 'beforeCreate', handle: handleSave })

The handle method receives the first argument as the event name, followed by the rest of the arguments supplied during runtime.

Running hooks

You can execute hooks using the Hooks Runner. You can create a new runner instance by calling the hooks.runner method and passing the event name for which you want to execute hooks.

1const hooks = new Hooks()
2
3const runner = hooks.runner('saving')
4await runner.run()

To run hooks in the reverse order, you can use the runner.runReverse method.

1const hooks = new Hooks()
2
3const runner = hooks.runner('saving')
4await runner.runReverse()

Passing data to hooks

You can pass one or more arguments to the runner.run method, which the runner will share with the hook callbacks. For example:

1const hooks = new Hooks()
2
3hooks.add('saving', function (model, transaction) {})
4
5const runner = hooks.runner('saving')
6await runner.run(model, transaction)

Cleanup functions

Cleanup functions allow hooks to clean up after themselves after the main action finishes successfully or with an error. Let's consider a real-world example of saving a model to the database.

• You will first run the saving hooks.
• Assume one of the saving hooks writes some files to the disk.
• Next, you issue the insert query to the database, and the query fails.
• The hook that has written files to the disk would want to remove those files as the main operation got canceled with an error.

Following is how you can express that with cleanup functions.

1hooks.add('saving', function () {
2  await fs.writeFile()
3
4  // Return the cleanup function
5  return (error) => {
6    // In case of an error, remove the file
7    if (error) {
8      await fs.unlink()
9    }
10  }
11})

The code responsible for issuing the insert query should run hooks as follows.

1const runner = hooks.runner('saving')
2
3try {
4  await runner.run(model)
5  await model.save()
6} catch (error) {
7  // Perform cleanup and pass error
8  await runner.cleanup(error)
9  throw error
10}
11
12// Perform cleanup in case of success as well
13await runner.cleanup()

Note: The runner.cleanup method is idempotent. Therefore you can call it multiple times, yet it will run the underlying cleanup methods only once.

Exclude certain hook handlers

You can exclude certain hook handlers from executing using the without method.

In the following example, we run hooks without executing the generateDefaultAvatar hook handler. As you can notice, you can specify the function name as a string.

1hooks.add('saving', function hashPassword() {})
2hooks.add('saving', function generateDefaultAvatar() {})
3
4await hooks.runner('saving').without(['generateDefaultAvatar']).run()

Event types

You can also specify the types of supported events and their arguments well in advance as follows.

The first step is to define a type for all the events.

1type Events = {
2  saving: [
3    [BaseModel], // for hook handler
4    [error: Error | null, BaseModel], // for cleanup function
5  ]
6  finding: [
7    [QueryBuilder], // for hook handler
8    [error: Error | null, QueryBuilder], // for cleanup function
9  ]
10}

And then pass it as a generic to the Hooks class.

1const hooks = new Hooks<Events>()