At the heart of KOA is the middleware mechanism, where CTX objects are eventually returned after each middleware process. We can use the official koa-compose for this chain treatment.

The use of koa – compose

Let’s take a look at how libraries are used in general. The simplest interface to get a list of users:

async function findUsers(ctx) {
  try {
    ctx.body = awaitCall another method to the database to find the user ({query: ctx.request.query })
  } catch (err) {
    ctx.status = 404
    ctx.body = { err, message: err.message }
  }
}
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We want to check the interface’s query parameters, and the code we use to check should be generic, abstracting a validatorMiddleware:

async function validatorMiddleware(ctx, next) {
    // verify the query parameters
    await next() // Call the next middleware
}
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When we export the interface we will use koa-compose to do this:

const compose = require('koa-compose')

module.exports = compose([
  validatorMiddleware,
  findUsers
])
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The source code

How does KoA-Compose combine multiple middleware components? The source code for Koa-compose is very simple, with only a few dozen lines (source address: github.com/koajs/compo…). :

'use strict'

/** * Expose compositor. */

module.exports = compose

/** * Compose `middleware` returning * a fully valid middleware comprised * of all those which are passed. * * @param {Array} middleware * @return {Function} * @api public */

function compose (middleware) {
  if (!Array.isArray(middleware)) throw new TypeError('Middleware stack must be an array! ')
  for (const fn of middleware) {
    if (typeoffn ! = ='function') throw new TypeError('Middleware must be composed of functions! ')}/** * @param {Object} context * @return {Promise} * @api public */

  return function (context, next) {
    // last called middleware #
    let index = - 1
    return dispatch(0)
    function dispatch (i) {
      if (i <= index) return Promise.reject(new Error('next() called multiple times'))
      index = i
      let fn = middleware[i]
      if (i === middleware.length) fn = next
      if(! fn)return Promise.resolve()
      try {
        return Promise.resolve(fn(context, dispatch.bind(null, i + 1)));
      } catch (err) {
        return Promise.reject(err)
      }
    }
  }
}
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  1. Use the index variable to cache the number of middleware calls to prevent multiple calls to next() in some middleware:
let index = - 1
return dispatch(0)
function dispatch (i) {
  if (i <= index) return Promise.reject(new Error('next() called multiple times'))... }Copy the code
  1. To recursively call the middleware in the Middleware array:
function dispatch (i) {... let fn = middleware[i] ... try {return Promise.resolve(fn(context, dispatch.bind(null, i + 1)));
  } catch (err) {
    return Promise.reject(err)
  }
}
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  1. According to the array subscript, judge that the last middleware has been called (there is no next middleware), return a Promise object with the depressing state, and return the middleware of the next layer (function unstack) :
if (i === middleware.length) fn = next
if(! fn)return Promise.resolve()
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reference

Take you into the world of KOA2 (KOA2 source talk) : imhjm.com/article/591…