member-hooks

Add before and after hooks to an object

Installation

yarn add member-hooks
npm install member-hooks

Usage

A hook is a function that defines a set of before/at/after functions on methods of an object. The before functions, when present, are executed before the method in order of priority. The after functions, when present, are executed after the method in order of priority. The at function with the highest priority, when present, replaces the method

Hooks must be registered by calling the register method on an instance of the Hooks class.

To install/uninstall an array of hooks to an object, call the install/uninstall method. The hooks to install are specified as an array of ['hookName', hookOptions] arrays. Where hookName is the name under which the hook was registered.

The after hook can return a context object that will be passed to the after hook as 3rd argument.

A hook function can optionally return a create function that will be called at install. This create function can optionally return a destroy function that will be call at uninstall. The create function receives the target and optional context argument passed to install.

Example:

import test from 'ava'
import sinon from 'sinon'
import { HookMethods, Hooks, TCreate } from '.'

function ensureNumber(methods: HookMethods, options: { defaultValue: number, createSpy: TCreate }): TCreate {
  const { defaultValue, createSpy } = { defaultValue: 0, ...options }
  // add before hook to 'negative' method at priority 5
  methods.before('negative', 5, function (args: any[]): void {
    const value = args[0]
    args[0] = typeof value === 'number' ? value : defaultValue
  })
  return createSpy
}

function minMax(methods: HookMethods, options: { min: number, max: number }): void {
  const { min, max } = { min: 0, max: 100, ...options }
  // add before hook to 'negative' method at priority 10 (executed after ensureNumber)
  methods.before('negative', 10, function (args: any[]): void {
    args[0] = Math.max(Math.min(args[0], max), min)
  })
  // add after hook to 'add' method at priority -100
  methods.after('add', -100, function (result: number): number {
    return Math.max(Math.min(result, max), min)
  })
}

class TestClass {
  public negative(value: any): any {
    return -value
  }

  public add(a: number, b: number): number {
    return a + b
  }
}

test('TestClass', t => {
  // register hooks
  const hooks = new Hooks()
  hooks.register('ensureNumber', ensureNumber)
  hooks.register('minMax', minMax)

  // install/uninstall the hooks on test object
  const testInstance = new TestClass()
  // The hooks to install are specified as an array of array where the inner array
  // has the format ['hookName', hookOptions]
  const destroySpy = sinon.spy()
  const createSpy = sinon.spy(() => destroySpy)
  hooks.install(testInstance, [
    ['minMax', { min: 0, max: 100 }],
    ['ensureNumber', { defaultValue: 200, createSpy }]
  ])

  t.assert(testInstance.negative(undefined) === -100)
  t.assert(testInstance.add(100, 200) === 100)

  hooks.uninstall(testInstance)
  t.assert(destroySpy.calledOnce)
})

Before hook

A before-hook is a function executed before a method. It receives all arguments passed to the method as an array. This allows the before-hook to change the arguments passed to the method before it is called. When defined as a normal function (not arrow function), 'this' refers to the object of the method.

Example: A before hook that ensures the first argument to the method is an object with an id field

function beforeHook(args: any[]): void {
  let doc = args[0]
  if (!isObject(doc)) {
    doc = {}
  }
  if (!('id' in doc)) {
    doc.id = uuid()
  }
  args[0] = doc
}

After hook

A after-hook is a function executed after a method. It receives the value returned from the method and an array of arguments passed to the method. This allows the after-hook to modify the returned value. When defined as a normal function (not arrow function), 'this' refers to the object of the method.

Example: afterHook validates wether returned value is smaller then then the first method argument

function afterHook(result: number, args: any[]): number {
  const max = args[0]
  if (result > max) {
    throw new Error(`${result} should be smaller then ${max}`)
  }
  return result
}