vmo-onion

vmo-onion

vmo-onion is a middleware processing class based on the onion model, designed to execute a series of middleware functions in sequence. This library is suitable for scenarios where requests or data streams need to be processed step-by-step, with each processing step controlling the execution of subsequent steps.

Installation

npm i vmo-onion

Usage

Import the Library

Ensure your project supports ES6 module syntax. You can import the library using the following code:

import { VmoOnion } from 'vmo-onion'

Define Middleware

Middleware is a function return a function that takes two parameters: a context object and a next function. Each middleware can choose whether to call next to pass control to the next middleware.

const middleware1 = async function (context, next) {
  console.log('Middleware 1 start')
  context.value = 1
  await next()
  context.value = 1
  console.log('Middleware 1 end')
}

const middleware2 = async function (context, next) {
  console.log('Middleware 2 start')
  context.value = 2
  context.value = 2
  console.log('Middleware 2 end')
}

Note: Middleware functions can be synchronous or asynchronous (returning a Promise).

Create vmo-onion Instance

You can initialize a vmo-onion instance with an array of middleware functions.

const onion = new VmoOnion([middleware1, middleware2])

Alternatively, you can initialize an empty instance and add middleware using the use method:

const onion = new VmoOnion()
onion.use(middleware1)
onion.use(middleware2)

Process Data

Use the pipingData method to process data. This method takes a context object and an optional array of middleware functions, and returns a Promise that resolves to the processed context object.

const contextData = { value: 0 }

onion
  .pipingData(contextData)
  .then(data => console.log('Final Context:', data))
  .catch(err => console.error('Error:', err))

In this example, control will flow through the middleware in the following order: middleware1 -> middleware2. Each middleware will return from its subsequent middleware in reverse order.

Example

A simple application of the processing flow might look like this:

import { VmoOnion } from 'vmo-onion'
import type { MiddleWare } from 'vmo-onion'

// create an instance of VmoOnion
const onion = new VmoOnion<{ value: number }>()

// 定义中间件
const middleware1: MiddleWare<{ value: number }> = async function (context, next) {
  console.log('Middleware 1: first step')
  context.value += 1
  await next()
  console.log('Middleware 1: final step')
}

const middleware2: MiddleWare<{ value: number }> = async function (context, next) {
  console.log('Middleware 2: first step')
  context.value += 1
  console.log('Middleware 2: final step')
}

// use middleware
onion.use(middleware1)
onion.use(middleware2)

// init context
const initialContext = { value: 0 }

onion
  .pipingData(initialContext)
  .then(context => {
    console.log('Processed Context:', context) // { value: 2 }
  })
  .catch(err => {
    console.error('Error:', err)
  })

Expected output:

Middleware 1: first step
Middleware 2: first step
Middleware 2: final step
Middleware 1: final step
Processed Context: { value: 2 }

Error Handling

If any middleware throws an exception or returns a rejected Promise, the error will be caught in the catch handler of the pipingData method.

const errorMiddleware = (context, next) => {
  console.log('Error Middleware')
  throw new Error('Something went wrong!')
}

onion.use(errorMiddleware)

onion
  .pipingData({})
  .then(() => console.log('Process completed successfully.'))
  .catch(error => console.error('Error caught:', error)) // Error caught: Error: Something went wrong!

API

Class VmoOnion

Constructor

• new VmoOnion([middlewares]): Creates a VmoOnion instance and optionally initializes an array of middleware functions.
  • middlewares: An array of middleware functions to be executed in sequence. If not provided, it initializes as an empty array.
• Also, you can define the Context type with TS by following the steps below

import { VmoOnion } from 'vmo-onion'
import type { MiddleWare } from 'vmo-onion'

const onion = new VmoOnion<{ name: string; age: number }>([])
// From now on, the middleware must accept parameters with the Context type as {name: string, age: number}
// or
const sampleMiddleWare: MiddleWare<{ name: string; age: number }> = (context, next) => {
  //.....
}

Instance Methods

• use(func): Adds a middleware function to the middleware stack.

  • func: The middleware function.

• pipingData(data, [middlewares]): Processes the context object through the middleware stack.

  • data: The context object to be modified step-by-step by the middleware.
  • middlewares: Optional, an array of middleware functions to be executed. Defaults to the middleware array initialized in the instance.

Private Methods

• checkMiddleWares(middlewares): Checks if the middleware array is valid, i.e., whether it is an array of functions.

  • middlewares: The middleware array.

• compose(middlewares): Composes the middleware array into a single function that executes all middleware in sequence.

  -middlewares: The middleware array.

Error Handling Mechanism

The library ensures that next() is not called multiple times, preventing infinite recursion. When this occurs, the library rejects the Promise with the error message next() called multiple times.