简介

主要是koa用来组合中间件的插件

使用

  1. $ npm install koa-compose
  3. fn = compose([a, b, c, ...])
  4. // Compose the given middleware and return middleware.

源码阅读

package.json

  1. {
  2. ...
  3. "devDependencies": {
  4.   "codecov": "^3.0.0",
  5.   "jest": "^27.0.6",
  6.   "standard": "^16.0.3"
  7.  },
  8. ...
  9. }

codecov:测试结果展示平台（此包已经被移除了）
jest：此包可以用来对JavaScript进行测试
standard：JavaScript的标准

index.js

  1. 'use strict'
  3. /**
  4.  * Expose compositor.
  5.  */
  7. // module.exports = compose
  9. /**
  10.  * Compose `middleware` returning
  11.  * a fully valid middleware comprised
  12.  * of all those which are passed.
  13.  *
  14.  * @param {Array} middleware
  15.  * @return {Function}
  16.  * @api public
  17.  */
  19. function compose (middleware) {
  20.   if (!Array.isArray(middleware)) throw new TypeError('Middleware stack must be an array!')
  21.   for (const fn of middleware) {
  22.     if (typeof fn !== 'function') throw new TypeError('Middleware must be composed of functions!')
  23.   }
  25.   /**
  26.    * @param {Object} context
  27.    * @return {Promise}
  28.    * @api public
  29.    */
  31.   return function (context, next) {
  32.     // last called middleware #
  33.     let index = -1
  34.     return dispatch(0)
  35.     function dispatch (i) {
  36.       if (i <= index) return Promise.reject(new Error('next() called multiple times'))
  37.       index = i
  38.       let fn = middleware[i]
  39.       if (i === middleware.length) fn = next
  40.       if (!fn) return Promise.resolve()
  41.       try {
  42.         return Promise.resolve(fn(context, dispatch.bind(null, i + 1)))
  43.       } catch (err) {
  44.         return Promise.reject(err)
  45.       }
  46.     }
  47.   }
  48. }

执行过程
image.png
其他的一些例子，可以参照test/test.js的测试用例来调试

总结

1.了解了compose执行的原理，get到了闭包的实际使用场景