co 库实现原理

Question

const fs = require('fs');

function fn1 () {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      console.log(100)
      resolve(100)
    }, 1000);
  })
}

function fn2 () {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      console.log(200)
      resolve(200)
    }, 1000);
  })
}

function* gen () {
  yield fn1()
  yield fn2()
}

function co (it) {
  return new Promise((resolve, reject) => {
    function next (r) {
      let {value, done} = it.next(r)
      if (!done) {
        value.then(r => {
          next(r)
        }, reject)
      } else {
        resolve(r)
      }
    }
    next()
  })
}

co(gen())