异步

Promise

• 模块用法

import axios from 'axios'

export function fetchHtml(url) {
  return new Promise(resolve => {
    axios.get(url).then(response => resolve(response.data))
  })
}

• 在 then 里面（第二个 fn）处理 error，与在外面 catch 有何区别？

  • catch()即Promise.prototype.catch(onRejected)相当于Promise.prototype.then(undefined, onRejected)
  • catch 能捕获 then 中抛出的错误，比在 then 中处理错误更好
  • 示例

  apiCall()
    .then(function() {
      throw new Error('uh oh 1')
    })
    .catch(function(e) {
      log('this will catch the error')
    })

• Promise.all 可以并行请求，也就是说多个请求依赖时，都尽量写成 all 形式

function test() {
  return Promise.all([1, 2]).then((a, b) => ({ a, b }))
}
console.log(test().then(r => console.log(r)))

// bad
async function someAsyncFunc() {
  const user = await asyncGetUser()
  const categories = await asyncGetCategories()
  const mapping = await asyncMapUserWithCategory(user, categories)
}

// good
async function someAsyncFunc() {
  const [user, categories] = await Promise.all([
    asyncGetUser(),
    asyncGetCategories()
  ])
  const mapping = await asyncMapUserWithCategory(user, categories)
}

参考

JavaScript Promise 迷你书（中文版）