等待仅在异步功能中有效

Question

我在 lib/helper.jsvar myfunction = async function(x,y) {   ....   reutrn [variableA, variableB]}exports.myfunction = myfunction;然后我试图在另一个文件中使用它 var helper = require('./helper.js');    var start = function(a,b){     ....     const result = await helper.myfunction('test','test'); } exports.start = start;我有一个错误“等待仅在异步功能中有效”有什么问题

Answer 1

错误不是指myfunction而是start。

async function start() {

   ....

   const result = await helper.myfunction('test', 'test');

}

// My function

const myfunction = async function(x, y) {

  return [

    x,

    y,

  ];

}

// Start function

const start = async function(a, b) {

  const result = await myfunction('test', 'test');

  

  console.log(result);

}

// Call start

start();

我利用这个问题的机会来告诉你一个已知的反模式的使用await方法：return await。

错误

async function myfunction() {

  console.log('Inside of myfunction');

}

// Here we wait for the myfunction to finish

// and then returns a promise that'll be waited for aswell

// It's useless to wait the myfunction to finish before to return

// we can simply returns a promise that will be resolved later

async function start() {

  // useless await here

  return await myfunction();

}

// Call start

(async() => {

  console.log('before start');

  await start();

  

  console.log('after start');

})();

正确

async function myfunction() {

  console.log('Inside of myfunction');

}

// Here we wait for the myfunction to finish

// and then returns a promise that'll be waited for aswell

// It's useless to wait the myfunction to finish before to return

// we can simply returns a promise that will be resolved later

async function start() {

  return myfunction();

}

// Call start

(async() => {

  console.log('before start');

  await start();

  

  console.log('after start');

})();

Answer 2

当我收到此错误时，事实证明我在“异步”函数中调用了map函数，因此此错误消息实际上是指未标记为“异步”的map函数。通过从map函数中取消“ await”调用并提出了一些其他实现预期行为的方法，我解决了这个问题。

var myfunction = async function(x,y) {

    ....

    someArray.map(someVariable => { // <- This was the function giving the error

        return await someFunction(someVariable);

    });

}

Answer 3

真正的问题是您需要了解异步/等待如何在这里工作。错误在于start function()

您需要定义功能Async函数的使用await作为

await 使用承诺/未来/任务返回方法/功能

async 将方法/功能标记为能够使用等待。

Await实际上是在执行promise / resolve的相同过程，并且由于该功能是async其他任务，因此正在并行处理

有关更多信息，请参考 MDN DOCS

async function foo(){

let myAsyncCall = await .... ('/endpoint') // hitting on some api

console.log(myAsyncCall) // myAsyncCall will log out whenever the request get resolved

}

foo()