javascript - 异步函数与返回 New Promise

Question

更新

我已经阅读了十几篇关于这个主题的文章，但没有一篇涉及这个基本问题。我将在本文末尾开始列出资源部分。

原帖

我对 async 函数的理解是它返回一个 promise 。

MDN 文档:https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function

在我的程序中我可以这样写:

function testPromise() {
  return new Promise((resolve, reject) => {
    // DO WORK
    reject() // IF WORK FAILS
    resolve() // IF WORK IS SUCCESSFUL        
  })
}

async function mainFunction() {
  let variable
  try {
    variable = await testPromise()
  } catch(e) {
    throw e
  }
  return variable
}

我还可以将 testPromise 编写为异步函数，并在相同的上下文中编写 await。

async function testAsyncFunction() {
  //DO WORK AND THROW ERROR IF THEY OCCUR      
}

async function mainFunction() {
  let variable
  try {
    variable = await testAsyncFunction()
  } catch(e) {
    throw e
  }
  return variable
}

哪个被认为是最佳实践？如果我想创建异步操作，该函数应该使用 return New Promise 并在 async 函数中等待，还是正在等待来自 async 的函数async 功能相同区别？

资源

JavaScript ES 2017:通过示例学习 Async/Await https://codeburst.io/javascript-es-2017-learn-async-await-by-example-48acc58bad65

Javascript — ES8引入async/await函数 https://medium.com/@reasoncode/javascript-es8-introducing-async-await-functions-7a471ec7de8a

JavaScript 的 Async/Await 破坏 Promises 的 6 个原因(教程) https://hackernoon.com/6-reasons-why-javascripts-async-await-blows-promises-away-tutorial-c7ec10518dd9

--------------------电流--------------------

export default function time_neo4jUpdate({ store, action, change, args, }) {
  return new Promise(async (resolve, reject) => {
    try {
      const {
        thing: { type },
        nonValidatedArgs: { leapYear, root, start, end },
          _neo4j,
          _cypherReducers,
          _neo4jCreateReduce,
          _timetreeSteps: { update }
      } = store.getState()
      let results = []
      for (let i = 0; i < _neo4jCreateReduce.length; i++) {
        const result = await _neo4j.session(
          _neo4jCreateReduce[i],
          _cypherReducers.runQuery(update, i, root, start, end))
        results = [...results, result]
      }
      resolve({
        store,
        action: 'NEO4J_UPDATE',
        change: results,
        args
      })
    } catch (e) {
      const m = `NEO4J TIMETREE UPDATE: Unable to complete the UPDATE step for the timetree. WHAT: ${e}`
      reject(m)
    }
  })
}

--------------------作为异步函数------------------------

export default async function time_neo4jUpdate({ store, action, change, args, }) {
  try {
    const {
      thing: { type },
      nonValidatedArgs: { leapYear, root, start, end },
      _neo4j,
      _cypherReducers,
      _neo4jCreateReduce,
      _timetreeSteps: { update }
    } = store.getState()
    let results = []
    for (let i = 0; i < _neo4jCreateReduce.length; i++) {
      const result = await _neo4j.session(
        _neo4jCreateReduce[i],
        _cypherReducers.runQuery(update, i, root, start, end))
      results = [...results, result]
    }
    return {
      store,
      action: 'NEO4J_UPDATE',
      change: results,
      args
    }
  } catch (e) {
    const m = `NEO4J TIMETREE UPDATE: Unable to complete the UPDATE step for the timetree. WHAT: ${e}`
    throw m
  }
}

Answer 1

即使没有可用的async/await，you should very rarely need to use new Promise() .如果您经常使用它，这通常是一种代码味道。

async/await 的全部意义在于，它可以让您避免很多需要显式处理 promises 的情况。

因此，如果您的目标环境支持它(Internet Explorer 不支持 async/await)或者您正在使用转译器，请继续使用它可以在任何地方。这就是它的用途。

请记住，这是毫无意义的:

catch(e) {
    throw e;
}

捕捉错误只是为了重新抛出它是没有意义的。因此，如果您实际上没有对捕获的错误执行任何操作，请不要捕获它:

async function testAsyncFunction() {
  //DO WORK AND THROW ERROR IF THEY OCCUR
  return value
}

编辑:既然您已经提供了代码示例，我可以更加肯定地回答:如果您的功能是基于现有的 promise ，那么无论如何，它很好用async/await 并且您通常不应该使用new Promise():

export default async function time_neo4jUpdate({
    store,
    action,
    change,
    args,
  }) {
    try {
      const {
        thing: {
          type
        },
        nonValidatedArgs: {
          leapYear,
          root,
          start,
          end
          },
          _neo4j,
          _cypherReducers,
          _neo4jCreateReduce,
          _timetreeSteps: {
            update
        }
      } = store.getState()

      const results = await _neo4jCreateReduce.reduce(async function (acc, el) {
        const result = await _neo4j.session(
          el,
          _cypherReducers.runQuery(
            update,
            i,
            root,
            start,
            end
          )
        )

        return [...(await acc), result]
      }, []);

      return {
        store,
        action: 'NEO4J_UPDATE',
        change: results,
        args
      };
    } catch (e) {
      const m = `NEO4J TIMETREE UPDATE: Unable to complete the UPDATE step for the timetree. WHAT: ${e}`
      throw m;
    }
}